/*
* Copyright (c) 2001, 2019, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#include "precompiled.hpp"
#include "classfile/classLoaderDataGraph.hpp"
#include "classfile/systemDictionary.hpp"
#include "code/codeCache.hpp"
#include "gc/cms/cmsGCStats.hpp"
#include "gc/cms/cmsHeap.hpp"
#include "gc/cms/cmsOopClosures.inline.hpp"
#include "gc/cms/cmsVMOperations.hpp"
#include "gc/cms/compactibleFreeListSpace.hpp"
#include "gc/cms/concurrentMarkSweepGeneration.inline.hpp"
#include "gc/cms/concurrentMarkSweepThread.hpp"
#include "gc/cms/parNewGeneration.hpp"
#include "gc/cms/promotionInfo.inline.hpp"
#include "gc/serial/genMarkSweep.hpp"
#include "gc/serial/tenuredGeneration.hpp"
#include "gc/shared/adaptiveSizePolicy.hpp"
#include "gc/shared/cardGeneration.inline.hpp"
#include "gc/shared/cardTableRS.hpp"
#include "gc/shared/collectedHeap.inline.hpp"
#include "gc/shared/collectorCounters.hpp"
#include "gc/shared/gcLocker.hpp"
#include "gc/shared/gcPolicyCounters.hpp"
#include "gc/shared/gcTimer.hpp"
#include "gc/shared/gcTrace.hpp"
#include "gc/shared/gcTraceTime.inline.hpp"
#include "gc/shared/genCollectedHeap.hpp"
#include "gc/shared/genOopClosures.inline.hpp"
#include "gc/shared/isGCActiveMark.hpp"
#include "gc/shared/owstTaskTerminator.hpp"
#include "gc/shared/referencePolicy.hpp"
#include "gc/shared/referenceProcessorPhaseTimes.hpp"
#include "gc/shared/space.inline.hpp"
#include "gc/shared/strongRootsScope.hpp"
#include "gc/shared/taskqueue.inline.hpp"
#include "gc/shared/weakProcessor.hpp"
#include "gc/shared/workerPolicy.hpp"
#include "logging/log.hpp"
#include "logging/logStream.hpp"
#include "memory/allocation.hpp"
#include "memory/binaryTreeDictionary.inline.hpp"
#include "memory/iterator.inline.hpp"
#include "memory/padded.hpp"
#include "memory/resourceArea.hpp"
#include "memory/universe.hpp"
#include "oops/access.inline.hpp"
#include "oops/oop.inline.hpp"
#include "prims/jvmtiExport.hpp"
#include "runtime/atomic.hpp"
#include "runtime/flags/flagSetting.hpp"
#include "runtime/globals_extension.hpp"
#include "runtime/handles.inline.hpp"
#include "runtime/java.hpp"
#include "runtime/orderAccess.hpp"
#include "runtime/timer.hpp"
#include "runtime/vmThread.hpp"
#include "services/memoryService.hpp"
#include "services/runtimeService.hpp"
#include "utilities/align.hpp"
#include "utilities/stack.inline.hpp"
#if INCLUDE_JVMCI
#include "jvmci/jvmci.hpp"
#endif
// statics
CMSCollector* ConcurrentMarkSweepGeneration::_collector = NULL;
bool CMSCollector::_full_gc_requested = false;
GCCause::Cause CMSCollector::_full_gc_cause = GCCause::_no_gc;
//////////////////////////////////////////////////////////////////
// In support of CMS/VM thread synchronization
//////////////////////////////////////////////////////////////////
// We split use of the CGC_lock into 2 "levels".
// The low-level locking is of the usual CGC_lock monitor. We introduce
// a higher level "token" (hereafter "CMS token") built on top of the
// low level monitor (hereafter "CGC lock").
// The token-passing protocol gives priority to the VM thread. The
// CMS-lock doesn't provide any fairness guarantees, but clients
// should ensure that it is only held for very short, bounded
// durations.
//
// When either of the CMS thread or the VM thread is involved in
// collection operations during which it does not want the other
// thread to interfere, it obtains the CMS token.
//
// If either thread tries to get the token while the other has
// it, that thread waits. However, if the VM thread and CMS thread
// both want the token, then the VM thread gets priority while the
// CMS thread waits. This ensures, for instance, that the "concurrent"
// phases of the CMS thread's work do not block out the VM thread
// for long periods of time as the CMS thread continues to hog
// the token. (See bug 4616232).
//
// The baton-passing functions are, however, controlled by the
// flags _foregroundGCShouldWait and _foregroundGCIsActive,
// and here the low-level CMS lock, not the high level token,
// ensures mutual exclusion.
//
// Two important conditions that we have to satisfy:
// 1. if a thread does a low-level wait on the CMS lock, then it
// relinquishes the CMS token if it were holding that token
// when it acquired the low-level CMS lock.
// 2. any low-level notifications on the low-level lock
// should only be sent when a thread has relinquished the token.
//
// In the absence of either property, we'd have potential deadlock.
//
// We protect each of the CMS (concurrent and sequential) phases
// with the CMS _token_, not the CMS _lock_.
//
// The only code protected by CMS lock is the token acquisition code
// itself, see ConcurrentMarkSweepThread::[de]synchronize(), and the
// baton-passing code.
//
// Unfortunately, i couldn't come up with a good abstraction to factor and
// hide the naked CGC_lock manipulation in the baton-passing code
// further below. That's something we should try to do. Also, the proof
// of correctness of this 2-level locking scheme is far from obvious,
// and potentially quite slippery. We have an uneasy suspicion, for instance,
// that there may be a theoretical possibility of delay/starvation in the
// low-level lock/wait/notify scheme used for the baton-passing because of
// potential interference with the priority scheme embodied in the
// CMS-token-passing protocol. See related comments at a CGC_lock->wait()
// invocation further below and marked with "XXX 20011219YSR".
// Indeed, as we note elsewhere, this may become yet more slippery
// in the presence of multiple CMS and/or multiple VM threads. XXX
class CMSTokenSync: public StackObj {
private:
bool _is_cms_thread;
public:
CMSTokenSync(bool is_cms_thread):
_is_cms_thread(is_cms_thread) {
assert(is_cms_thread == Thread::current()->is_ConcurrentGC_thread(),
"Incorrect argument to constructor");
ConcurrentMarkSweepThread::synchronize(_is_cms_thread);
}
~CMSTokenSync() {
assert(_is_cms_thread ?
ConcurrentMarkSweepThread::cms_thread_has_cms_token() :
ConcurrentMarkSweepThread::vm_thread_has_cms_token(),
"Incorrect state");
ConcurrentMarkSweepThread::desynchronize(_is_cms_thread);
}
};
// Convenience class that does a CMSTokenSync, and then acquires
// upto three locks.
class CMSTokenSyncWithLocks: public CMSTokenSync {
private:
// Note: locks are acquired in textual declaration order
// and released in the opposite order
MutexLocker _locker1, _locker2, _locker3;
public:
CMSTokenSyncWithLocks(bool is_cms_thread, Mutex* mutex1,
Mutex* mutex2 = NULL, Mutex* mutex3 = NULL):
CMSTokenSync(is_cms_thread),
_locker1(mutex1, Mutex::_no_safepoint_check_flag),
_locker2(mutex2, Mutex::_no_safepoint_check_flag),
_locker3(mutex3, Mutex::_no_safepoint_check_flag)
{ }
};
//////////////////////////////////////////////////////////////////
// Concurrent Mark-Sweep Generation /////////////////////////////
//////////////////////////////////////////////////////////////////
NOT_PRODUCT(CompactibleFreeListSpace* debug_cms_space;)
// This struct contains per-thread things necessary to support parallel
// young-gen collection.
class CMSParGCThreadState: public CHeapObj<mtGC> {
public:
CompactibleFreeListSpaceLAB lab;
PromotionInfo promo;
// Constructor.
CMSParGCThreadState(CompactibleFreeListSpace* cfls) : lab(cfls) {
promo.setSpace(cfls);
}
};
ConcurrentMarkSweepGeneration::ConcurrentMarkSweepGeneration(
ReservedSpace rs,
size_t initial_byte_size,
size_t min_byte_size,
size_t max_byte_size,
CardTableRS* ct) :
CardGeneration(rs, initial_byte_size, ct),
_dilatation_factor(((double)MinChunkSize)/((double)(CollectedHeap::min_fill_size()))),
_did_compact(false)
{
HeapWord* bottom = (HeapWord*) _virtual_space.low();
HeapWord* end = (HeapWord*) _virtual_space.high();
_direct_allocated_words = 0;
NOT_PRODUCT(
_numObjectsPromoted = 0;
_numWordsPromoted = 0;
_numObjectsAllocated = 0;
_numWordsAllocated = 0;
)
_cmsSpace = new CompactibleFreeListSpace(_bts, MemRegion(bottom, end));
NOT_PRODUCT(debug_cms_space = _cmsSpace;)
_cmsSpace->_old_gen = this;
_gc_stats = new CMSGCStats();
// Verify the assumption that FreeChunk::_prev and OopDesc::_klass
// offsets match. The ability to tell free chunks from objects
// depends on this property.
debug_only(
FreeChunk* junk = NULL;
assert(UseCompressedClassPointers ||
junk->prev_addr() == (void*)(oop(junk)->klass_addr()),
"Offset of FreeChunk::_prev within FreeChunk must match"
" that of OopDesc::_klass within OopDesc");
)
_par_gc_thread_states = NEW_C_HEAP_ARRAY(CMSParGCThreadState*, ParallelGCThreads, mtGC);
for (uint i = 0; i < ParallelGCThreads; i++) {
_par_gc_thread_states[i] = new CMSParGCThreadState(cmsSpace());
}
_incremental_collection_failed = false;
// The "dilatation_factor" is the expansion that can occur on
// account of the fact that the minimum object size in the CMS
// generation may be larger than that in, say, a contiguous young
// generation.
// Ideally, in the calculation below, we'd compute the dilatation
// factor as: MinChunkSize/(promoting_gen's min object size)
// Since we do not have such a general query interface for the
// promoting generation, we'll instead just use the minimum
// object size (which today is a header's worth of space);
// note that all arithmetic is in units of HeapWords.
assert(MinChunkSize >= CollectedHeap::min_fill_size(), "just checking");
assert(_dilatation_factor >= 1.0, "from previous assert");
initialize_performance_counters(min_byte_size, max_byte_size);
}
// The field "_initiating_occupancy" represents the occupancy percentage
// at which we trigger a new collection cycle. Unless explicitly specified
// via CMSInitiatingOccupancyFraction (argument "io" below), it
// is calculated by:
//
// Let "f" be MinHeapFreeRatio in
//
// _initiating_occupancy = 100-f +
// f * (CMSTriggerRatio/100)
// where CMSTriggerRatio is the argument "tr" below.
//
// That is, if we assume the heap is at its desired maximum occupancy at the
// end of a collection, we let CMSTriggerRatio of the (purported) free
// space be allocated before initiating a new collection cycle.
//
void ConcurrentMarkSweepGeneration::init_initiating_occupancy(intx io, uintx tr) {
assert(io <= 100 && tr <= 100, "Check the arguments");
if (io >= 0) {
_initiating_occupancy = (double)io / 100.0;
} else {
_initiating_occupancy = ((100 - MinHeapFreeRatio) +
(double)(tr * MinHeapFreeRatio) / 100.0)
/ 100.0;
}
}
void ConcurrentMarkSweepGeneration::ref_processor_init() {
assert(collector() != NULL, "no collector");
collector()->ref_processor_init();
}
void CMSCollector::ref_processor_init() {
if (_ref_processor == NULL) {
// Allocate and initialize a reference processor
_ref_processor =
new ReferenceProcessor(&_span_based_discoverer,
(ParallelGCThreads > 1) && ParallelRefProcEnabled, // mt processing
ParallelGCThreads, // mt processing degree
_cmsGen->refs_discovery_is_mt(), // mt discovery
MAX2(ConcGCThreads, ParallelGCThreads), // mt discovery degree
_cmsGen->refs_discovery_is_atomic(), // discovery is not atomic
&_is_alive_closure, // closure for liveness info
false); // disable adjusting number of processing threads
// Initialize the _ref_processor field of CMSGen
_cmsGen->set_ref_processor(_ref_processor);
}
}
AdaptiveSizePolicy* CMSCollector::size_policy() {
return CMSHeap::heap()->size_policy();
}
void ConcurrentMarkSweepGeneration::initialize_performance_counters(size_t min_old_size,
size_t max_old_size) {
const char* gen_name = "old";
// Generation Counters - generation 1, 1 subspace
_gen_counters = new GenerationCounters(gen_name, 1, 1,
min_old_size, max_old_size, &_virtual_space);
_space_counters = new GSpaceCounters(gen_name, 0,
_virtual_space.reserved_size(),
this, _gen_counters);
}
CMSStats::CMSStats(ConcurrentMarkSweepGeneration* cms_gen, unsigned int alpha):
_cms_gen(cms_gen)
{
assert(alpha <= 100, "bad value");
_saved_alpha = alpha;
// Initialize the alphas to the bootstrap value of 100.
_gc0_alpha = _cms_alpha = 100;
_cms_begin_time.update();
_cms_end_time.update();
_gc0_duration = 0.0;
_gc0_period = 0.0;
_gc0_promoted = 0;
_cms_duration = 0.0;
_cms_period = 0.0;
_cms_allocated = 0;
_cms_used_at_gc0_begin = 0;
_cms_used_at_gc0_end = 0;
_allow_duty_cycle_reduction = false;
_valid_bits = 0;
}
double CMSStats::cms_free_adjustment_factor(size_t free) const {
// TBD: CR 6909490
return 1.0;
}
void CMSStats::adjust_cms_free_adjustment_factor(bool fail, size_t free) {
}
// If promotion failure handling is on use
// the padded average size of the promotion for each
// young generation collection.
double CMSStats::time_until_cms_gen_full() const {
size_t cms_free = _cms_gen->cmsSpace()->free();
CMSHeap* heap = CMSHeap::heap();
size_t expected_promotion = MIN2(heap->young_gen()->capacity(),
(size_t) _cms_gen->gc_stats()->avg_promoted()->padded_average());
if (cms_free > expected_promotion) {
// Start a cms collection if there isn't enough space to promote
// for the next young collection. Use the padded average as
// a safety factor.
cms_free -= expected_promotion;
// Adjust by the safety factor.
double cms_free_dbl = (double)cms_free;
double cms_adjustment = (100.0 - CMSIncrementalSafetyFactor) / 100.0;
// Apply a further correction factor which tries to adjust
// for recent occurance of concurrent mode failures.
cms_adjustment = cms_adjustment * cms_free_adjustment_factor(cms_free);
cms_free_dbl = cms_free_dbl * cms_adjustment;
log_trace(gc)("CMSStats::time_until_cms_gen_full: cms_free " SIZE_FORMAT " expected_promotion " SIZE_FORMAT,
cms_free, expected_promotion);
log_trace(gc)(" cms_free_dbl %f cms_consumption_rate %f", cms_free_dbl, cms_consumption_rate() + 1.0);
// Add 1 in case the consumption rate goes to zero.
return cms_free_dbl / (cms_consumption_rate() + 1.0);
}
return 0.0;
}
// Compare the duration of the cms collection to the
// time remaining before the cms generation is empty.
// Note that the time from the start of the cms collection
// to the start of the cms sweep (less than the total
// duration of the cms collection) can be used. This
// has been tried and some applications experienced
// promotion failures early in execution. This was
// possibly because the averages were not accurate
// enough at the beginning.
double CMSStats::time_until_cms_start() const {
// We add "gc0_period" to the "work" calculation
// below because this query is done (mostly) at the
// end of a scavenge, so we need to conservatively
// account for that much possible delay
// in the query so as to avoid concurrent mode failures
// due to starting the collection just a wee bit too
// late.
double work = cms_duration() + gc0_period();
double deadline = time_until_cms_gen_full();
// If a concurrent mode failure occurred recently, we want to be
// more conservative and halve our expected time_until_cms_gen_full()
if (work > deadline) {
log_develop_trace(gc)("CMSCollector: collect because of anticipated promotion before full %3.7f + %3.7f > %3.7f ",
cms_duration(), gc0_period(), time_until_cms_gen_full());
return 0.0;
}
return work - deadline;
}
#ifndef PRODUCT
void CMSStats::print_on(outputStream *st) const {
st->print(" gc0_alpha=%d,cms_alpha=%d", _gc0_alpha, _cms_alpha);
st->print(",gc0_dur=%g,gc0_per=%g,gc0_promo=" SIZE_FORMAT,
gc0_duration(), gc0_period(), gc0_promoted());
st->print(",cms_dur=%g,cms_per=%g,cms_alloc=" SIZE_FORMAT,
cms_duration(), cms_period(), cms_allocated());
st->print(",cms_since_beg=%g,cms_since_end=%g",
cms_time_since_begin(), cms_time_since_end());
st->print(",cms_used_beg=" SIZE_FORMAT ",cms_used_end=" SIZE_FORMAT,
_cms_used_at_gc0_begin, _cms_used_at_gc0_end);
if (valid()) {
st->print(",promo_rate=%g,cms_alloc_rate=%g",
promotion_rate(), cms_allocation_rate());
st->print(",cms_consumption_rate=%g,time_until_full=%g",
cms_consumption_rate(), time_until_cms_gen_full());
}
st->cr();
}
#endif // #ifndef PRODUCT
CMSCollector::CollectorState CMSCollector::_collectorState =
CMSCollector::Idling;
bool CMSCollector::_foregroundGCIsActive = false;
bool CMSCollector::_foregroundGCShouldWait = false;
CMSCollector::CMSCollector(ConcurrentMarkSweepGeneration* cmsGen,
CardTableRS* ct):
_overflow_list(NULL),
_conc_workers(NULL), // may be set later
_completed_initialization(false),
_collection_count_start(0),
_should_unload_classes(CMSClassUnloadingEnabled),
_concurrent_cycles_since_last_unload(0),
_roots_scanning_options(GenCollectedHeap::SO_None),
_verification_mark_bm(0, Mutex::leaf + 1, "CMS_verification_mark_bm_lock"),
_verifying(false),
_inter_sweep_estimate(CMS_SweepWeight, CMS_SweepPadding),
_intra_sweep_estimate(CMS_SweepWeight, CMS_SweepPadding),
_gc_tracer_cm(new (ResourceObj::C_HEAP, mtGC) CMSTracer()),
_gc_timer_cm(new (ResourceObj::C_HEAP, mtGC) ConcurrentGCTimer()),
_cms_start_registered(false),
_cmsGen(cmsGen),
// Adjust span to cover old (cms) gen
_span(cmsGen->reserved()),
_ct(ct),
_markBitMap(0, Mutex::leaf + 1, "CMS_markBitMap_lock"),
_modUnionTable((CardTable::card_shift - LogHeapWordSize),
-1 /* lock-free */, "No_lock" /* dummy */),
_restart_addr(NULL),
_ser_pmc_preclean_ovflw(0),
_ser_pmc_remark_ovflw(0),
_par_pmc_remark_ovflw(0),
_ser_kac_preclean_ovflw(0),
_ser_kac_ovflw(0),
_par_kac_ovflw(0),
#ifndef PRODUCT
_num_par_pushes(0),
#endif
_span_based_discoverer(_span),
_ref_processor(NULL), // will be set later
// Construct the is_alive_closure with _span & markBitMap
_is_alive_closure(_span, &_markBitMap),
_modUnionClosurePar(&_modUnionTable),
_between_prologue_and_epilogue(false),
_abort_preclean(false),
_start_sampling(false),
_stats(cmsGen),
_eden_chunk_lock(new Mutex(Mutex::leaf + 1, "CMS_eden_chunk_lock", true,
//verify that this lock should be acquired with safepoint check.
Monitor::_safepoint_check_never)),
_eden_chunk_array(NULL), // may be set in ctor body
_eden_chunk_index(0), // -- ditto --
_eden_chunk_capacity(0), // -- ditto --
_survivor_chunk_array(NULL), // -- ditto --
_survivor_chunk_index(0), // -- ditto --
_survivor_chunk_capacity(0), // -- ditto --
_survivor_plab_array(NULL) // -- ditto --
{
// Now expand the span and allocate the collection support structures
// (MUT, marking bit map etc.) to cover both generations subject to
// collection.
// For use by dirty card to oop closures.
_cmsGen->cmsSpace()->set_collector(this);
// Allocate MUT and marking bit map
{
MutexLocker x(_markBitMap.lock(), Mutex::_no_safepoint_check_flag);
if (!_markBitMap.allocate(_span)) {
log_warning(gc)("Failed to allocate CMS Bit Map");
return;
}
assert(_markBitMap.covers(_span), "_markBitMap inconsistency?");
}
{
_modUnionTable.allocate(_span);
assert(_modUnionTable.covers(_span), "_modUnionTable inconsistency?");
}
if (!_markStack.allocate(MarkStackSize)) {
log_warning(gc)("Failed to allocate CMS Marking Stack");
return;
}
// Support for multi-threaded concurrent phases
if (CMSConcurrentMTEnabled) {
if (FLAG_IS_DEFAULT(ConcGCThreads)) {
// just for now
FLAG_SET_DEFAULT(ConcGCThreads, (ParallelGCThreads + 3) / 4);
}
if (ConcGCThreads > 1) {
_conc_workers = new YieldingFlexibleWorkGang("CMS Thread",
ConcGCThreads, true);
if (_conc_workers == NULL) {
log_warning(gc)("GC/CMS: _conc_workers allocation failure: forcing -CMSConcurrentMTEnabled");
CMSConcurrentMTEnabled = false;
} else {
_conc_workers->initialize_workers();
}
} else {
CMSConcurrentMTEnabled = false;
}
}
if (!CMSConcurrentMTEnabled) {
ConcGCThreads = 0;
} else {
// Turn off CMSCleanOnEnter optimization temporarily for
// the MT case where it's not fixed yet; see 6178663.
CMSCleanOnEnter = false;
}
assert((_conc_workers != NULL) == (ConcGCThreads > 1),
"Inconsistency");
log_debug(gc)("ConcGCThreads: %u", ConcGCThreads);
log_debug(gc)("ParallelGCThreads: %u", ParallelGCThreads);
// Parallel task queues; these are shared for the
// concurrent and stop-world phases of CMS, but
// are not shared with parallel scavenge (ParNew).
{
uint i;
uint num_queues = MAX2(ParallelGCThreads, ConcGCThreads);
if ((CMSParallelRemarkEnabled || CMSConcurrentMTEnabled
|| ParallelRefProcEnabled)
&& num_queues > 0) {
_task_queues = new OopTaskQueueSet(num_queues);
if (_task_queues == NULL) {
log_warning(gc)("task_queues allocation failure.");
return;
}
typedef Padded<OopTaskQueue> PaddedOopTaskQueue;
for (i = 0; i < num_queues; i++) {
PaddedOopTaskQueue *q = new PaddedOopTaskQueue();
if (q == NULL) {
log_warning(gc)("work_queue allocation failure.");
return;
}
_task_queues->register_queue(i, q);
}
for (i = 0; i < num_queues; i++) {
_task_queues->queue(i)->initialize();
}
}
}
_cmsGen ->init_initiating_occupancy(CMSInitiatingOccupancyFraction, CMSTriggerRatio);
// Clip CMSBootstrapOccupancy between 0 and 100.
_bootstrap_occupancy = CMSBootstrapOccupancy / 100.0;
// Now tell CMS generations the identity of their collector
ConcurrentMarkSweepGeneration::set_collector(this);
// Create & start a CMS thread for this CMS collector
_cmsThread = ConcurrentMarkSweepThread::start(this);
assert(cmsThread() != NULL, "CMS Thread should have been created");
assert(cmsThread()->collector() == this,
"CMS Thread should refer to this gen");
assert(CGC_lock != NULL, "Where's the CGC_lock?");
// Support for parallelizing young gen rescan
CMSHeap* heap = CMSHeap::heap();
_young_gen = heap->young_gen();
if (heap->supports_inline_contig_alloc()) {
_top_addr = heap->top_addr();
_end_addr = heap->end_addr();
assert(_young_gen != NULL, "no _young_gen");
_eden_chunk_index = 0;
_eden_chunk_capacity = (_young_gen->max_capacity() + CMSSamplingGrain) / CMSSamplingGrain;
_eden_chunk_array = NEW_C_HEAP_ARRAY(HeapWord*, _eden_chunk_capacity, mtGC);
}
// Support for parallelizing survivor space rescan
if ((CMSParallelRemarkEnabled && CMSParallelSurvivorRemarkEnabled) || CMSParallelInitialMarkEnabled) {
const size_t max_plab_samples =
_young_gen->max_survivor_size() / (PLAB::min_size() * HeapWordSize);
_survivor_plab_array = NEW_C_HEAP_ARRAY(ChunkArray, ParallelGCThreads, mtGC);
_survivor_chunk_array = NEW_C_HEAP_ARRAY(HeapWord*, max_plab_samples, mtGC);
_cursor = NEW_C_HEAP_ARRAY(size_t, ParallelGCThreads, mtGC);
_survivor_chunk_capacity = max_plab_samples;
for (uint i = 0; i < ParallelGCThreads; i++) {
HeapWord** vec = NEW_C_HEAP_ARRAY(HeapWord*, max_plab_samples, mtGC);
ChunkArray* cur = ::new (&_survivor_plab_array[i]) ChunkArray(vec, max_plab_samples);
assert(cur->end() == 0, "Should be 0");
assert(cur->array() == vec, "Should be vec");
assert(cur->capacity() == max_plab_samples, "Error");
}
}
NOT_PRODUCT(_overflow_counter = CMSMarkStackOverflowInterval;)
_gc_counters = new CollectorCounters("CMS full collection pauses", 1);
_cgc_counters = new CollectorCounters("CMS concurrent cycle pauses", 2);
_completed_initialization = true;
_inter_sweep_timer.start(); // start of time
}
const char* ConcurrentMarkSweepGeneration::name() const {
return "concurrent mark-sweep generation";
}
void ConcurrentMarkSweepGeneration::update_counters() {
if (UsePerfData) {
_space_counters->update_all();
_gen_counters->update_all();
}
}
// this is an optimized version of update_counters(). it takes the
// used value as a parameter rather than computing it.
//
void ConcurrentMarkSweepGeneration::update_counters(size_t used) {
if (UsePerfData) {
_space_counters->update_used(used);
_space_counters->update_capacity();
_gen_counters->update_all();
}
}
void ConcurrentMarkSweepGeneration::print() const {
Generation::print();
cmsSpace()->print();
}
#ifndef PRODUCT
void ConcurrentMarkSweepGeneration::print_statistics() {
cmsSpace()->printFLCensus(0);
}
#endif
size_t
ConcurrentMarkSweepGeneration::contiguous_available() const {
// dld proposes an improvement in precision here. If the committed
// part of the space ends in a free block we should add that to
// uncommitted size in the calculation below. Will make this
// change later, staying with the approximation below for the
// time being. -- ysr.
return MAX2(_virtual_space.uncommitted_size(), unsafe_max_alloc_nogc());
}
size_t
ConcurrentMarkSweepGeneration::unsafe_max_alloc_nogc() const {
return _cmsSpace->max_alloc_in_words() * HeapWordSize;
}
size_t ConcurrentMarkSweepGeneration::used_stable() const {
return cmsSpace()->used_stable();
}
size_t ConcurrentMarkSweepGeneration::max_available() const {
return free() + _virtual_space.uncommitted_size();
}
bool ConcurrentMarkSweepGeneration::promotion_attempt_is_safe(size_t max_promotion_in_bytes) const {
size_t available = max_available();
size_t av_promo = (size_t)gc_stats()->avg_promoted()->padded_average();
bool res = (available >= av_promo) || (available >= max_promotion_in_bytes);
log_trace(gc, promotion)("CMS: promo attempt is%s safe: available(" SIZE_FORMAT ") %s av_promo(" SIZE_FORMAT "), max_promo(" SIZE_FORMAT ")",
res? "":" not", available, res? ">=":"<", av_promo, max_promotion_in_bytes);
return res;
}
// At a promotion failure dump information on block layout in heap
// (cms old generation).
void ConcurrentMarkSweepGeneration::promotion_failure_occurred() {
Log(gc, promotion) log;
if (log.is_trace()) {
LogStream ls(log.trace());
cmsSpace()->dump_at_safepoint_with_locks(collector(), &ls);
}
}
void ConcurrentMarkSweepGeneration::reset_after_compaction() {
// Clear the promotion information. These pointers can be adjusted
// along with all the other pointers into the heap but
// compaction is expected to be a rare event with
// a heap using cms so don't do it without seeing the need.
for (uint i = 0; i < ParallelGCThreads; i++) {
_par_gc_thread_states[i]->promo.reset();
}
}
void ConcurrentMarkSweepGeneration::compute_new_size() {
assert_locked_or_safepoint(Heap_lock);
// If incremental collection failed, we just want to expand
// to the limit.
if (incremental_collection_failed()) {
clear_incremental_collection_failed();
grow_to_reserved();
return;
}
// The heap has been compacted but not reset yet.
// Any metric such as free() or used() will be incorrect.
CardGeneration::compute_new_size();
// Reset again after a possible resizing
if (did_compact()) {
cmsSpace()->reset_after_compaction();
}
}
void ConcurrentMarkSweepGeneration::compute_new_size_free_list() {
assert_locked_or_safepoint(Heap_lock);
// If incremental collection failed, we just want to expand
// to the limit.
if (incremental_collection_failed()) {
clear_incremental_collection_failed();
grow_to_reserved();
return;
}
double free_percentage = ((double) free()) / capacity();
double desired_free_percentage = (double) MinHeapFreeRatio / 100;
double maximum_free_percentage = (double) MaxHeapFreeRatio / 100;
// compute expansion delta needed for reaching desired free percentage
if (free_percentage < desired_free_percentage) {
size_t desired_capacity = (size_t)(used() / ((double) 1 - desired_free_percentage));
assert(desired_capacity >= capacity(), "invalid expansion size");
size_t expand_bytes = MAX2(desired_capacity - capacity(), MinHeapDeltaBytes);
Log(gc) log;
if (log.is_trace()) {
size_t desired_capacity = (size_t)(used() / ((double) 1 - desired_free_percentage));
log.trace("From compute_new_size: ");
log.trace(" Free fraction %f", free_percentage);
log.trace(" Desired free fraction %f", desired_free_percentage);
log.trace(" Maximum free fraction %f", maximum_free_percentage);
log.trace(" Capacity " SIZE_FORMAT, capacity() / 1000);
log.trace(" Desired capacity " SIZE_FORMAT, desired_capacity / 1000);
CMSHeap* heap = CMSHeap::heap();
size_t young_size = heap->young_gen()->capacity();
log.trace(" Young gen size " SIZE_FORMAT, young_size / 1000);
log.trace(" unsafe_max_alloc_nogc " SIZE_FORMAT, unsafe_max_alloc_nogc() / 1000);
log.trace(" contiguous available " SIZE_FORMAT, contiguous_available() / 1000);
log.trace(" Expand by " SIZE_FORMAT " (bytes)", expand_bytes);
}
// safe if expansion fails
expand_for_gc_cause(expand_bytes, 0, CMSExpansionCause::_satisfy_free_ratio);
log.trace(" Expanded free fraction %f", ((double) free()) / capacity());
} else {
size_t desired_capacity = (size_t)(used() / ((double) 1 - desired_free_percentage));
assert(desired_capacity <= capacity(), "invalid expansion size");
size_t shrink_bytes = capacity() - desired_capacity;
// Don't shrink unless the delta is greater than the minimum shrink we want
if (shrink_bytes >= MinHeapDeltaBytes) {
shrink_free_list_by(shrink_bytes);
}
}
}
Mutex* ConcurrentMarkSweepGeneration::freelistLock() const {
return cmsSpace()->freelistLock();
}
HeapWord* ConcurrentMarkSweepGeneration::allocate(size_t size, bool tlab) {
CMSSynchronousYieldRequest yr;
MutexLocker x(freelistLock(), Mutex::_no_safepoint_check_flag);
return have_lock_and_allocate(size, tlab);
}
HeapWord* ConcurrentMarkSweepGeneration::have_lock_and_allocate(size_t size,
bool tlab /* ignored */) {
assert_lock_strong(freelistLock());
size_t adjustedSize = CompactibleFreeListSpace::adjustObjectSize(size);
HeapWord* res = cmsSpace()->allocate(adjustedSize);
// Allocate the object live (grey) if the background collector has
// started marking. This is necessary because the marker may
// have passed this address and consequently this object will
// not otherwise be greyed and would be incorrectly swept up.
// Note that if this object contains references, the writing
// of those references will dirty the card containing this object
// allowing the object to be blackened (and its references scanned)
// either during a preclean phase or at the final checkpoint.
if (res != NULL) {
// We may block here with an uninitialized object with
// its mark-bit or P-bits not yet set. Such objects need
// to be safely navigable by block_start().
assert(oop(res)->klass_or_null() == NULL, "Object should be uninitialized here.");
assert(!((FreeChunk*)res)->is_free(), "Error, block will look free but show wrong size");
collector()->direct_allocated(res, adjustedSize);
_direct_allocated_words += adjustedSize;
// allocation counters
NOT_PRODUCT(
_numObjectsAllocated++;
_numWordsAllocated += (int)adjustedSize;
)
}
return res;
}
// In the case of direct allocation by mutators in a generation that
// is being concurrently collected, the object must be allocated
// live (grey) if the background collector has started marking.
// This is necessary because the marker may
// have passed this address and consequently this object will
// not otherwise be greyed and would be incorrectly swept up.
// Note that if this object contains references, the writing
// of those references will dirty the card containing this object
// allowing the object to be blackened (and its references scanned)
// either during a preclean phase or at the final checkpoint.
void CMSCollector::direct_allocated(HeapWord* start, size_t size) {
assert(_markBitMap.covers(start, size), "Out of bounds");
if (_collectorState >= Marking) {
MutexLocker y(_markBitMap.lock(),
Mutex::_no_safepoint_check_flag);
// [see comments preceding SweepClosure::do_blk() below for details]
//
// Can the P-bits be deleted now? JJJ
//
// 1. need to mark the object as live so it isn't collected
// 2. need to mark the 2nd bit to indicate the object may be uninitialized
// 3. need to mark the end of the object so marking, precleaning or sweeping
// can skip over uninitialized or unparsable objects. An allocated
// object is considered uninitialized for our purposes as long as
// its klass word is NULL. All old gen objects are parsable
// as soon as they are initialized.)
_markBitMap.mark(start); // object is live
_markBitMap.mark(start + 1); // object is potentially uninitialized?
_markBitMap.mark(start + size - 1);
// mark end of object
}
// check that oop looks uninitialized
assert(oop(start)->klass_or_null() == NULL, "_klass should be NULL");
}
void CMSCollector::promoted(bool par, HeapWord* start,
bool is_obj_array, size_t obj_size) {
assert(_markBitMap.covers(start), "Out of bounds");
// See comment in direct_allocated() about when objects should
// be allocated live.
if (_collectorState >= Marking) {
// we already hold the marking bit map lock, taken in
// the prologue
if (par) {
_markBitMap.par_mark(start);
} else {
_markBitMap.mark(start);
}
// We don't need to mark the object as uninitialized (as
// in direct_allocated above) because this is being done with the
// world stopped and the object will be initialized by the
// time the marking, precleaning or sweeping get to look at it.
// But see the code for copying objects into the CMS generation,
// where we need to ensure that concurrent readers of the
// block offset table are able to safely navigate a block that
// is in flux from being free to being allocated (and in
// transition while being copied into) and subsequently
// becoming a bona-fide object when the copy/promotion is complete.
assert(SafepointSynchronize::is_at_safepoint(),
"expect promotion only at safepoints");
if (_collectorState < Sweeping) {
// Mark the appropriate cards in the modUnionTable, so that
// this object gets scanned before the sweep. If this is
// not done, CMS generation references in the object might
// not get marked.
// For the case of arrays, which are otherwise precisely
// marked, we need to dirty the entire array, not just its head.
if (is_obj_array) {
// The [par_]mark_range() method expects mr.end() below to
// be aligned to the granularity of a bit's representation
// in the heap. In the case of the MUT below, that's a
// card size.
MemRegion mr(start,
align_up(start + obj_size,
CardTable::card_size /* bytes */));
if (par) {
_modUnionTable.par_mark_range(mr);
} else {
_modUnionTable.mark_range(mr);
}
} else { // not an obj array; we can just mark the head
if (par) {
_modUnionTable.par_mark(start);
} else {
_modUnionTable.mark(start);
}
}
}
}
}
oop ConcurrentMarkSweepGeneration::promote(oop obj, size_t obj_size) {
assert(obj_size == (size_t)obj->size(), "bad obj_size passed in");
// allocate, copy and if necessary update promoinfo --
// delegate to underlying space.
assert_lock_strong(freelistLock());
#ifndef PRODUCT
if (CMSHeap::heap()->promotion_should_fail()) {
return NULL;
}
#endif // #ifndef PRODUCT
oop res = _cmsSpace->promote(obj, obj_size);
if (res == NULL) {
// expand and retry
size_t s = _cmsSpace->expansionSpaceRequired(obj_size); // HeapWords
expand_for_gc_cause(s*HeapWordSize, MinHeapDeltaBytes, CMSExpansionCause::_satisfy_promotion);
// Since this is the old generation, we don't try to promote
// into a more senior generation.
res = _cmsSpace->promote(obj, obj_size);
}
if (res != NULL) {
// See comment in allocate() about when objects should
// be allocated live.
assert(oopDesc::is_oop(obj), "Will dereference klass pointer below");
collector()->promoted(false, // Not parallel
(HeapWord*)res, obj->is_objArray(), obj_size);
// promotion counters
NOT_PRODUCT(
_numObjectsPromoted++;
_numWordsPromoted +=
(int)(CompactibleFreeListSpace::adjustObjectSize(obj->size()));
)
}
return res;
}
// IMPORTANT: Notes on object size recognition in CMS.
// ---------------------------------------------------
// A block of storage in the CMS generation is always in
// one of three states. A free block (FREE), an allocated
// object (OBJECT) whose size() method reports the correct size,
// and an intermediate state (TRANSIENT) in which its size cannot
// be accurately determined.
// STATE IDENTIFICATION: (32 bit and 64 bit w/o COOPS)
// -----------------------------------------------------
// FREE: klass_word & 1 == 1; mark_word holds block size
//
// OBJECT: klass_word installed; klass_word != 0 && klass_word & 1 == 0;
// obj->size() computes correct size
//
// TRANSIENT: klass_word == 0; size is indeterminate until we become an OBJECT
//
// STATE IDENTIFICATION: (64 bit+COOPS)
// ------------------------------------
// FREE: mark_word & CMS_FREE_BIT == 1; mark_word & ~CMS_FREE_BIT gives block_size
//
// OBJECT: klass_word installed; klass_word != 0;
// obj->size() computes correct size
//
// TRANSIENT: klass_word == 0; size is indeterminate until we become an OBJECT
//
//
// STATE TRANSITION DIAGRAM
//
// mut / parnew mut / parnew
// FREE --------------------> TRANSIENT ---------------------> OBJECT --|
// ^ |
// |------------------------ DEAD <------------------------------------|
// sweep mut
//
// While a block is in TRANSIENT state its size cannot be determined
// so readers will either need to come back later or stall until
// the size can be determined. Note that for the case of direct
// allocation, P-bits, when available, may be used to determine the
// size of an object that may not yet have been initialized.
// Things to support parallel young-gen collection.
oop
ConcurrentMarkSweepGeneration::par_promote(int thread_num,
oop old, markWord m,
size_t word_sz) {
#ifndef PRODUCT
if (CMSHeap::heap()->promotion_should_fail()) {
return NULL;
}
#endif // #ifndef PRODUCT
CMSParGCThreadState* ps = _par_gc_thread_states[thread_num];
PromotionInfo* promoInfo = &ps->promo;
// if we are tracking promotions, then first ensure space for
// promotion (including spooling space for saving header if necessary).
// then allocate and copy, then track promoted info if needed.
// When tracking (see PromotionInfo::track()), the mark word may
// be displaced and in this case restoration of the mark word
// occurs in the (oop_since_save_marks_)iterate phase.
if (promoInfo->tracking() && !promoInfo->ensure_spooling_space()) {
// Out of space for allocating spooling buffers;
// try expanding and allocating spooling buffers.
if (!expand_and_ensure_spooling_space(promoInfo)) {
return NULL;
}
}
assert(!promoInfo->tracking() || promoInfo->has_spooling_space(), "Control point invariant");
const size_t alloc_sz = CompactibleFreeListSpace::adjustObjectSize(word_sz);
HeapWord* obj_ptr = ps->lab.alloc(alloc_sz);
if (obj_ptr == NULL) {
obj_ptr = expand_and_par_lab_allocate(ps, alloc_sz);
if (obj_ptr == NULL) {
return NULL;
}
}
oop obj = oop(obj_ptr);
OrderAccess::storestore();
assert(obj->klass_or_null() == NULL, "Object should be uninitialized here.");
assert(!((FreeChunk*)obj_ptr)->is_free(), "Error, block will look free but show wrong size");
// IMPORTANT: See note on object initialization for CMS above.
// Otherwise, copy the object. Here we must be careful to insert the
// klass pointer last, since this marks the block as an allocated object.
// Except with compressed oops it's the mark word.
HeapWord* old_ptr = (HeapWord*)old;
// Restore the mark word copied above.
obj->set_mark_raw(m);
assert(obj->klass_or_null() == NULL, "Object should be uninitialized here.");
assert(!((FreeChunk*)obj_ptr)->is_free(), "Error, block will look free but show wrong size");
OrderAccess::storestore();
if (UseCompressedClassPointers) {
// Copy gap missed by (aligned) header size calculation below
obj->set_klass_gap(old->klass_gap());
}
if (word_sz > (size_t)oopDesc::header_size()) {
Copy::aligned_disjoint_words(old_ptr + oopDesc::header_size(),
obj_ptr + oopDesc::header_size(),
word_sz - oopDesc::header_size());
}
// Now we can track the promoted object, if necessary. We take care
// to delay the transition from uninitialized to full object
// (i.e., insertion of klass pointer) until after, so that it
// atomically becomes a promoted object.
if (promoInfo->tracking()) {
promoInfo->track((PromotedObject*)obj, old->klass());
}
assert(obj->klass_or_null() == NULL, "Object should be uninitialized here.");
assert(!((FreeChunk*)obj_ptr)->is_free(), "Error, block will look free but show wrong size");
assert(oopDesc::is_oop(old), "Will use and dereference old klass ptr below");
// Finally, install the klass pointer (this should be volatile).
OrderAccess::storestore();
obj->set_klass(old->klass());
// We should now be able to calculate the right size for this object
assert(oopDesc::is_oop(obj) && obj->size() == (int)word_sz, "Error, incorrect size computed for promoted object");
collector()->promoted(true, // parallel
obj_ptr, old->is_objArray(), word_sz);
NOT_PRODUCT(
Atomic::inc(&_numObjectsPromoted);
Atomic::add(alloc_sz, &_numWordsPromoted);
)
return obj;
}
void
ConcurrentMarkSweepGeneration::
par_promote_alloc_done(int thread_num) {
CMSParGCThreadState* ps = _par_gc_thread_states[thread_num];
ps->lab.retire(thread_num);
}
void
ConcurrentMarkSweepGeneration::
par_oop_since_save_marks_iterate_done(int thread_num) {
CMSParGCThreadState* ps = _par_gc_thread_states[thread_num];
ParScanWithoutBarrierClosure* dummy_cl = NULL;
ps->promo.promoted_oops_iterate(dummy_cl);
// Because card-scanning has been completed, subsequent phases
// (e.g., reference processing) will not need to recognize which
// objects have been promoted during this GC. So, we can now disable
// promotion tracking.
ps->promo.stopTrackingPromotions();
}
bool ConcurrentMarkSweepGeneration::should_collect(bool full,
size_t size,
bool tlab)
{
// We allow a STW collection only if a full
// collection was requested.
return full || should_allocate(size, tlab); // FIX ME !!!
// This and promotion failure handling are connected at the
// hip and should be fixed by untying them.
}
bool CMSCollector::shouldConcurrentCollect() {
LogTarget(Trace, gc) log;
if (_full_gc_requested) {
log.print("CMSCollector: collect because of explicit gc request (or GCLocker)");
return true;
}
FreelistLocker x(this);
// ------------------------------------------------------------------
// Print out lots of information which affects the initiation of
// a collection.
if (log.is_enabled() && stats().valid()) {
log.print("CMSCollector shouldConcurrentCollect: ");
LogStream out(log);
stats().print_on(&out);
log.print("time_until_cms_gen_full %3.7f", stats().time_until_cms_gen_full());
log.print("free=" SIZE_FORMAT, _cmsGen->free());
log.print("contiguous_available=" SIZE_FORMAT, _cmsGen->contiguous_available());
log.print("promotion_rate=%g", stats().promotion_rate());
log.print("cms_allocation_rate=%g", stats().cms_allocation_rate());
log.print("occupancy=%3.7f", _cmsGen->occupancy());
log.print("initiatingOccupancy=%3.7f", _cmsGen->initiating_occupancy());
log.print("cms_time_since_begin=%3.7f", stats().cms_time_since_begin());
log.print("cms_time_since_end=%3.7f", stats().cms_time_since_end());
log.print("metadata initialized %d", MetaspaceGC::should_concurrent_collect());
}
// ------------------------------------------------------------------
// If the estimated time to complete a cms collection (cms_duration())
// is less than the estimated time remaining until the cms generation
// is full, start a collection.
if (!UseCMSInitiatingOccupancyOnly) {
if (stats().valid()) {
if (stats().time_until_cms_start() == 0.0) {
return true;
}
} else {
// We want to conservatively collect somewhat early in order
// to try and "bootstrap" our CMS/promotion statistics;
// this branch will not fire after the first successful CMS
// collection because the stats should then be valid.
if (_cmsGen->occupancy() >= _bootstrap_occupancy) {
log.print(" CMSCollector: collect for bootstrapping statistics: occupancy = %f, boot occupancy = %f",
_cmsGen->occupancy(), _bootstrap_occupancy);
return true;
}
}
}
// Otherwise, we start a collection cycle if
// old gen want a collection cycle started. Each may use
// an appropriate criterion for making this decision.
// XXX We need to make sure that the gen expansion
// criterion dovetails well with this. XXX NEED TO FIX THIS
if (_cmsGen->should_concurrent_collect()) {
log.print("CMS old gen initiated");
return true;
}
// We start a collection if we believe an incremental collection may fail;
// this is not likely to be productive in practice because it's probably too
// late anyway.
CMSHeap* heap = CMSHeap::heap();
if (heap->incremental_collection_will_fail(true /* consult_young */)) {
log.print("CMSCollector: collect because incremental collection will fail ");
return true;
}
if (MetaspaceGC::should_concurrent_collect()) {
log.print("CMSCollector: collect for metadata allocation ");
return true;
}
// CMSTriggerInterval starts a CMS cycle if enough time has passed.
if (CMSTriggerInterval >= 0) {
if (CMSTriggerInterval == 0) {
// Trigger always
return true;
}
// Check the CMS time since begin (we do not check the stats validity
// as we want to be able to trigger the first CMS cycle as well)
if (stats().cms_time_since_begin() >= (CMSTriggerInterval / ((double) MILLIUNITS))) {
if (stats().valid()) {
log.print("CMSCollector: collect because of trigger interval (time since last begin %3.7f secs)",
stats().cms_time_since_begin());
} else {
log.print("CMSCollector: collect because of trigger interval (first collection)");
}
return true;
}
}
return false;
}
void CMSCollector::set_did_compact(bool v) { _cmsGen->set_did_compact(v); }
// Clear _expansion_cause fields of constituent generations
void CMSCollector::clear_expansion_cause() {
_cmsGen->clear_expansion_cause();
}
// We should be conservative in starting a collection cycle. To
// start too eagerly runs the risk of collecting too often in the
// extreme. To collect too rarely falls back on full collections,
// which works, even if not optimum in terms of concurrent work.
// As a work around for too eagerly collecting, use the flag
// UseCMSInitiatingOccupancyOnly. This also has the advantage of
// giving the user an easily understandable way of controlling the
// collections.
// We want to start a new collection cycle if any of the following
// conditions hold:
// . our current occupancy exceeds the configured initiating occupancy
// for this generation, or
// . we recently needed to expand this space and have not, since that
// expansion, done a collection of this generation, or
// . the underlying space believes that it may be a good idea to initiate
// a concurrent collection (this may be based on criteria such as the
// following: the space uses linear allocation and linear allocation is
// going to fail, or there is believed to be excessive fragmentation in
// the generation, etc... or ...
// [.(currently done by CMSCollector::shouldConcurrentCollect() only for
// the case of the old generation; see CR 6543076):
// we may be approaching a point at which allocation requests may fail because
// we will be out of sufficient free space given allocation rate estimates.]
bool ConcurrentMarkSweepGeneration::should_concurrent_collect() const {
assert_lock_strong(freelistLock());
if (occupancy() > initiating_occupancy()) {
log_trace(gc)(" %s: collect because of occupancy %f / %f ",
short_name(), occupancy(), initiating_occupancy());
return true;
}
if (UseCMSInitiatingOccupancyOnly) {
return false;
}
if (expansion_cause() == CMSExpansionCause::_satisfy_allocation) {
log_trace(gc)(" %s: collect because expanded for allocation ", short_name());
return true;
}
return false;
}
void ConcurrentMarkSweepGeneration::collect(bool full,
bool clear_all_soft_refs,
size_t size,
bool tlab)
{
collector()->collect(full, clear_all_soft_refs, size, tlab);
}
void CMSCollector::collect(bool full,
bool clear_all_soft_refs,
size_t size,
bool tlab)
{
// The following "if" branch is present for defensive reasons.
// In the current uses of this interface, it can be replaced with:
// assert(!GCLocker.is_active(), "Can't be called otherwise");
// But I am not placing that assert here to allow future
// generality in invoking this interface.
if (GCLocker::is_active()) {
// A consistency test for GCLocker
assert(GCLocker::needs_gc(), "Should have been set already");
// Skip this foreground collection, instead
// expanding the heap if necessary.
// Need the free list locks for the call to free() in compute_new_size()
compute_new_size();
return;
}
acquire_control_and_collect(full, clear_all_soft_refs);
}
void CMSCollector::request_full_gc(unsigned int full_gc_count, GCCause::Cause cause) {
CMSHeap* heap = CMSHeap::heap();
unsigned int gc_count = heap->total_full_collections();
if (gc_count == full_gc_count) {
MutexLocker y(CGC_lock, Mutex::_no_safepoint_check_flag);
_full_gc_requested = true;
_full_gc_cause = cause;
CGC_lock->notify(); // nudge CMS thread
} else {
assert(gc_count > full_gc_count, "Error: causal loop");
}
}
bool CMSCollector::is_external_interruption() {
GCCause::Cause cause = CMSHeap::heap()->gc_cause();
return GCCause::is_user_requested_gc(cause) ||
GCCause::is_serviceability_requested_gc(cause);
}
void CMSCollector::report_concurrent_mode_interruption() {
if (is_external_interruption()) {
log_debug(gc)("Concurrent mode interrupted");
} else {
log_debug(gc)("Concurrent mode failure");
_gc_tracer_cm->report_concurrent_mode_failure();
}
}
// The foreground and background collectors need to coordinate in order
// to make sure that they do not mutually interfere with CMS collections.
// When a background collection is active,
// the foreground collector may need to take over (preempt) and
// synchronously complete an ongoing collection. Depending on the
// frequency of the background collections and the heap usage
// of the application, this preemption can be seldom or frequent.
// There are only certain
// points in the background collection that the "collection-baton"
// can be passed to the foreground collector.
//
// The foreground collector will wait for the baton before
// starting any part of the collection. The foreground collector
// will only wait at one location.
//
// The background collector will yield the baton before starting a new
// phase of the collection (e.g., before initial marking, marking from roots,
// precleaning, final re-mark, sweep etc.) This is normally done at the head
// of the loop which switches the phases. The background collector does some
// of the phases (initial mark, final re-mark) with the world stopped.
// Because of locking involved in stopping the world,
// the foreground collector should not block waiting for the background
// collector when it is doing a stop-the-world phase. The background
// collector will yield the baton at an additional point just before
// it enters a stop-the-world phase. Once the world is stopped, the
// background collector checks the phase of the collection. If the
// phase has not changed, it proceeds with the collection. If the
// phase has changed, it skips that phase of the collection. See
// the comments on the use of the Heap_lock in collect_in_background().
//
// Variable used in baton passing.
// _foregroundGCIsActive - Set to true by the foreground collector when
// it wants the baton. The foreground clears it when it has finished
// the collection.
// _foregroundGCShouldWait - Set to true by the background collector
// when it is running. The foreground collector waits while
// _foregroundGCShouldWait is true.
// CGC_lock - monitor used to protect access to the above variables
// and to notify the foreground and background collectors.
// _collectorState - current state of the CMS collection.
//
// The foreground collector
// acquires the CGC_lock
// sets _foregroundGCIsActive
// waits on the CGC_lock for _foregroundGCShouldWait to be false
// various locks acquired in preparation for the collection
// are released so as not to block the background collector
// that is in the midst of a collection
// proceeds with the collection
// clears _foregroundGCIsActive
// returns
//
// The background collector in a loop iterating on the phases of the
// collection
// acquires the CGC_lock
// sets _foregroundGCShouldWait
// if _foregroundGCIsActive is set
// clears _foregroundGCShouldWait, notifies _CGC_lock
// waits on _CGC_lock for _foregroundGCIsActive to become false
// and exits the loop.
// otherwise
// proceed with that phase of the collection
// if the phase is a stop-the-world phase,
// yield the baton once more just before enqueueing
// the stop-world CMS operation (executed by the VM thread).
// returns after all phases of the collection are done
//
void CMSCollector::acquire_control_and_collect(bool full,
bool clear_all_soft_refs) {
assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
assert(!Thread::current()->is_ConcurrentGC_thread(),
"shouldn't try to acquire control from self!");
// Start the protocol for acquiring control of the
// collection from the background collector (aka CMS thread).
assert(ConcurrentMarkSweepThread::vm_thread_has_cms_token(),
"VM thread should have CMS token");
// Remember the possibly interrupted state of an ongoing
// concurrent collection
CollectorState first_state = _collectorState;
// Signal to a possibly ongoing concurrent collection that
// we want to do a foreground collection.
_foregroundGCIsActive = true;
// release locks and wait for a notify from the background collector
// releasing the locks in only necessary for phases which
// do yields to improve the granularity of the collection.
assert_lock_strong(bitMapLock());
// We need to lock the Free list lock for the space that we are
// currently collecting.
assert(haveFreelistLocks(), "Must be holding free list locks");
bitMapLock()->unlock();
releaseFreelistLocks();
{
MutexLocker x(CGC_lock, Mutex::_no_safepoint_check_flag);
if (_foregroundGCShouldWait) {
// We are going to be waiting for action for the CMS thread;
// it had better not be gone (for instance at shutdown)!
assert(ConcurrentMarkSweepThread::cmst() != NULL && !ConcurrentMarkSweepThread::cmst()->has_terminated(),
"CMS thread must be running");
// Wait here until the background collector gives us the go-ahead
ConcurrentMarkSweepThread::clear_CMS_flag(
ConcurrentMarkSweepThread::CMS_vm_has_token); // release token
// Get a possibly blocked CMS thread going:
// Note that we set _foregroundGCIsActive true above,
// without protection of the CGC_lock.
CGC_lock->notify();
assert(!ConcurrentMarkSweepThread::vm_thread_wants_cms_token(),
"Possible deadlock");
while (_foregroundGCShouldWait) {
// wait for notification
CGC_lock->wait_without_safepoint_check();
// Possibility of delay/starvation here, since CMS token does
// not know to give priority to VM thread? Actually, i think
// there wouldn't be any delay/starvation, but the proof of
// that "fact" (?) appears non-trivial. XXX 20011219YSR
}
ConcurrentMarkSweepThread::set_CMS_flag(
ConcurrentMarkSweepThread::CMS_vm_has_token);
}
}
// The CMS_token is already held. Get back the other locks.
assert(ConcurrentMarkSweepThread::vm_thread_has_cms_token(),
"VM thread should have CMS token");
getFreelistLocks();
bitMapLock()->lock_without_safepoint_check();
log_debug(gc, state)("CMS foreground collector has asked for control " INTPTR_FORMAT " with first state %d",
p2i(Thread::current()), first_state);
log_debug(gc, state)(" gets control with state %d", _collectorState);
// Inform cms gen if this was due to partial collection failing.
// The CMS gen may use this fact to determine its expansion policy.
CMSHeap* heap = CMSHeap::heap();
if (heap->incremental_collection_will_fail(false /* don't consult_young */)) {
assert(!_cmsGen->incremental_collection_failed(),
"Should have been noticed, reacted to and cleared");
_cmsGen->set_incremental_collection_failed();
}
if (first_state > Idling) {
report_concurrent_mode_interruption();
}
set_did_compact(true);
// If the collection is being acquired from the background
// collector, there may be references on the discovered
// references lists. Abandon those references, since some
// of them may have become unreachable after concurrent
// discovery; the STW compacting collector will redo discovery
// more precisely, without being subject to floating garbage.
// Leaving otherwise unreachable references in the discovered
// lists would require special handling.
ref_processor()->disable_discovery();
ref_processor()->abandon_partial_discovery();
ref_processor()->verify_no_references_recorded();
if (first_state > Idling) {
save_heap_summary();
}
do_compaction_work(clear_all_soft_refs);
// Has the GC time limit been exceeded?
size_t max_eden_size = _young_gen->max_eden_size();
GCCause::Cause gc_cause = heap->gc_cause();
size_policy()->check_gc_overhead_limit(_young_gen->eden()->used(),
_cmsGen->max_capacity(),
max_eden_size,
full,
gc_cause,
heap->soft_ref_policy());
// Reset the expansion cause, now that we just completed
// a collection cycle.
clear_expansion_cause();
_foregroundGCIsActive = false;
return;
}
// Resize the tenured generation
// after obtaining the free list locks for the
// two generations.
void CMSCollector::compute_new_size() {
assert_locked_or_safepoint(Heap_lock);
FreelistLocker z(this);
MetaspaceGC::compute_new_size();
_cmsGen->compute_new_size_free_list();
// recalculate CMS used space after CMS collection
_cmsGen->cmsSpace()->recalculate_used_stable();
}
// A work method used by the foreground collector to do
// a mark-sweep-compact.
void CMSCollector::do_compaction_work(bool clear_all_soft_refs) {
CMSHeap* heap = CMSHeap::heap();
STWGCTimer* gc_timer = GenMarkSweep::gc_timer();
gc_timer->register_gc_start();
SerialOldTracer* gc_tracer = GenMarkSweep::gc_tracer();
gc_tracer->report_gc_start(heap->gc_cause(), gc_timer->gc_start());
heap->pre_full_gc_dump(gc_timer);
GCTraceTime(Trace, gc, phases) t("CMS:MSC");
// Temporarily widen the span of the weak reference processing to
// the entire heap.
MemRegion new_span(CMSHeap::heap()->reserved_region());
ReferenceProcessorSpanMutator rp_mut_span(ref_processor(), new_span);
// Temporarily, clear the "is_alive_non_header" field of the
// reference processor.
ReferenceProcessorIsAliveMutator rp_mut_closure(ref_processor(), NULL);
// Temporarily make reference _processing_ single threaded (non-MT).
ReferenceProcessorMTProcMutator rp_mut_mt_processing(ref_processor(), false);
// Temporarily make refs discovery atomic
ReferenceProcessorAtomicMutator rp_mut_atomic(ref_processor(), true);
// Temporarily make reference _discovery_ single threaded (non-MT)
ReferenceProcessorMTDiscoveryMutator rp_mut_discovery(ref_processor(), false);
ref_processor()->set_enqueuing_is_done(false);
ref_processor()->enable_discovery();
ref_processor()->setup_policy(clear_all_soft_refs);
// If an asynchronous collection finishes, the _modUnionTable is
// all clear. If we are assuming the collection from an asynchronous
// collection, clear the _modUnionTable.
assert(_collectorState != Idling || _modUnionTable.isAllClear(),
"_modUnionTable should be clear if the baton was not passed");
_modUnionTable.clear_all();
assert(_collectorState != Idling || _ct->cld_rem_set()->mod_union_is_clear(),
"mod union for klasses should be clear if the baton was passed");
_ct->cld_rem_set()->clear_mod_union();
// We must adjust the allocation statistics being maintained
// in the free list space. We do so by reading and clearing
// the sweep timer and updating the block flux rate estimates below.
assert(!_intra_sweep_timer.is_active(), "_intra_sweep_timer should be inactive");
if (_inter_sweep_timer.is_active()) {
_inter_sweep_timer.stop();
// Note that we do not use this sample to update the _inter_sweep_estimate.
_cmsGen->cmsSpace()->beginSweepFLCensus((float)(_inter_sweep_timer.seconds()),
_inter_sweep_estimate.padded_average(),
_intra_sweep_estimate.padded_average());
}
GenMarkSweep::invoke_at_safepoint(ref_processor(), clear_all_soft_refs);
#ifdef ASSERT
CompactibleFreeListSpace* cms_space = _cmsGen->cmsSpace();
size_t free_size = cms_space->free();
assert(free_size ==
pointer_delta(cms_space->end(), cms_space->compaction_top())
* HeapWordSize,
"All the free space should be compacted into one chunk at top");
assert(cms_space->dictionary()->total_chunk_size(
debug_only(cms_space->freelistLock())) == 0 ||
cms_space->totalSizeInIndexedFreeLists() == 0,
"All the free space should be in a single chunk");
size_t num = cms_space->totalCount();
assert((free_size == 0 && num == 0) ||
(free_size > 0 && (num == 1 || num == 2)),
"There should be at most 2 free chunks after compaction");
#endif // ASSERT
_collectorState = Resetting;
assert(_restart_addr == NULL,
"Should have been NULL'd before baton was passed");
reset_stw();
_cmsGen->reset_after_compaction();
_concurrent_cycles_since_last_unload = 0;
// Clear any data recorded in the PLAB chunk arrays.
if (_survivor_plab_array != NULL) {
reset_survivor_plab_arrays();
}
// Adjust the per-size allocation stats for the next epoch.
_cmsGen->cmsSpace()->endSweepFLCensus(sweep_count() /* fake */);
// Restart the "inter sweep timer" for the next epoch.
_inter_sweep_timer.reset();
_inter_sweep_timer.start();
// No longer a need to do a concurrent collection for Metaspace.
MetaspaceGC::set_should_concurrent_collect(false);
heap->post_full_gc_dump(gc_timer);
gc_timer->register_gc_end();
gc_tracer->report_gc_end(gc_timer->gc_end(), gc_timer->time_partitions());
// For a mark-sweep-compact, compute_new_size() will be called
// in the heap's do_collection() method.
}
void CMSCollector::print_eden_and_survivor_chunk_arrays() {
Log(gc, heap) log;
if (!log.is_trace()) {
return;
}
ContiguousSpace* eden_space = _young_gen->eden();
ContiguousSpace* from_space = _young_gen->from();
ContiguousSpace* to_space = _young_gen->to();
// Eden
if (_eden_chunk_array != NULL) {
log.trace("eden " PTR_FORMAT "-" PTR_FORMAT "-" PTR_FORMAT "(" SIZE_FORMAT ")",
p2i(eden_space->bottom()), p2i(eden_space->top()),
p2i(eden_space->end()), eden_space->capacity());
log.trace("_eden_chunk_index=" SIZE_FORMAT ", _eden_chunk_capacity=" SIZE_FORMAT,
_eden_chunk_index, _eden_chunk_capacity);
for (size_t i = 0; i < _eden_chunk_index; i++) {
log.trace("_eden_chunk_array[" SIZE_FORMAT "]=" PTR_FORMAT, i, p2i(_eden_chunk_array[i]));
}
}
// Survivor
if (_survivor_chunk_array != NULL) {
log.trace("survivor " PTR_FORMAT "-" PTR_FORMAT "-" PTR_FORMAT "(" SIZE_FORMAT ")",
p2i(from_space->bottom()), p2i(from_space->top()),
p2i(from_space->end()), from_space->capacity());
log.trace("_survivor_chunk_index=" SIZE_FORMAT ", _survivor_chunk_capacity=" SIZE_FORMAT,
_survivor_chunk_index, _survivor_chunk_capacity);
for (size_t i = 0; i < _survivor_chunk_index; i++) {
log.trace("_survivor_chunk_array[" SIZE_FORMAT "]=" PTR_FORMAT, i, p2i(_survivor_chunk_array[i]));
}
}
}
void CMSCollector::getFreelistLocks() const {
// Get locks for all free lists in all generations that this
// collector is responsible for
_cmsGen->freelistLock()->lock_without_safepoint_check();
}
void CMSCollector::releaseFreelistLocks() const {
// Release locks for all free lists in all generations that this
// collector is responsible for
_cmsGen->freelistLock()->unlock();
}
bool CMSCollector::haveFreelistLocks() const {
// Check locks for all free lists in all generations that this
// collector is responsible for
assert_lock_strong(_cmsGen->freelistLock());
PRODUCT_ONLY(ShouldNotReachHere());
return true;
}
// A utility class that is used by the CMS collector to
// temporarily "release" the foreground collector from its
// usual obligation to wait for the background collector to
// complete an ongoing phase before proceeding.
class ReleaseForegroundGC: public StackObj {
private:
CMSCollector* _c;
public:
ReleaseForegroundGC(CMSCollector* c) : _c(c) {
assert(_c->_foregroundGCShouldWait, "Else should not need to call");
MutexLocker x(CGC_lock, Mutex::_no_safepoint_check_flag);
// allow a potentially blocked foreground collector to proceed
_c->_foregroundGCShouldWait = false;
if (_c->_foregroundGCIsActive) {
CGC_lock->notify();
}
assert(!ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"Possible deadlock");
}
~ReleaseForegroundGC() {
assert(!_c->_foregroundGCShouldWait, "Usage protocol violation?");
MutexLocker x(CGC_lock, Mutex::_no_safepoint_check_flag);
_c->_foregroundGCShouldWait = true;
}
};
void CMSCollector::collect_in_background(GCCause::Cause cause) {
assert(Thread::current()->is_ConcurrentGC_thread(),
"A CMS asynchronous collection is only allowed on a CMS thread.");
CMSHeap* heap = CMSHeap::heap();
{
MutexLocker hl(Heap_lock, Mutex::_no_safepoint_check_flag);
FreelistLocker fll(this);
MutexLocker x(CGC_lock, Mutex::_no_safepoint_check_flag);
if (_foregroundGCIsActive) {
// The foreground collector is. Skip this
// background collection.
assert(!_foregroundGCShouldWait, "Should be clear");
return;
} else {
assert(_collectorState == Idling, "Should be idling before start.");
_collectorState = InitialMarking;
register_gc_start(cause);
// Reset the expansion cause, now that we are about to begin
// a new cycle.
clear_expansion_cause();
// Clear the MetaspaceGC flag since a concurrent collection
// is starting but also clear it after the collection.
MetaspaceGC::set_should_concurrent_collect(false);
}
// Decide if we want to enable class unloading as part of the
// ensuing concurrent GC cycle.
update_should_unload_classes();
_full_gc_requested = false; // acks all outstanding full gc requests
_full_gc_cause = GCCause::_no_gc;
// Signal that we are about to start a collection
heap->increment_total_full_collections(); // ... starting a collection cycle
_collection_count_start = heap->total_full_collections();
}
size_t prev_used = _cmsGen->used();
// The change of the collection state is normally done at this level;
// the exceptions are phases that are executed while the world is
// stopped. For those phases the change of state is done while the
// world is stopped. For baton passing purposes this allows the
// background collector to finish the phase and change state atomically.
// The foreground collector cannot wait on a phase that is done
// while the world is stopped because the foreground collector already
// has the world stopped and would deadlock.
while (_collectorState != Idling) {
log_debug(gc, state)("Thread " INTPTR_FORMAT " in CMS state %d",
p2i(Thread::current()), _collectorState);
// The foreground collector
// holds the Heap_lock throughout its collection.
// holds the CMS token (but not the lock)
// except while it is waiting for the background collector to yield.
//
// The foreground collector should be blocked (not for long)
// if the background collector is about to start a phase
// executed with world stopped. If the background
// collector has already started such a phase, the
// foreground collector is blocked waiting for the
// Heap_lock. The stop-world phases (InitialMarking and FinalMarking)
// are executed in the VM thread.
//
// The locking order is
// PendingListLock (PLL) -- if applicable (FinalMarking)
// Heap_lock (both this & PLL locked in VM_CMS_Operation::prologue())
// CMS token (claimed in
// stop_world_and_do() -->
// safepoint_synchronize() -->
// CMSThread::synchronize())
{
// Check if the FG collector wants us to yield.
CMSTokenSync x(true); // is cms thread
if (waitForForegroundGC()) {
// We yielded to a foreground GC, nothing more to be
// done this round.
assert(_foregroundGCShouldWait == false, "We set it to false in "
"waitForForegroundGC()");
log_debug(gc, state)("CMS Thread " INTPTR_FORMAT " exiting collection CMS state %d",
p2i(Thread::current()), _collectorState);
return;
} else {
// The background collector can run but check to see if the
// foreground collector has done a collection while the
// background collector was waiting to get the CGC_lock
// above. If yes, break so that _foregroundGCShouldWait
// is cleared before returning.
if (_collectorState == Idling) {
break;
}
}
}
assert(_foregroundGCShouldWait, "Foreground collector, if active, "
"should be waiting");
switch (_collectorState) {
case InitialMarking:
{
ReleaseForegroundGC x(this);
stats().record_cms_begin();
VM_CMS_Initial_Mark initial_mark_op(this);
VMThread::execute(&initial_mark_op);
}
// The collector state may be any legal state at this point
// since the background collector may have yielded to the
// foreground collector.
break;
case Marking:
// initial marking in checkpointRootsInitialWork has been completed
if (markFromRoots()) { // we were successful
assert(_collectorState == Precleaning, "Collector state should "
"have changed");
} else {
assert(_foregroundGCIsActive, "Internal state inconsistency");
}
break;
case Precleaning:
// marking from roots in markFromRoots has been completed
preclean();
assert(_collectorState == AbortablePreclean ||
_collectorState == FinalMarking,
"Collector state should have changed");
break;
case AbortablePreclean:
abortable_preclean();
assert(_collectorState == FinalMarking, "Collector state should "
"have changed");
break;
case FinalMarking:
{
ReleaseForegroundGC x(this);
VM_CMS_Final_Remark final_remark_op(this);
VMThread::execute(&final_remark_op);
}
assert(_foregroundGCShouldWait, "block post-condition");
break;
case Sweeping:
// final marking in checkpointRootsFinal has been completed
sweep();
assert(_collectorState == Resizing, "Collector state change "
"to Resizing must be done under the free_list_lock");
case Resizing: {
// Sweeping has been completed...
// At this point the background collection has completed.
// Don't move the call to compute_new_size() down
// into code that might be executed if the background
// collection was preempted.
{
ReleaseForegroundGC x(this); // unblock FG collection
MutexLocker y(Heap_lock, Mutex::_no_safepoint_check_flag);
CMSTokenSync z(true); // not strictly needed.
if (_collectorState == Resizing) {
compute_new_size();
save_heap_summary();
_collectorState = Resetting;
} else {
assert(_collectorState == Idling, "The state should only change"
" because the foreground collector has finished the collection");
}
}
break;
}
case Resetting:
// CMS heap resizing has been completed
reset_concurrent();
assert(_collectorState == Idling, "Collector state should "
"have changed");
MetaspaceGC::set_should_concurrent_collect(false);
stats().record_cms_end();
// Don't move the concurrent_phases_end() and compute_new_size()
// calls to here because a preempted background collection
// has it's state set to "Resetting".
break;
case Idling:
default:
ShouldNotReachHere();
break;
}
log_debug(gc, state)(" Thread " INTPTR_FORMAT " done - next CMS state %d",
p2i(Thread::current()), _collectorState);
assert(_foregroundGCShouldWait, "block post-condition");
}
// Should this be in gc_epilogue?
heap->counters()->update_counters();
{
// Clear _foregroundGCShouldWait and, in the event that the
// foreground collector is waiting, notify it, before
// returning.
MutexLocker x(CGC_lock, Mutex::_no_safepoint_check_flag);
_foregroundGCShouldWait = false;
if (_foregroundGCIsActive) {
CGC_lock->notify();
}
assert(!ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"Possible deadlock");
}
log_debug(gc, state)("CMS Thread " INTPTR_FORMAT " exiting collection CMS state %d",
p2i(Thread::current()), _collectorState);
log_info(gc, heap)("Old: " SIZE_FORMAT "K->" SIZE_FORMAT "K(" SIZE_FORMAT "K)",
prev_used / K, _cmsGen->used()/K, _cmsGen->capacity() /K);
}
void CMSCollector::register_gc_start(GCCause::Cause cause) {
_cms_start_registered = true;
_gc_timer_cm->register_gc_start();
_gc_tracer_cm->report_gc_start(cause, _gc_timer_cm->gc_start());
}
void CMSCollector::register_gc_end() {
if (_cms_start_registered) {
report_heap_summary(GCWhen::AfterGC);
_gc_timer_cm->register_gc_end();
_gc_tracer_cm->report_gc_end(_gc_timer_cm->gc_end(), _gc_timer_cm->time_partitions());
_cms_start_registered = false;
}
}
void CMSCollector::save_heap_summary() {
CMSHeap* heap = CMSHeap::heap();
_last_heap_summary = heap->create_heap_summary();
_last_metaspace_summary = heap->create_metaspace_summary();
}
void CMSCollector::report_heap_summary(GCWhen::Type when) {
_gc_tracer_cm->report_gc_heap_summary(when, _last_heap_summary);
_gc_tracer_cm->report_metaspace_summary(when, _last_metaspace_summary);
}
bool CMSCollector::waitForForegroundGC() {
bool res = false;
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should have CMS token");
// Block the foreground collector until the
// background collectors decides whether to
// yield.
MutexLocker x(CGC_lock, Mutex::_no_safepoint_check_flag);
_foregroundGCShouldWait = true;
if (_foregroundGCIsActive) {
// The background collector yields to the
// foreground collector and returns a value
// indicating that it has yielded. The foreground
// collector can proceed.
res = true;
_foregroundGCShouldWait = false;
ConcurrentMarkSweepThread::clear_CMS_flag(
ConcurrentMarkSweepThread::CMS_cms_has_token);
ConcurrentMarkSweepThread::set_CMS_flag(
ConcurrentMarkSweepThread::CMS_cms_wants_token);
// Get a possibly blocked foreground thread going
CGC_lock->notify();
log_debug(gc, state)("CMS Thread " INTPTR_FORMAT " waiting at CMS state %d",
p2i(Thread::current()), _collectorState);
while (_foregroundGCIsActive) {
CGC_lock->wait_without_safepoint_check();
}
ConcurrentMarkSweepThread::set_CMS_flag(
ConcurrentMarkSweepThread::CMS_cms_has_token);
ConcurrentMarkSweepThread::clear_CMS_flag(
ConcurrentMarkSweepThread::CMS_cms_wants_token);
}
log_debug(gc, state)("CMS Thread " INTPTR_FORMAT " continuing at CMS state %d",
p2i(Thread::current()), _collectorState);
return res;
}
// Because of the need to lock the free lists and other structures in
// the collector, common to all the generations that the collector is
// collecting, we need the gc_prologues of individual CMS generations
// delegate to their collector. It may have been simpler had the
// current infrastructure allowed one to call a prologue on a
// collector. In the absence of that we have the generation's
// prologue delegate to the collector, which delegates back
// some "local" work to a worker method in the individual generations
// that it's responsible for collecting, while itself doing any
// work common to all generations it's responsible for. A similar
// comment applies to the gc_epilogue()'s.
// The role of the variable _between_prologue_and_epilogue is to
// enforce the invocation protocol.
void CMSCollector::gc_prologue(bool full) {
// Call gc_prologue_work() for the CMSGen
// we are responsible for.
// The following locking discipline assumes that we are only called
// when the world is stopped.
assert(SafepointSynchronize::is_at_safepoint(), "world is stopped assumption");
// The CMSCollector prologue must call the gc_prologues for the
// "generations" that it's responsible
// for.
assert( Thread::current()->is_VM_thread()
|| ( CMSScavengeBeforeRemark
&& Thread::current()->is_ConcurrentGC_thread()),
"Incorrect thread type for prologue execution");
if (_between_prologue_and_epilogue) {
// We have already been invoked; this is a gc_prologue delegation
// from yet another CMS generation that we are responsible for, just
// ignore it since all relevant work has already been done.
return;
}
// set a bit saying prologue has been called; cleared in epilogue
_between_prologue_and_epilogue = true;
// Claim locks for common data structures, then call gc_prologue_work()
// for each CMSGen.
getFreelistLocks(); // gets free list locks on constituent spaces
bitMapLock()->lock_without_safepoint_check();
// Should call gc_prologue_work() for all cms gens we are responsible for
bool duringMarking = _collectorState >= Marking
&& _collectorState < Sweeping;
// The young collections clear the modified oops state, which tells if
// there are any modified oops in the class. The remark phase also needs
// that information. Tell the young collection to save the union of all
// modified klasses.
if (duringMarking) {
_ct->cld_rem_set()->set_accumulate_modified_oops(true);
}
bool registerClosure = duringMarking;
_cmsGen->gc_prologue_work(full, registerClosure, &_modUnionClosurePar);
if (!full) {
stats().record_gc0_begin();
}
}
void ConcurrentMarkSweepGeneration::gc_prologue(bool full) {
_capacity_at_prologue = capacity();
_used_at_prologue = used();
_cmsSpace->recalculate_used_stable();
// We enable promotion tracking so that card-scanning can recognize
// which objects have been promoted during this GC and skip them.
for (uint i = 0; i < ParallelGCThreads; i++) {
_par_gc_thread_states[i]->promo.startTrackingPromotions();
}
// Delegate to CMScollector which knows how to coordinate between
// this and any other CMS generations that it is responsible for
// collecting.
collector()->gc_prologue(full);
}
// This is a "private" interface for use by this generation's CMSCollector.
// Not to be called directly by any other entity (for instance,
// GenCollectedHeap, which calls the "public" gc_prologue method above).
void ConcurrentMarkSweepGeneration::gc_prologue_work(bool full,
bool registerClosure, ModUnionClosure* modUnionClosure) {
assert(!incremental_collection_failed(), "Shouldn't be set yet");
assert(cmsSpace()->preconsumptionDirtyCardClosure() == NULL,
"Should be NULL");
if (registerClosure) {
cmsSpace()->setPreconsumptionDirtyCardClosure(modUnionClosure);
}
cmsSpace()->gc_prologue();
// Clear stat counters
NOT_PRODUCT(
assert(_numObjectsPromoted == 0, "check");
assert(_numWordsPromoted == 0, "check");
log_develop_trace(gc, alloc)("Allocated " SIZE_FORMAT " objects, " SIZE_FORMAT " bytes concurrently",
_numObjectsAllocated, _numWordsAllocated*sizeof(HeapWord));
_numObjectsAllocated = 0;
_numWordsAllocated = 0;
)
}
void CMSCollector::gc_epilogue(bool full) {
// The following locking discipline assumes that we are only called
// when the world is stopped.
assert(SafepointSynchronize::is_at_safepoint(),
"world is stopped assumption");
// Currently the CMS epilogue (see CompactibleFreeListSpace) merely checks
// if linear allocation blocks need to be appropriately marked to allow the
// the blocks to be parsable. We also check here whether we need to nudge the
// CMS collector thread to start a new cycle (if it's not already active).
assert( Thread::current()->is_VM_thread()
|| ( CMSScavengeBeforeRemark
&& Thread::current()->is_ConcurrentGC_thread()),
"Incorrect thread type for epilogue execution");
if (!_between_prologue_and_epilogue) {
// We have already been invoked; this is a gc_epilogue delegation
// from yet another CMS generation that we are responsible for, just
// ignore it since all relevant work has already been done.
return;
}
assert(haveFreelistLocks(), "must have freelist locks");
assert_lock_strong(bitMapLock());
_ct->cld_rem_set()->set_accumulate_modified_oops(false);
_cmsGen->gc_epilogue_work(full);
if (_collectorState == AbortablePreclean || _collectorState == Precleaning) {
// in case sampling was not already enabled, enable it
_start_sampling = true;
}
// reset _eden_chunk_array so sampling starts afresh
_eden_chunk_index = 0;
size_t cms_used = _cmsGen->cmsSpace()->used();
_cmsGen->cmsSpace()->recalculate_used_stable();
// update performance counters - this uses a special version of
// update_counters() that allows the utilization to be passed as a
// parameter, avoiding multiple calls to used().
//
_cmsGen->update_counters(cms_used);
bitMapLock()->unlock();
releaseFreelistLocks();
if (!CleanChunkPoolAsync) {
Chunk::clean_chunk_pool();
}
set_did_compact(false);
_between_prologue_and_epilogue = false; // ready for next cycle
}
void ConcurrentMarkSweepGeneration::gc_epilogue(bool full) {
collector()->gc_epilogue(full);
// When using ParNew, promotion tracking should have already been
// disabled. However, the prologue (which enables promotion
// tracking) and epilogue are called irrespective of the type of
// GC. So they will also be called before and after Full GCs, during
// which promotion tracking will not be explicitly disabled. So,
// it's safer to also disable it here too (to be symmetric with
// enabling it in the prologue).
for (uint i = 0; i < ParallelGCThreads; i++) {
_par_gc_thread_states[i]->promo.stopTrackingPromotions();
}
}
void ConcurrentMarkSweepGeneration::gc_epilogue_work(bool full) {
assert(!incremental_collection_failed(), "Should have been cleared");
cmsSpace()->setPreconsumptionDirtyCardClosure(NULL);
cmsSpace()->gc_epilogue();
// Print stat counters
NOT_PRODUCT(
assert(_numObjectsAllocated == 0, "check");
assert(_numWordsAllocated == 0, "check");
log_develop_trace(gc, promotion)("Promoted " SIZE_FORMAT " objects, " SIZE_FORMAT " bytes",
_numObjectsPromoted, _numWordsPromoted*sizeof(HeapWord));
_numObjectsPromoted = 0;
_numWordsPromoted = 0;
)
// Call down the chain in contiguous_available needs the freelistLock
// so print this out before releasing the freeListLock.
log_develop_trace(gc)(" Contiguous available " SIZE_FORMAT " bytes ", contiguous_available());
}
#ifndef PRODUCT
bool CMSCollector::have_cms_token() {
Thread* thr = Thread::current();
if (thr->is_VM_thread()) {
return ConcurrentMarkSweepThread::vm_thread_has_cms_token();
} else if (thr->is_ConcurrentGC_thread()) {
return ConcurrentMarkSweepThread::cms_thread_has_cms_token();
} else if (thr->is_GC_task_thread()) {
return ConcurrentMarkSweepThread::vm_thread_has_cms_token() &&
ParGCRareEvent_lock->owned_by_self();
}
return false;
}
// Check reachability of the given heap address in CMS generation,
// treating all other generations as roots.
bool CMSCollector::is_cms_reachable(HeapWord* addr) {
// We could "guarantee" below, rather than assert, but I'll
// leave these as "asserts" so that an adventurous debugger
// could try this in the product build provided some subset of
// the conditions were met, provided they were interested in the
// results and knew that the computation below wouldn't interfere
// with other concurrent computations mutating the structures
// being read or written.
assert(SafepointSynchronize::is_at_safepoint(),
"Else mutations in object graph will make answer suspect");
assert(have_cms_token(), "Should hold cms token");
assert(haveFreelistLocks(), "must hold free list locks");
assert_lock_strong(bitMapLock());
// Clear the marking bit map array before starting, but, just
// for kicks, first report if the given address is already marked
tty->print_cr("Start: Address " PTR_FORMAT " is%s marked", p2i(addr),
_markBitMap.isMarked(addr) ? "" : " not");
if (verify_after_remark()) {
MutexLocker x(verification_mark_bm()->lock(), Mutex::_no_safepoint_check_flag);
bool result = verification_mark_bm()->isMarked(addr);
tty->print_cr("TransitiveMark: Address " PTR_FORMAT " %s marked", p2i(addr),
result ? "IS" : "is NOT");
return result;
} else {
tty->print_cr("Could not compute result");
return false;
}
}
#endif
void
CMSCollector::print_on_error(outputStream* st) {
CMSCollector* collector = ConcurrentMarkSweepGeneration::_collector;
if (collector != NULL) {
CMSBitMap* bitmap = &collector->_markBitMap;
st->print_cr("Marking Bits: (CMSBitMap*) " PTR_FORMAT, p2i(bitmap));
bitmap->print_on_error(st, " Bits: ");
st->cr();
CMSBitMap* mut_bitmap = &collector->_modUnionTable;
st->print_cr("Mod Union Table: (CMSBitMap*) " PTR_FORMAT, p2i(mut_bitmap));
mut_bitmap->print_on_error(st, " Bits: ");
}
}
////////////////////////////////////////////////////////
// CMS Verification Support
////////////////////////////////////////////////////////
// Following the remark phase, the following invariant
// should hold -- each object in the CMS heap which is
// marked in markBitMap() should be marked in the verification_mark_bm().
class VerifyMarkedClosure: public BitMapClosure {
CMSBitMap* _marks;
bool _failed;
public:
VerifyMarkedClosure(CMSBitMap* bm): _marks(bm), _failed(false) {}
bool do_bit(size_t offset) {
HeapWord* addr = _marks->offsetToHeapWord(offset);
if (!_marks->isMarked(addr)) {
Log(gc, verify) log;
ResourceMark rm;
LogStream ls(log.error());
oop(addr)->print_on(&ls);
log.error(" (" INTPTR_FORMAT " should have been marked)", p2i(addr));
_failed = true;
}
return true;
}
bool failed() { return _failed; }
};
bool CMSCollector::verify_after_remark() {
GCTraceTime(Info, gc, phases, verify) tm("Verifying CMS Marking.");
MutexLocker ml(verification_mark_bm()->lock(), Mutex::_no_safepoint_check_flag);
static bool init = false;
assert(SafepointSynchronize::is_at_safepoint(),
"Else mutations in object graph will make answer suspect");
assert(have_cms_token(),
"Else there may be mutual interference in use of "
" verification data structures");
assert(_collectorState > Marking && _collectorState <= Sweeping,
"Else marking info checked here may be obsolete");
assert(haveFreelistLocks(), "must hold free list locks");
assert_lock_strong(bitMapLock());
// Allocate marking bit map if not already allocated
if (!init) { // first time
if (!verification_mark_bm()->allocate(_span)) {
return false;
}
init = true;
}
assert(verification_mark_stack()->isEmpty(), "Should be empty");
// Turn off refs discovery -- so we will be tracing through refs.
// This is as intended, because by this time
// GC must already have cleared any refs that need to be cleared,
// and traced those that need to be marked; moreover,
// the marking done here is not going to interfere in any
// way with the marking information used by GC.
NoRefDiscovery no_discovery(ref_processor());
#if COMPILER2_OR_JVMCI
DerivedPointerTableDeactivate dpt_deact;
#endif
// Clear any marks from a previous round
verification_mark_bm()->clear_all();
assert(verification_mark_stack()->isEmpty(), "markStack should be empty");
verify_work_stacks_empty();
CMSHeap* heap = CMSHeap::heap();
heap->ensure_parsability(false); // fill TLABs, but no need to retire them
// Update the saved marks which may affect the root scans.
heap->save_marks();
if (CMSRemarkVerifyVariant == 1) {
// In this first variant of verification, we complete
// all marking, then check if the new marks-vector is
// a subset of the CMS marks-vector.
verify_after_remark_work_1();
} else {
guarantee(CMSRemarkVerifyVariant == 2, "Range checking for CMSRemarkVerifyVariant should guarantee 1 or 2");
// In this second variant of verification, we flag an error
// (i.e. an object reachable in the new marks-vector not reachable
// in the CMS marks-vector) immediately, also indicating the
// identify of an object (A) that references the unmarked object (B) --
// presumably, a mutation to A failed to be picked up by preclean/remark?
verify_after_remark_work_2();
}
return true;
}
void CMSCollector::verify_after_remark_work_1() {
ResourceMark rm;
HandleMark hm;
CMSHeap* heap = CMSHeap::heap();
// Get a clear set of claim bits for the roots processing to work with.
ClassLoaderDataGraph::clear_claimed_marks();
// Mark from roots one level into CMS
MarkRefsIntoClosure notOlder(_span, verification_mark_bm());
heap->rem_set()->prepare_for_younger_refs_iterate(false); // Not parallel.
{
StrongRootsScope srs(1);
heap->cms_process_roots(&srs,
true, // young gen as roots
GenCollectedHeap::ScanningOption(roots_scanning_options()),
should_unload_classes(),
¬Older,
NULL);
}
// Now mark from the roots
MarkFromRootsClosure markFromRootsClosure(this, _span,
verification_mark_bm(), verification_mark_stack(),
false /* don't yield */, true /* verifying */);
assert(_restart_addr == NULL, "Expected pre-condition");
verification_mark_bm()->iterate(&markFromRootsClosure);
while (_restart_addr != NULL) {
// Deal with stack overflow: by restarting at the indicated
// address.
HeapWord* ra = _restart_addr;
markFromRootsClosure.reset(ra);
_restart_addr = NULL;
verification_mark_bm()->iterate(&markFromRootsClosure, ra, _span.end());
}
assert(verification_mark_stack()->isEmpty(), "Should have been drained");
verify_work_stacks_empty();
// Marking completed -- now verify that each bit marked in
// verification_mark_bm() is also marked in markBitMap(); flag all
// errors by printing corresponding objects.
VerifyMarkedClosure vcl(markBitMap());
verification_mark_bm()->iterate(&vcl);
if (vcl.failed()) {
Log(gc, verify) log;
log.error("Failed marking verification after remark");
ResourceMark rm;
LogStream ls(log.error());
heap->print_on(&ls);
fatal("CMS: failed marking verification after remark");
}
}
class VerifyCLDOopsCLDClosure : public CLDClosure {
class VerifyCLDOopsClosure : public OopClosure {
CMSBitMap* _bitmap;
public:
VerifyCLDOopsClosure(CMSBitMap* bitmap) : _bitmap(bitmap) { }
void do_oop(oop* p) { guarantee(*p == NULL || _bitmap->isMarked((HeapWord*) *p), "Should be marked"); }
void do_oop(narrowOop* p) { ShouldNotReachHere(); }
} _oop_closure;
public:
VerifyCLDOopsCLDClosure(CMSBitMap* bitmap) : _oop_closure(bitmap) {}
void do_cld(ClassLoaderData* cld) {
cld->oops_do(&_oop_closure, ClassLoaderData::_claim_none, false);
}
};
void CMSCollector::verify_after_remark_work_2() {
ResourceMark rm;
HandleMark hm;
CMSHeap* heap = CMSHeap::heap();
// Get a clear set of claim bits for the roots processing to work with.
ClassLoaderDataGraph::clear_claimed_marks();
// Mark from roots one level into CMS
MarkRefsIntoVerifyClosure notOlder(_span, verification_mark_bm(),
markBitMap());
CLDToOopClosure cld_closure(¬Older, ClassLoaderData::_claim_strong);
heap->rem_set()->prepare_for_younger_refs_iterate(false); // Not parallel.
{
StrongRootsScope srs(1);
heap->cms_process_roots(&srs,
true, // young gen as roots
GenCollectedHeap::ScanningOption(roots_scanning_options()),
should_unload_classes(),
¬Older,
&cld_closure);
}
// Now mark from the roots
MarkFromRootsVerifyClosure markFromRootsClosure(this, _span,
verification_mark_bm(), markBitMap(), verification_mark_stack());
assert(_restart_addr == NULL, "Expected pre-condition");
verification_mark_bm()->iterate(&markFromRootsClosure);
while (_restart_addr != NULL) {
// Deal with stack overflow: by restarting at the indicated
// address.
HeapWord* ra = _restart_addr;
markFromRootsClosure.reset(ra);
_restart_addr = NULL;
verification_mark_bm()->iterate(&markFromRootsClosure, ra, _span.end());
}
assert(verification_mark_stack()->isEmpty(), "Should have been drained");
verify_work_stacks_empty();
VerifyCLDOopsCLDClosure verify_cld_oops(verification_mark_bm());
ClassLoaderDataGraph::cld_do(&verify_cld_oops);
// Marking completed -- now verify that each bit marked in
// verification_mark_bm() is also marked in markBitMap(); flag all
// errors by printing corresponding objects.
VerifyMarkedClosure vcl(markBitMap());
verification_mark_bm()->iterate(&vcl);
assert(!vcl.failed(), "Else verification above should not have succeeded");
}
void ConcurrentMarkSweepGeneration::save_marks() {
// delegate to CMS space
cmsSpace()->save_marks();
}
bool ConcurrentMarkSweepGeneration::no_allocs_since_save_marks() {
return cmsSpace()->no_allocs_since_save_marks();
}
void
ConcurrentMarkSweepGeneration::oop_iterate(OopIterateClosure* cl) {
if (freelistLock()->owned_by_self()) {
Generation::oop_iterate(cl);
} else {
MutexLocker x(freelistLock(), Mutex::_no_safepoint_check_flag);
Generation::oop_iterate(cl);
}
}
void
ConcurrentMarkSweepGeneration::object_iterate(ObjectClosure* cl) {
if (freelistLock()->owned_by_self()) {
Generation::object_iterate(cl);
} else {
MutexLocker x(freelistLock(), Mutex::_no_safepoint_check_flag);
Generation::object_iterate(cl);
}
}
void
ConcurrentMarkSweepGeneration::safe_object_iterate(ObjectClosure* cl) {
if (freelistLock()->owned_by_self()) {
Generation::safe_object_iterate(cl);
} else {
MutexLocker x(freelistLock(), Mutex::_no_safepoint_check_flag);
Generation::safe_object_iterate(cl);
}
}
void
ConcurrentMarkSweepGeneration::post_compact() {
}
void
ConcurrentMarkSweepGeneration::prepare_for_verify() {
// Fix the linear allocation blocks to look like free blocks.
// Locks are normally acquired/released in gc_prologue/gc_epilogue, but those
// are not called when the heap is verified during universe initialization and
// at vm shutdown.
if (freelistLock()->owned_by_self()) {
cmsSpace()->prepare_for_verify();
} else {
MutexLocker fll(freelistLock(), Mutex::_no_safepoint_check_flag);
cmsSpace()->prepare_for_verify();
}
}
void
ConcurrentMarkSweepGeneration::verify() {
// Locks are normally acquired/released in gc_prologue/gc_epilogue, but those
// are not called when the heap is verified during universe initialization and
// at vm shutdown.
if (freelistLock()->owned_by_self()) {
cmsSpace()->verify();
} else {
MutexLocker fll(freelistLock(), Mutex::_no_safepoint_check_flag);
cmsSpace()->verify();
}
}
void CMSCollector::verify() {
_cmsGen->verify();
}
#ifndef PRODUCT
bool CMSCollector::overflow_list_is_empty() const {
assert(_num_par_pushes >= 0, "Inconsistency");
if (_overflow_list == NULL) {
assert(_num_par_pushes == 0, "Inconsistency");
}
return _overflow_list == NULL;
}
// The methods verify_work_stacks_empty() and verify_overflow_empty()
// merely consolidate assertion checks that appear to occur together frequently.
void CMSCollector::verify_work_stacks_empty() const {
assert(_markStack.isEmpty(), "Marking stack should be empty");
assert(overflow_list_is_empty(), "Overflow list should be empty");
}
void CMSCollector::verify_overflow_empty() const {
assert(overflow_list_is_empty(), "Overflow list should be empty");
assert(no_preserved_marks(), "No preserved marks");
}
#endif // PRODUCT
// Decide if we want to enable class unloading as part of the
// ensuing concurrent GC cycle. We will collect and
// unload classes if it's the case that:
// (a) class unloading is enabled at the command line, and
// (b) old gen is getting really full
// NOTE: Provided there is no change in the state of the heap between
// calls to this method, it should have idempotent results. Moreover,
// its results should be monotonically increasing (i.e. going from 0 to 1,
// but not 1 to 0) between successive calls between which the heap was
// not collected. For the implementation below, it must thus rely on
// the property that concurrent_cycles_since_last_unload()
// will not decrease unless a collection cycle happened and that
// _cmsGen->is_too_full() are
// themselves also monotonic in that sense. See check_monotonicity()
// below.
void CMSCollector::update_should_unload_classes() {
_should_unload_classes = false;
if (CMSClassUnloadingEnabled) {
_should_unload_classes = (concurrent_cycles_since_last_unload() >=
CMSClassUnloadingMaxInterval)
|| _cmsGen->is_too_full();
}
}
bool ConcurrentMarkSweepGeneration::is_too_full() const {
bool res = should_concurrent_collect();
res = res && (occupancy() > (double)CMSIsTooFullPercentage/100.0);
return res;
}
void CMSCollector::setup_cms_unloading_and_verification_state() {
const bool should_verify = VerifyBeforeGC || VerifyAfterGC || VerifyDuringGC
|| VerifyBeforeExit;
const int rso = GenCollectedHeap::SO_AllCodeCache;
// We set the proper root for this CMS cycle here.
if (should_unload_classes()) { // Should unload classes this cycle
remove_root_scanning_option(rso); // Shrink the root set appropriately
set_verifying(should_verify); // Set verification state for this cycle
return; // Nothing else needs to be done at this time
}
// Not unloading classes this cycle
assert(!should_unload_classes(), "Inconsistency!");
// If we are not unloading classes then add SO_AllCodeCache to root
// scanning options.
add_root_scanning_option(rso);
if ((!verifying() || unloaded_classes_last_cycle()) && should_verify) {
set_verifying(true);
} else if (verifying() && !should_verify) {
// We were verifying, but some verification flags got disabled.
set_verifying(false);
// Exclude symbols, strings and code cache elements from root scanning to
// reduce IM and RM pauses.
remove_root_scanning_option(rso);
}
}
#ifndef PRODUCT
HeapWord* CMSCollector::block_start(const void* p) const {
const HeapWord* addr = (HeapWord*)p;
if (_span.contains(p)) {
if (_cmsGen->cmsSpace()->is_in_reserved(addr)) {
return _cmsGen->cmsSpace()->block_start(p);
}
}
return NULL;
}
#endif
HeapWord*
ConcurrentMarkSweepGeneration::expand_and_allocate(size_t word_size,
bool tlab,
bool parallel) {
CMSSynchronousYieldRequest yr;
assert(!tlab, "Can't deal with TLAB allocation");
MutexLocker x(freelistLock(), Mutex::_no_safepoint_check_flag);
expand_for_gc_cause(word_size*HeapWordSize, MinHeapDeltaBytes, CMSExpansionCause::_satisfy_allocation);
if (GCExpandToAllocateDelayMillis > 0) {
os::naked_sleep(GCExpandToAllocateDelayMillis);
}
return have_lock_and_allocate(word_size, tlab);
}
void ConcurrentMarkSweepGeneration::expand_for_gc_cause(
size_t bytes,
size_t expand_bytes,
CMSExpansionCause::Cause cause)
{
bool success = expand(bytes, expand_bytes);
// remember why we expanded; this information is used
// by shouldConcurrentCollect() when making decisions on whether to start
// a new CMS cycle.
if (success) {
set_expansion_cause(cause);
log_trace(gc)("Expanded CMS gen for %s", CMSExpansionCause::to_string(cause));
}
}
HeapWord* ConcurrentMarkSweepGeneration::expand_and_par_lab_allocate(CMSParGCThreadState* ps, size_t word_sz) {
HeapWord* res = NULL;
MutexLocker x(ParGCRareEvent_lock);
while (true) {
// Expansion by some other thread might make alloc OK now:
res = ps->lab.alloc(word_sz);
if (res != NULL) return res;
// If there's not enough expansion space available, give up.
if (_virtual_space.uncommitted_size() < (word_sz * HeapWordSize)) {
return NULL;
}
// Otherwise, we try expansion.
expand_for_gc_cause(word_sz*HeapWordSize, MinHeapDeltaBytes, CMSExpansionCause::_allocate_par_lab);
// Now go around the loop and try alloc again;
// A competing par_promote might beat us to the expansion space,
// so we may go around the loop again if promotion fails again.
if (GCExpandToAllocateDelayMillis > 0) {
os::naked_sleep(GCExpandToAllocateDelayMillis);
}
}
}
bool ConcurrentMarkSweepGeneration::expand_and_ensure_spooling_space(
PromotionInfo* promo) {
MutexLocker x(ParGCRareEvent_lock);
size_t refill_size_bytes = promo->refillSize() * HeapWordSize;
while (true) {
// Expansion by some other thread might make alloc OK now:
if (promo->ensure_spooling_space()) {
assert(promo->has_spooling_space(),
"Post-condition of successful ensure_spooling_space()");
return true;
}
// If there's not enough expansion space available, give up.
if (_virtual_space.uncommitted_size() < refill_size_bytes) {
return false;
}
// Otherwise, we try expansion.
expand_for_gc_cause(refill_size_bytes, MinHeapDeltaBytes, CMSExpansionCause::_allocate_par_spooling_space);
// Now go around the loop and try alloc again;
// A competing allocation might beat us to the expansion space,
// so we may go around the loop again if allocation fails again.
if (GCExpandToAllocateDelayMillis > 0) {
os::naked_sleep(GCExpandToAllocateDelayMillis);
}
}
}
void ConcurrentMarkSweepGeneration::shrink(size_t bytes) {
// Only shrink if a compaction was done so that all the free space
// in the generation is in a contiguous block at the end.
if (did_compact()) {
CardGeneration::shrink(bytes);
}
}
void ConcurrentMarkSweepGeneration::assert_correct_size_change_locking() {
assert_locked_or_safepoint(Heap_lock);
}
void ConcurrentMarkSweepGeneration::shrink_free_list_by(size_t bytes) {
assert_locked_or_safepoint(Heap_lock);
assert_lock_strong(freelistLock());
log_trace(gc)("Shrinking of CMS not yet implemented");
return;
}
// Simple ctor/dtor wrapper for accounting & timer chores around concurrent
// phases.
class CMSPhaseAccounting: public StackObj {
public:
CMSPhaseAccounting(CMSCollector *collector,
const char *title);
~CMSPhaseAccounting();
private:
CMSCollector *_collector;
const char *_title;
GCTraceConcTime(Info, gc) _trace_time;
public:
// Not MT-safe; so do not pass around these StackObj's
// where they may be accessed by other threads.
double wallclock_millis() {
return TimeHelper::counter_to_millis(os::elapsed_counter() - _trace_time.start_time());
}
};
CMSPhaseAccounting::CMSPhaseAccounting(CMSCollector *collector,
const char *title) :
_collector(collector), _title(title), _trace_time(title) {
_collector->resetYields();
_collector->resetTimer();
_collector->startTimer();
_collector->gc_timer_cm()->register_gc_concurrent_start(title);
}
CMSPhaseAccounting::~CMSPhaseAccounting() {
_collector->gc_timer_cm()->register_gc_concurrent_end();
_collector->stopTimer();
log_debug(gc)("Concurrent active time: %.3fms", TimeHelper::counter_to_millis(_collector->timerTicks()));
log_trace(gc)(" (CMS %s yielded %d times)", _title, _collector->yields());
}
// CMS work
// The common parts of CMSParInitialMarkTask and CMSParRemarkTask.
class CMSParMarkTask : public AbstractGangTask {
protected:
CMSCollector* _collector;
uint _n_workers;
CMSParMarkTask(const char* name, CMSCollector* collector, uint n_workers) :
AbstractGangTask(name),
_collector(collector),
_n_workers(n_workers) {}
// Work method in support of parallel rescan ... of young gen spaces
void do_young_space_rescan(OopsInGenClosure* cl,
ContiguousSpace* space,
HeapWord** chunk_array, size_t chunk_top);
void work_on_young_gen_roots(OopsInGenClosure* cl);
};
// Parallel initial mark task
class CMSParInitialMarkTask: public CMSParMarkTask {
StrongRootsScope* _strong_roots_scope;
public:
CMSParInitialMarkTask(CMSCollector* collector, StrongRootsScope* strong_roots_scope, uint n_workers) :
CMSParMarkTask("Scan roots and young gen for initial mark in parallel", collector, n_workers),
_strong_roots_scope(strong_roots_scope) {}
void work(uint worker_id);
};
// Checkpoint the roots into this generation from outside
// this generation. [Note this initial checkpoint need only
// be approximate -- we'll do a catch up phase subsequently.]
void CMSCollector::checkpointRootsInitial() {
assert(_collectorState == InitialMarking, "Wrong collector state");
check_correct_thread_executing();
TraceCMSMemoryManagerStats tms(_collectorState, CMSHeap::heap()->gc_cause());
save_heap_summary();
report_heap_summary(GCWhen::BeforeGC);
ReferenceProcessor* rp = ref_processor();
assert(_restart_addr == NULL, "Control point invariant");
{
// acquire locks for subsequent manipulations
MutexLocker x(bitMapLock(),
Mutex::_no_safepoint_check_flag);
checkpointRootsInitialWork();
// enable ("weak") refs discovery
rp->enable_discovery();
_collectorState = Marking;
}
_cmsGen->cmsSpace()->recalculate_used_stable();
}
void CMSCollector::checkpointRootsInitialWork() {
assert(SafepointSynchronize::is_at_safepoint(), "world should be stopped");
assert(_collectorState == InitialMarking, "just checking");
// Already have locks.
assert_lock_strong(bitMapLock());
assert(_markBitMap.isAllClear(), "was reset at end of previous cycle");
// Setup the verification and class unloading state for this
// CMS collection cycle.
setup_cms_unloading_and_verification_state();
GCTraceTime(Trace, gc, phases) ts("checkpointRootsInitialWork", _gc_timer_cm);
// Reset all the PLAB chunk arrays if necessary.
if (_survivor_plab_array != NULL && !CMSPLABRecordAlways) {
reset_survivor_plab_arrays();
}
ResourceMark rm;
HandleMark hm;
MarkRefsIntoClosure notOlder(_span, &_markBitMap);
CMSHeap* heap = CMSHeap::heap();
verify_work_stacks_empty();
verify_overflow_empty();
heap->ensure_parsability(false); // fill TLABs, but no need to retire them
// Update the saved marks which may affect the root scans.
heap->save_marks();
// weak reference processing has not started yet.
ref_processor()->set_enqueuing_is_done(false);
// Need to remember all newly created CLDs,
// so that we can guarantee that the remark finds them.
ClassLoaderDataGraph::remember_new_clds(true);
// Whenever a CLD is found, it will be claimed before proceeding to mark
// the klasses. The claimed marks need to be cleared before marking starts.
ClassLoaderDataGraph::clear_claimed_marks();
print_eden_and_survivor_chunk_arrays();
{
#if COMPILER2_OR_JVMCI
DerivedPointerTableDeactivate dpt_deact;
#endif
if (CMSParallelInitialMarkEnabled) {
// The parallel version.
WorkGang* workers = heap->workers();
assert(workers != NULL, "Need parallel worker threads.");
uint n_workers = workers->active_workers();
StrongRootsScope srs(n_workers);
CMSParInitialMarkTask tsk(this, &srs, n_workers);
initialize_sequential_subtasks_for_young_gen_rescan(n_workers);
// If the total workers is greater than 1, then multiple workers
// may be used at some time and the initialization has been set
// such that the single threaded path cannot be used.
if (workers->total_workers() > 1) {
workers->run_task(&tsk);
} else {
tsk.work(0);
}
} else {
// The serial version.
CLDToOopClosure cld_closure(¬Older, ClassLoaderData::_claim_strong);
heap->rem_set()->prepare_for_younger_refs_iterate(false); // Not parallel.
StrongRootsScope srs(1);
heap->cms_process_roots(&srs,
true, // young gen as roots
GenCollectedHeap::ScanningOption(roots_scanning_options()),
should_unload_classes(),
¬Older,
&cld_closure);
}
}
// Clear mod-union table; it will be dirtied in the prologue of
// CMS generation per each young generation collection.
assert(_modUnionTable.isAllClear(),
"Was cleared in most recent final checkpoint phase"
" or no bits are set in the gc_prologue before the start of the next "
"subsequent marking phase.");
assert(_ct->cld_rem_set()->mod_union_is_clear(), "Must be");
// Save the end of the used_region of the constituent generations
// to be used to limit the extent of sweep in each generation.
save_sweep_limits();
verify_overflow_empty();
}
bool CMSCollector::markFromRoots() {
// we might be tempted to assert that:
// assert(!SafepointSynchronize::is_at_safepoint(),
// "inconsistent argument?");
// However that wouldn't be right, because it's possible that
// a safepoint is indeed in progress as a young generation
// stop-the-world GC happens even as we mark in this generation.
assert(_collectorState == Marking, "inconsistent state?");
check_correct_thread_executing();
verify_overflow_empty();
// Weak ref discovery note: We may be discovering weak
// refs in this generation concurrent (but interleaved) with
// weak ref discovery by the young generation collector.
CMSTokenSyncWithLocks ts(true, bitMapLock());
GCTraceCPUTime tcpu;
CMSPhaseAccounting pa(this, "Concurrent Mark");
bool res = markFromRootsWork();
if (res) {
_collectorState = Precleaning;
} else { // We failed and a foreground collection wants to take over
assert(_foregroundGCIsActive, "internal state inconsistency");
assert(_restart_addr == NULL, "foreground will restart from scratch");
log_debug(gc)("bailing out to foreground collection");
}
verify_overflow_empty();
return res;
}
bool CMSCollector::markFromRootsWork() {
// iterate over marked bits in bit map, doing a full scan and mark
// from these roots using the following algorithm:
// . if oop is to the right of the current scan pointer,
// mark corresponding bit (we'll process it later)
// . else (oop is to left of current scan pointer)
// push oop on marking stack
// . drain the marking stack
// Note that when we do a marking step we need to hold the
// bit map lock -- recall that direct allocation (by mutators)
// and promotion (by the young generation collector) is also
// marking the bit map. [the so-called allocate live policy.]
// Because the implementation of bit map marking is not
// robust wrt simultaneous marking of bits in the same word,
// we need to make sure that there is no such interference
// between concurrent such updates.
// already have locks
assert_lock_strong(bitMapLock());
verify_work_stacks_empty();
verify_overflow_empty();
bool result = false;
if (CMSConcurrentMTEnabled && ConcGCThreads > 0) {
result = do_marking_mt();
} else {
result = do_marking_st();
}
return result;
}
// Forward decl
class CMSConcMarkingTask;
class CMSConcMarkingParallelTerminator: public ParallelTaskTerminator {
CMSCollector* _collector;
CMSConcMarkingTask* _task;
public:
virtual void yield();
// "n_threads" is the number of threads to be terminated.
// "queue_set" is a set of work queues of other threads.
// "collector" is the CMS collector associated with this task terminator.
// "yield" indicates whether we need the gang as a whole to yield.
CMSConcMarkingParallelTerminator(int n_threads, TaskQueueSetSuper* queue_set, CMSCollector* collector) :
ParallelTaskTerminator(n_threads, queue_set),
_collector(collector) { }
void set_task(CMSConcMarkingTask* task) {
_task = task;
}
};
class CMSConcMarkingOWSTTerminator: public OWSTTaskTerminator {
CMSCollector* _collector;
CMSConcMarkingTask* _task;
public:
virtual void yield();
// "n_threads" is the number of threads to be terminated.
// "queue_set" is a set of work queues of other threads.
// "collector" is the CMS collector associated with this task terminator.
// "yield" indicates whether we need the gang as a whole to yield.
CMSConcMarkingOWSTTerminator(int n_threads, TaskQueueSetSuper* queue_set, CMSCollector* collector) :
OWSTTaskTerminator(n_threads, queue_set),
_collector(collector) { }
void set_task(CMSConcMarkingTask* task) {
_task = task;
}
};
class CMSConcMarkingTaskTerminator {
private:
ParallelTaskTerminator* _term;
public:
CMSConcMarkingTaskTerminator(int n_threads, TaskQueueSetSuper* queue_set, CMSCollector* collector) {
if (UseOWSTTaskTerminator) {
_term = new CMSConcMarkingOWSTTerminator(n_threads, queue_set, collector);
} else {
_term = new CMSConcMarkingParallelTerminator(n_threads, queue_set, collector);
}
}
~CMSConcMarkingTaskTerminator() {
assert(_term != NULL, "Must not be NULL");
delete _term;
}
void set_task(CMSConcMarkingTask* task);
ParallelTaskTerminator* terminator() const { return _term; }
};
class CMSConcMarkingTerminatorTerminator: public TerminatorTerminator {
CMSConcMarkingTask* _task;
public:
bool should_exit_termination();
void set_task(CMSConcMarkingTask* task) {
_task = task;
}
};
// MT Concurrent Marking Task
class CMSConcMarkingTask: public YieldingFlexibleGangTask {
CMSCollector* _collector;
uint _n_workers; // requested/desired # workers
bool _result;
CompactibleFreeListSpace* _cms_space;
char _pad_front[64]; // padding to ...
HeapWord* volatile _global_finger; // ... avoid sharing cache line
char _pad_back[64];
HeapWord* _restart_addr;
// Exposed here for yielding support
Mutex* const _bit_map_lock;
// The per thread work queues, available here for stealing
OopTaskQueueSet* _task_queues;
// Termination (and yielding) support
CMSConcMarkingTaskTerminator _term;
CMSConcMarkingTerminatorTerminator _term_term;
public:
CMSConcMarkingTask(CMSCollector* collector,
CompactibleFreeListSpace* cms_space,
YieldingFlexibleWorkGang* workers,
OopTaskQueueSet* task_queues):
YieldingFlexibleGangTask("Concurrent marking done multi-threaded"),
_collector(collector),
_n_workers(0),
_result(true),
_cms_space(cms_space),
_bit_map_lock(collector->bitMapLock()),
_task_queues(task_queues),
_term(_n_workers, task_queues, _collector)
{
_requested_size = _n_workers;
_term.set_task(this);
_term_term.set_task(this);
_restart_addr = _global_finger = _cms_space->bottom();
}
OopTaskQueueSet* task_queues() { return _task_queues; }
OopTaskQueue* work_queue(int i) { return task_queues()->queue(i); }
HeapWord* volatile* global_finger_addr() { return &_global_finger; }
ParallelTaskTerminator* terminator() { return _term.terminator(); }
virtual void set_for_termination(uint active_workers) {
terminator()->reset_for_reuse(active_workers);
}
void work(uint worker_id);
bool should_yield() {
return ConcurrentMarkSweepThread::should_yield()
&& !_collector->foregroundGCIsActive();
}
virtual void coordinator_yield(); // stuff done by coordinator
bool result() { return _result; }
void reset(HeapWord* ra) {
assert(_global_finger >= _cms_space->end(), "Postcondition of ::work(i)");
_restart_addr = _global_finger = ra;
_term.terminator()->reset_for_reuse();
}
static bool get_work_from_overflow_stack(CMSMarkStack* ovflw_stk,
OopTaskQueue* work_q);
private:
void do_scan_and_mark(int i, CompactibleFreeListSpace* sp);
void do_work_steal(int i);
void bump_global_finger(HeapWord* f);
};
bool CMSConcMarkingTerminatorTerminator::should_exit_termination() {
assert(_task != NULL, "Error");
return _task->yielding();
// Note that we do not need the disjunct || _task->should_yield() above
// because we want terminating threads to yield only if the task
// is already in the midst of yielding, which happens only after at least one
// thread has yielded.
}
void CMSConcMarkingParallelTerminator::yield() {
if (_task->should_yield()) {
_task->yield();
} else {
ParallelTaskTerminator::yield();
}
}
void CMSConcMarkingOWSTTerminator::yield() {
if (_task->should_yield()) {
_task->yield();
} else {
OWSTTaskTerminator::yield();
}
}
void CMSConcMarkingTaskTerminator::set_task(CMSConcMarkingTask* task) {
if (UseOWSTTaskTerminator) {
((CMSConcMarkingOWSTTerminator*)_term)->set_task(task);
} else {
((CMSConcMarkingParallelTerminator*)_term)->set_task(task);
}
}
////////////////////////////////////////////////////////////////
// Concurrent Marking Algorithm Sketch
////////////////////////////////////////////////////////////////
// Until all tasks exhausted (both spaces):
// -- claim next available chunk
// -- bump global finger via CAS
// -- find first object that starts in this chunk
// and start scanning bitmap from that position
// -- scan marked objects for oops
// -- CAS-mark target, and if successful:
// . if target oop is above global finger (volatile read)
// nothing to do
// . if target oop is in chunk and above local finger
// then nothing to do
// . else push on work-queue
// -- Deal with possible overflow issues:
// . local work-queue overflow causes stuff to be pushed on
// global (common) overflow queue
// . always first empty local work queue
// . then get a batch of oops from global work queue if any
// . then do work stealing
// -- When all tasks claimed (both spaces)
// and local work queue empty,
// then in a loop do:
// . check global overflow stack; steal a batch of oops and trace
// . try to steal from other threads oif GOS is empty
// . if neither is available, offer termination
// -- Terminate and return result
//
void CMSConcMarkingTask::work(uint worker_id) {
elapsedTimer _timer;
ResourceMark rm;
HandleMark hm;
DEBUG_ONLY(_collector->verify_overflow_empty();)
// Before we begin work, our work queue should be empty
assert(work_queue(worker_id)->size() == 0, "Expected to be empty");
// Scan the bitmap covering _cms_space, tracing through grey objects.
_timer.start();
do_scan_and_mark(worker_id, _cms_space);
_timer.stop();
log_trace(gc, task)("Finished cms space scanning in %dth thread: %3.3f sec", worker_id, _timer.seconds());
// ... do work stealing
_timer.reset();
_timer.start();
do_work_steal(worker_id);
_timer.stop();
log_trace(gc, task)("Finished work stealing in %dth thread: %3.3f sec", worker_id, _timer.seconds());
assert(_collector->_markStack.isEmpty(), "Should have been emptied");
assert(work_queue(worker_id)->size() == 0, "Should have been emptied");
// Note that under the current task protocol, the
// following assertion is true even of the spaces
// expanded since the completion of the concurrent
// marking. XXX This will likely change under a strict
// ABORT semantics.
// After perm removal the comparison was changed to
// greater than or equal to from strictly greater than.
// Before perm removal the highest address sweep would
// have been at the end of perm gen but now is at the
// end of the tenured gen.
assert(_global_finger >= _cms_space->end(),
"All tasks have been completed");
DEBUG_ONLY(_collector->verify_overflow_empty();)
}
void CMSConcMarkingTask::bump_global_finger(HeapWord* f) {
HeapWord* read = _global_finger;
HeapWord* cur = read;
while (f > read) {
cur = read;
read = Atomic::cmpxchg(f, &_global_finger, cur);
if (cur == read) {
// our cas succeeded
assert(_global_finger >= f, "protocol consistency");
break;
}
}
}
// This is really inefficient, and should be redone by
// using (not yet available) block-read and -write interfaces to the
// stack and the work_queue. XXX FIX ME !!!
bool CMSConcMarkingTask::get_work_from_overflow_stack(CMSMarkStack* ovflw_stk,
OopTaskQueue* work_q) {
// Fast lock-free check
if (ovflw_stk->length() == 0) {
return false;
}
assert(work_q->size() == 0, "Shouldn't steal");
MutexLocker ml(ovflw_stk->par_lock(),
Mutex::_no_safepoint_check_flag);
// Grab up to 1/4 the size of the work queue
size_t num = MIN2((size_t)(work_q->max_elems() - work_q->size())/4,
(size_t)ParGCDesiredObjsFromOverflowList);
num = MIN2(num, ovflw_stk->length());
for (int i = (int) num; i > 0; i--) {
oop cur = ovflw_stk->pop();
assert(cur != NULL, "Counted wrong?");
work_q->push(cur);
}
return num > 0;
}
void CMSConcMarkingTask::do_scan_and_mark(int i, CompactibleFreeListSpace* sp) {
SequentialSubTasksDone* pst = sp->conc_par_seq_tasks();
int n_tasks = pst->n_tasks();
// We allow that there may be no tasks to do here because
// we are restarting after a stack overflow.
assert(pst->valid() || n_tasks == 0, "Uninitialized use?");
uint nth_task = 0;
HeapWord* aligned_start = sp->bottom();
if (sp->used_region().contains(_restart_addr)) {
// Align down to a card boundary for the start of 0th task
// for this space.
aligned_start = align_down(_restart_addr, CardTable::card_size);
}
size_t chunk_size = sp->marking_task_size();
while (pst->try_claim_task(/* reference */ nth_task)) {
// Having claimed the nth task in this space,
// compute the chunk that it corresponds to:
MemRegion span = MemRegion(aligned_start + nth_task*chunk_size,
aligned_start + (nth_task+1)*chunk_size);
// Try and bump the global finger via a CAS;
// note that we need to do the global finger bump
// _before_ taking the intersection below, because
// the task corresponding to that region will be
// deemed done even if the used_region() expands
// because of allocation -- as it almost certainly will
// during start-up while the threads yield in the
// closure below.
HeapWord* finger = span.end();
bump_global_finger(finger); // atomically
// There are null tasks here corresponding to chunks
// beyond the "top" address of the space.
span = span.intersection(sp->used_region());
if (!span.is_empty()) { // Non-null task
HeapWord* prev_obj;
assert(!span.contains(_restart_addr) || nth_task == 0,
"Inconsistency");
if (nth_task == 0) {
// For the 0th task, we'll not need to compute a block_start.
if (span.contains(_restart_addr)) {
// In the case of a restart because of stack overflow,
// we might additionally skip a chunk prefix.
prev_obj = _restart_addr;
} else {
prev_obj = span.start();
}
} else {
// We want to skip the first object because
// the protocol is to scan any object in its entirety
// that _starts_ in this span; a fortiori, any
// object starting in an earlier span is scanned
// as part of an earlier claimed task.
// Below we use the "careful" version of block_start
// so we do not try to navigate uninitialized objects.
prev_obj = sp->block_start_careful(span.start());
// Below we use a variant of block_size that uses the
// Printezis bits to avoid waiting for allocated
// objects to become initialized/parsable.
while (prev_obj < span.start()) {
size_t sz = sp->block_size_no_stall(prev_obj, _collector);
if (sz > 0) {
prev_obj += sz;
} else {
// In this case we may end up doing a bit of redundant
// scanning, but that appears unavoidable, short of
// locking the free list locks; see bug 6324141.
break;
}
}
}
if (prev_obj < span.end()) {
MemRegion my_span = MemRegion(prev_obj, span.end());
// Do the marking work within a non-empty span --
// the last argument to the constructor indicates whether the
// iteration should be incremental with periodic yields.
ParMarkFromRootsClosure cl(this, _collector, my_span,
&_collector->_markBitMap,
work_queue(i),
&_collector->_markStack);
_collector->_markBitMap.iterate(&cl, my_span.start(), my_span.end());
} // else nothing to do for this task
} // else nothing to do for this task
}
// We'd be tempted to assert here that since there are no
// more tasks left to claim in this space, the global_finger
// must exceed space->top() and a fortiori space->end(). However,
// that would not quite be correct because the bumping of
// global_finger occurs strictly after the claiming of a task,
// so by the time we reach here the global finger may not yet
// have been bumped up by the thread that claimed the last
// task.
pst->all_tasks_completed();
}
class ParConcMarkingClosure: public MetadataVisitingOopIterateClosure {
private:
CMSCollector* _collector;
CMSConcMarkingTask* _task;
MemRegion _span;
CMSBitMap* _bit_map;
CMSMarkStack* _overflow_stack;
OopTaskQueue* _work_queue;
protected:
DO_OOP_WORK_DEFN
public:
ParConcMarkingClosure(CMSCollector* collector, CMSConcMarkingTask* task, OopTaskQueue* work_queue,
CMSBitMap* bit_map, CMSMarkStack* overflow_stack):
MetadataVisitingOopIterateClosure(collector->ref_processor()),
_collector(collector),
_task(task),
_span(collector->_span),
_bit_map(bit_map),
_overflow_stack(overflow_stack),
_work_queue(work_queue)
{ }
virtual void do_oop(oop* p);
virtual void do_oop(narrowOop* p);
void trim_queue(size_t max);
void handle_stack_overflow(HeapWord* lost);
void do_yield_check() {
if (_task->should_yield()) {
_task->yield();
}
}
};
DO_OOP_WORK_IMPL(ParConcMarkingClosure)
// Grey object scanning during work stealing phase --
// the salient assumption here is that any references
// that are in these stolen objects being scanned must
// already have been initialized (else they would not have
// been published), so we do not need to check for
// uninitialized objects before pushing here.
void ParConcMarkingClosure::do_oop(oop obj) {
assert(oopDesc::is_oop_or_null(obj, true), "Expected an oop or NULL at " PTR_FORMAT, p2i(obj));
HeapWord* addr = (HeapWord*)obj;
// Check if oop points into the CMS generation
// and is not marked
if (_span.contains(addr) && !_bit_map->isMarked(addr)) {
// a white object ...
// If we manage to "claim" the object, by being the
// first thread to mark it, then we push it on our
// marking stack
if (_bit_map->par_mark(addr)) { // ... now grey
// push on work queue (grey set)
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow ||
!(_work_queue->push(obj) || _overflow_stack->par_push(obj))) {
// stack overflow
log_trace(gc)("CMS marking stack overflow (benign) at " SIZE_FORMAT, _overflow_stack->capacity());
// We cannot assert that the overflow stack is full because
// it may have been emptied since.
assert(simulate_overflow ||
_work_queue->size() == _work_queue->max_elems(),
"Else push should have succeeded");
handle_stack_overflow(addr);
}
} // Else, some other thread got there first
do_yield_check();
}
}
void ParConcMarkingClosure::trim_queue(size_t max) {
while (_work_queue->size() > max) {
oop new_oop;
if (_work_queue->pop_local(new_oop)) {
assert(oopDesc::is_oop(new_oop), "Should be an oop");
assert(_bit_map->isMarked((HeapWord*)new_oop), "Grey object");
assert(_span.contains((HeapWord*)new_oop), "Not in span");
new_oop->oop_iterate(this); // do_oop() above
do_yield_check();
}
}
}
// Upon stack overflow, we discard (part of) the stack,
// remembering the least address amongst those discarded
// in CMSCollector's _restart_address.
void ParConcMarkingClosure::handle_stack_overflow(HeapWord* lost) {
// We need to do this under a mutex to prevent other
// workers from interfering with the work done below.
MutexLocker ml(_overflow_stack->par_lock(),
Mutex::_no_safepoint_check_flag);
// Remember the least grey address discarded
HeapWord* ra = (HeapWord*)_overflow_stack->least_value(lost);
_collector->lower_restart_addr(ra);
_overflow_stack->reset(); // discard stack contents
_overflow_stack->expand(); // expand the stack if possible
}
void CMSConcMarkingTask::do_work_steal(int i) {
OopTaskQueue* work_q = work_queue(i);
oop obj_to_scan;
CMSBitMap* bm = &(_collector->_markBitMap);
CMSMarkStack* ovflw = &(_collector->_markStack);
ParConcMarkingClosure cl(_collector, this, work_q, bm, ovflw);
while (true) {
cl.trim_queue(0);
assert(work_q->size() == 0, "Should have been emptied above");
if (get_work_from_overflow_stack(ovflw, work_q)) {
// Can't assert below because the work obtained from the
// overflow stack may already have been stolen from us.
// assert(work_q->size() > 0, "Work from overflow stack");
continue;
} else if (task_queues()->steal(i, /* reference */ obj_to_scan)) {
assert(oopDesc::is_oop(obj_to_scan), "Should be an oop");
assert(bm->isMarked((HeapWord*)obj_to_scan), "Grey object");
obj_to_scan->oop_iterate(&cl);
} else if (terminator()->offer_termination(&_term_term)) {
assert(work_q->size() == 0, "Impossible!");
break;
} else if (yielding() || should_yield()) {
yield();
}
}
}
// This is run by the CMS (coordinator) thread.
void CMSConcMarkingTask::coordinator_yield() {
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
// First give up the locks, then yield, then re-lock
// We should probably use a constructor/destructor idiom to
// do this unlock/lock or modify the MutexUnlocker class to
// serve our purpose. XXX
assert_lock_strong(_bit_map_lock);
_bit_map_lock->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// It is possible for whichever thread initiated the yield request
// not to get a chance to wake up and take the bitmap lock between
// this thread releasing it and reacquiring it. So, while the
// should_yield() flag is on, let's sleep for a bit to give the
// other thread a chance to wake up. The limit imposed on the number
// of iterations is defensive, to avoid any unforseen circumstances
// putting us into an infinite loop. Since it's always been this
// (coordinator_yield()) method that was observed to cause the
// problem, we are using a parameter (CMSCoordinatorYieldSleepCount)
// which is by default non-zero. For the other seven methods that
// also perform the yield operation, as are using a different
// parameter (CMSYieldSleepCount) which is by default zero. This way we
// can enable the sleeping for those methods too, if necessary.
// See 6442774.
//
// We really need to reconsider the synchronization between the GC
// thread and the yield-requesting threads in the future and we
// should really use wait/notify, which is the recommended
// way of doing this type of interaction. Additionally, we should
// consolidate the eight methods that do the yield operation and they
// are almost identical into one for better maintainability and
// readability. See 6445193.
//
// Tony 2006.06.29
for (unsigned i = 0; i < CMSCoordinatorYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
_bit_map_lock->lock_without_safepoint_check();
_collector->startTimer();
}
bool CMSCollector::do_marking_mt() {
assert(ConcGCThreads > 0 && conc_workers() != NULL, "precondition");
uint num_workers = WorkerPolicy::calc_active_conc_workers(conc_workers()->total_workers(),
conc_workers()->active_workers(),
Threads::number_of_non_daemon_threads());
num_workers = conc_workers()->update_active_workers(num_workers);
log_info(gc,task)("Using %u workers of %u for marking", num_workers, conc_workers()->total_workers());
CompactibleFreeListSpace* cms_space = _cmsGen->cmsSpace();
CMSConcMarkingTask tsk(this,
cms_space,
conc_workers(),
task_queues());
// Since the actual number of workers we get may be different
// from the number we requested above, do we need to do anything different
// below? In particular, may be we need to subclass the SequantialSubTasksDone
// class?? XXX
cms_space ->initialize_sequential_subtasks_for_marking(num_workers);
// Refs discovery is already non-atomic.
assert(!ref_processor()->discovery_is_atomic(), "Should be non-atomic");
assert(ref_processor()->discovery_is_mt(), "Discovery should be MT");
conc_workers()->start_task(&tsk);
while (tsk.yielded()) {
tsk.coordinator_yield();
conc_workers()->continue_task(&tsk);
}
// If the task was aborted, _restart_addr will be non-NULL
assert(tsk.completed() || _restart_addr != NULL, "Inconsistency");
while (_restart_addr != NULL) {
// XXX For now we do not make use of ABORTED state and have not
// yet implemented the right abort semantics (even in the original
// single-threaded CMS case). That needs some more investigation
// and is deferred for now; see CR# TBF. 07252005YSR. XXX
assert(!CMSAbortSemantics || tsk.aborted(), "Inconsistency");
// If _restart_addr is non-NULL, a marking stack overflow
// occurred; we need to do a fresh marking iteration from the
// indicated restart address.
if (_foregroundGCIsActive) {
// We may be running into repeated stack overflows, having
// reached the limit of the stack size, while making very
// slow forward progress. It may be best to bail out and
// let the foreground collector do its job.
// Clear _restart_addr, so that foreground GC
// works from scratch. This avoids the headache of
// a "rescan" which would otherwise be needed because
// of the dirty mod union table & card table.
_restart_addr = NULL;
return false;
}
// Adjust the task to restart from _restart_addr
tsk.reset(_restart_addr);
cms_space ->initialize_sequential_subtasks_for_marking(num_workers,
_restart_addr);
_restart_addr = NULL;
// Get the workers going again
conc_workers()->start_task(&tsk);
while (tsk.yielded()) {
tsk.coordinator_yield();
conc_workers()->continue_task(&tsk);
}
}
assert(tsk.completed(), "Inconsistency");
assert(tsk.result() == true, "Inconsistency");
return true;
}
bool CMSCollector::do_marking_st() {
ResourceMark rm;
HandleMark hm;
// Temporarily make refs discovery single threaded (non-MT)
ReferenceProcessorMTDiscoveryMutator rp_mut_discovery(ref_processor(), false);
MarkFromRootsClosure markFromRootsClosure(this, _span, &_markBitMap,
&_markStack, CMSYield);
// the last argument to iterate indicates whether the iteration
// should be incremental with periodic yields.
_markBitMap.iterate(&markFromRootsClosure);
// If _restart_addr is non-NULL, a marking stack overflow
// occurred; we need to do a fresh iteration from the
// indicated restart address.
while (_restart_addr != NULL) {
if (_foregroundGCIsActive) {
// We may be running into repeated stack overflows, having
// reached the limit of the stack size, while making very
// slow forward progress. It may be best to bail out and
// let the foreground collector do its job.
// Clear _restart_addr, so that foreground GC
// works from scratch. This avoids the headache of
// a "rescan" which would otherwise be needed because
// of the dirty mod union table & card table.
_restart_addr = NULL;
return false; // indicating failure to complete marking
}
// Deal with stack overflow:
// we restart marking from _restart_addr
HeapWord* ra = _restart_addr;
markFromRootsClosure.reset(ra);
_restart_addr = NULL;
_markBitMap.iterate(&markFromRootsClosure, ra, _span.end());
}
return true;
}
void CMSCollector::preclean() {
check_correct_thread_executing();
assert(Thread::current()->is_ConcurrentGC_thread(), "Wrong thread");
verify_work_stacks_empty();
verify_overflow_empty();
_abort_preclean = false;
if (CMSPrecleaningEnabled) {
if (!CMSEdenChunksRecordAlways) {
_eden_chunk_index = 0;
}
size_t used = get_eden_used();
size_t capacity = get_eden_capacity();
// Don't start sampling unless we will get sufficiently
// many samples.
if (used < (((capacity / CMSScheduleRemarkSamplingRatio) / 100)
* CMSScheduleRemarkEdenPenetration)) {
_start_sampling = true;
} else {
_start_sampling = false;
}
GCTraceCPUTime tcpu;
CMSPhaseAccounting pa(this, "Concurrent Preclean");
preclean_work(CMSPrecleanRefLists1, CMSPrecleanSurvivors1);
}
CMSTokenSync x(true); // is cms thread
if (CMSPrecleaningEnabled) {
sample_eden();
_collectorState = AbortablePreclean;
} else {
_collectorState = FinalMarking;
}
verify_work_stacks_empty();
verify_overflow_empty();
}
// Try and schedule the remark such that young gen
// occupancy is CMSScheduleRemarkEdenPenetration %.
void CMSCollector::abortable_preclean() {
check_correct_thread_executing();
assert(CMSPrecleaningEnabled, "Inconsistent control state");
assert(_collectorState == AbortablePreclean, "Inconsistent control state");
// If Eden's current occupancy is below this threshold,
// immediately schedule the remark; else preclean
// past the next scavenge in an effort to
// schedule the pause as described above. By choosing
// CMSScheduleRemarkEdenSizeThreshold >= max eden size
// we will never do an actual abortable preclean cycle.
if (get_eden_used() > CMSScheduleRemarkEdenSizeThreshold) {
GCTraceCPUTime tcpu;
CMSPhaseAccounting pa(this, "Concurrent Abortable Preclean");
// We need more smarts in the abortable preclean
// loop below to deal with cases where allocation
// in young gen is very very slow, and our precleaning
// is running a losing race against a horde of
// mutators intent on flooding us with CMS updates
// (dirty cards).
// One, admittedly dumb, strategy is to give up
// after a certain number of abortable precleaning loops
// or after a certain maximum time. We want to make
// this smarter in the next iteration.
// XXX FIX ME!!! YSR
size_t loops = 0, workdone = 0, cumworkdone = 0, waited = 0;
while (!(should_abort_preclean() ||
ConcurrentMarkSweepThread::cmst()->should_terminate())) {
workdone = preclean_work(CMSPrecleanRefLists2, CMSPrecleanSurvivors2);
cumworkdone += workdone;
loops++;
// Voluntarily terminate abortable preclean phase if we have
// been at it for too long.
if ((CMSMaxAbortablePrecleanLoops != 0) &&
loops >= CMSMaxAbortablePrecleanLoops) {
log_debug(gc)(" CMS: abort preclean due to loops ");
break;
}
if (pa.wallclock_millis() > CMSMaxAbortablePrecleanTime) {
log_debug(gc)(" CMS: abort preclean due to time ");
break;
}
// If we are doing little work each iteration, we should
// take a short break.
if (workdone < CMSAbortablePrecleanMinWorkPerIteration) {
// Sleep for some time, waiting for work to accumulate
stopTimer();
cmsThread()->wait_on_cms_lock(CMSAbortablePrecleanWaitMillis);
startTimer();
waited++;
}
}
log_trace(gc)(" [" SIZE_FORMAT " iterations, " SIZE_FORMAT " waits, " SIZE_FORMAT " cards)] ",
loops, waited, cumworkdone);
}
CMSTokenSync x(true); // is cms thread
if (_collectorState != Idling) {
assert(_collectorState == AbortablePreclean,
"Spontaneous state transition?");
_collectorState = FinalMarking;
} // Else, a foreground collection completed this CMS cycle.
return;
}
// Respond to an Eden sampling opportunity
void CMSCollector::sample_eden() {
// Make sure a young gc cannot sneak in between our
// reading and recording of a sample.
assert(Thread::current()->is_ConcurrentGC_thread(),
"Only the cms thread may collect Eden samples");
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"Should collect samples while holding CMS token");
if (!_start_sampling) {
return;
}
// When CMSEdenChunksRecordAlways is true, the eden chunk array
// is populated by the young generation.
if (_eden_chunk_array != NULL && !CMSEdenChunksRecordAlways) {
if (_eden_chunk_index < _eden_chunk_capacity) {
_eden_chunk_array[_eden_chunk_index] = *_top_addr; // take sample
assert(_eden_chunk_array[_eden_chunk_index] <= *_end_addr,
"Unexpected state of Eden");
// We'd like to check that what we just sampled is an oop-start address;
// however, we cannot do that here since the object may not yet have been
// initialized. So we'll instead do the check when we _use_ this sample
// later.
if (_eden_chunk_index == 0 ||
(pointer_delta(_eden_chunk_array[_eden_chunk_index],
_eden_chunk_array[_eden_chunk_index-1])
>= CMSSamplingGrain)) {
_eden_chunk_index++; // commit sample
}
}
}
if ((_collectorState == AbortablePreclean) && !_abort_preclean) {
size_t used = get_eden_used();
size_t capacity = get_eden_capacity();
assert(used <= capacity, "Unexpected state of Eden");
if (used > (capacity/100 * CMSScheduleRemarkEdenPenetration)) {
_abort_preclean = true;
}
}
}
size_t CMSCollector::preclean_work(bool clean_refs, bool clean_survivor) {
assert(_collectorState == Precleaning ||
_collectorState == AbortablePreclean, "incorrect state");
ResourceMark rm;
HandleMark hm;
// Precleaning is currently not MT but the reference processor
// may be set for MT. Disable it temporarily here.
ReferenceProcessor* rp = ref_processor();
ReferenceProcessorMTDiscoveryMutator rp_mut_discovery(rp, false);
// Do one pass of scrubbing the discovered reference lists
// to remove any reference objects with strongly-reachable
// referents.
if (clean_refs) {
CMSPrecleanRefsYieldClosure yield_cl(this);
assert(_span_based_discoverer.span().equals(_span), "Spans should be equal");
CMSKeepAliveClosure keep_alive(this, _span, &_markBitMap,
&_markStack, true /* preclean */);
CMSDrainMarkingStackClosure complete_trace(this,
_span, &_markBitMap, &_markStack,
&keep_alive, true /* preclean */);
// We don't want this step to interfere with a young
// collection because we don't want to take CPU
// or memory bandwidth away from the young GC threads
// (which may be as many as there are CPUs).
// Note that we don't need to protect ourselves from
// interference with mutators because they can't
// manipulate the discovered reference lists nor affect
// the computed reachability of the referents, the
// only properties manipulated by the precleaning
// of these reference lists.
stopTimer();
CMSTokenSyncWithLocks x(true /* is cms thread */,
bitMapLock());
startTimer();
sample_eden();
// The following will yield to allow foreground
// collection to proceed promptly. XXX YSR:
// The code in this method may need further
// tweaking for better performance and some restructuring
// for cleaner interfaces.
GCTimer *gc_timer = NULL; // Currently not tracing concurrent phases
rp->preclean_discovered_references(
rp->is_alive_non_header(), &keep_alive, &complete_trace, &yield_cl,
gc_timer);
}
if (clean_survivor) { // preclean the active survivor space(s)
PushAndMarkClosure pam_cl(this, _span, ref_processor(),
&_markBitMap, &_modUnionTable,
&_markStack, true /* precleaning phase */);
stopTimer();
CMSTokenSyncWithLocks ts(true /* is cms thread */,
bitMapLock());
startTimer();
unsigned int before_count =
CMSHeap::heap()->total_collections();
SurvivorSpacePrecleanClosure
sss_cl(this, _span, &_markBitMap, &_markStack,
&pam_cl, before_count, CMSYield);
_young_gen->from()->object_iterate_careful(&sss_cl);
_young_gen->to()->object_iterate_careful(&sss_cl);
}
MarkRefsIntoAndScanClosure
mrias_cl(_span, ref_processor(), &_markBitMap, &_modUnionTable,
&_markStack, this, CMSYield,
true /* precleaning phase */);
// CAUTION: The following closure has persistent state that may need to
// be reset upon a decrease in the sequence of addresses it
// processes.
ScanMarkedObjectsAgainCarefullyClosure
smoac_cl(this, _span,
&_markBitMap, &_markStack, &mrias_cl, CMSYield);
// Preclean dirty cards in ModUnionTable and CardTable using
// appropriate convergence criterion;
// repeat CMSPrecleanIter times unless we find that
// we are losing.
assert(CMSPrecleanIter < 10, "CMSPrecleanIter is too large");
assert(CMSPrecleanNumerator < CMSPrecleanDenominator,
"Bad convergence multiplier");
assert(CMSPrecleanThreshold >= 100,
"Unreasonably low CMSPrecleanThreshold");
size_t numIter, cumNumCards, lastNumCards, curNumCards;
for (numIter = 0, cumNumCards = lastNumCards = curNumCards = 0;
numIter < CMSPrecleanIter;
numIter++, lastNumCards = curNumCards, cumNumCards += curNumCards) {
curNumCards = preclean_mod_union_table(_cmsGen, &smoac_cl);
log_trace(gc)(" (modUnionTable: " SIZE_FORMAT " cards)", curNumCards);
// Either there are very few dirty cards, so re-mark
// pause will be small anyway, or our pre-cleaning isn't
// that much faster than the rate at which cards are being
// dirtied, so we might as well stop and re-mark since
// precleaning won't improve our re-mark time by much.
if (curNumCards <= CMSPrecleanThreshold ||
(numIter > 0 &&
(curNumCards * CMSPrecleanDenominator >
lastNumCards * CMSPrecleanNumerator))) {
numIter++;
cumNumCards += curNumCards;
break;
}
}
preclean_cld(&mrias_cl, _cmsGen->freelistLock());
curNumCards = preclean_card_table(_cmsGen, &smoac_cl);
cumNumCards += curNumCards;
log_trace(gc)(" (cardTable: " SIZE_FORMAT " cards, re-scanned " SIZE_FORMAT " cards, " SIZE_FORMAT " iterations)",
curNumCards, cumNumCards, numIter);
return cumNumCards; // as a measure of useful work done
}
// PRECLEANING NOTES:
// Precleaning involves:
// . reading the bits of the modUnionTable and clearing the set bits.
// . For the cards corresponding to the set bits, we scan the
// objects on those cards. This means we need the free_list_lock
// so that we can safely iterate over the CMS space when scanning
// for oops.
// . When we scan the objects, we'll be both reading and setting
// marks in the marking bit map, so we'll need the marking bit map.
// . For protecting _collector_state transitions, we take the CGC_lock.
// Note that any races in the reading of of card table entries by the
// CMS thread on the one hand and the clearing of those entries by the
// VM thread or the setting of those entries by the mutator threads on the
// other are quite benign. However, for efficiency it makes sense to keep
// the VM thread from racing with the CMS thread while the latter is
// dirty card info to the modUnionTable. We therefore also use the
// CGC_lock to protect the reading of the card table and the mod union
// table by the CM thread.
// . We run concurrently with mutator updates, so scanning
// needs to be done carefully -- we should not try to scan
// potentially uninitialized objects.
//
// Locking strategy: While holding the CGC_lock, we scan over and
// reset a maximal dirty range of the mod union / card tables, then lock
// the free_list_lock and bitmap lock to do a full marking, then
// release these locks; and repeat the cycle. This allows for a
// certain amount of fairness in the sharing of these locks between
// the CMS collector on the one hand, and the VM thread and the
// mutators on the other.
// NOTE: preclean_mod_union_table() and preclean_card_table()
// further below are largely identical; if you need to modify
// one of these methods, please check the other method too.
size_t CMSCollector::preclean_mod_union_table(
ConcurrentMarkSweepGeneration* old_gen,
ScanMarkedObjectsAgainCarefullyClosure* cl) {
verify_work_stacks_empty();
verify_overflow_empty();
// strategy: starting with the first card, accumulate contiguous
// ranges of dirty cards; clear these cards, then scan the region
// covered by these cards.
// Since all of the MUT is committed ahead, we can just use
// that, in case the generations expand while we are precleaning.
// It might also be fine to just use the committed part of the
// generation, but we might potentially miss cards when the
// generation is rapidly expanding while we are in the midst
// of precleaning.
HeapWord* startAddr = old_gen->reserved().start();
HeapWord* endAddr = old_gen->reserved().end();
cl->setFreelistLock(old_gen->freelistLock()); // needed for yielding
size_t numDirtyCards, cumNumDirtyCards;
HeapWord *nextAddr, *lastAddr;
for (cumNumDirtyCards = numDirtyCards = 0,
nextAddr = lastAddr = startAddr;
nextAddr < endAddr;
nextAddr = lastAddr, cumNumDirtyCards += numDirtyCards) {
ResourceMark rm;
HandleMark hm;
MemRegion dirtyRegion;
{
stopTimer();
// Potential yield point
CMSTokenSync ts(true);
startTimer();
sample_eden();
// Get dirty region starting at nextOffset (inclusive),
// simultaneously clearing it.
dirtyRegion =
_modUnionTable.getAndClearMarkedRegion(nextAddr, endAddr);
assert(dirtyRegion.start() >= nextAddr,
"returned region inconsistent?");
}
// Remember where the next search should begin.
// The returned region (if non-empty) is a right open interval,
// so lastOffset is obtained from the right end of that
// interval.
lastAddr = dirtyRegion.end();
// Should do something more transparent and less hacky XXX
numDirtyCards =
_modUnionTable.heapWordDiffToOffsetDiff(dirtyRegion.word_size());
// We'll scan the cards in the dirty region (with periodic
// yields for foreground GC as needed).
if (!dirtyRegion.is_empty()) {
assert(numDirtyCards > 0, "consistency check");
HeapWord* stop_point = NULL;
stopTimer();
// Potential yield point
CMSTokenSyncWithLocks ts(true, old_gen->freelistLock(),
bitMapLock());
startTimer();
{
verify_work_stacks_empty();
verify_overflow_empty();
sample_eden();
stop_point =
old_gen->cmsSpace()->object_iterate_careful_m(dirtyRegion, cl);
}
if (stop_point != NULL) {
// The careful iteration stopped early either because it found an
// uninitialized object, or because we were in the midst of an
// "abortable preclean", which should now be aborted. Redirty
// the bits corresponding to the partially-scanned or unscanned
// cards. We'll either restart at the next block boundary or
// abort the preclean.
assert((_collectorState == AbortablePreclean && should_abort_preclean()),
"Should only be AbortablePreclean.");
_modUnionTable.mark_range(MemRegion(stop_point, dirtyRegion.end()));
if (should_abort_preclean()) {
break; // out of preclean loop
} else {
// Compute the next address at which preclean should pick up;
// might need bitMapLock in order to read P-bits.
lastAddr = next_card_start_after_block(stop_point);
}
}
} else {
assert(lastAddr == endAddr, "consistency check");
assert(numDirtyCards == 0, "consistency check");
break;
}
}
verify_work_stacks_empty();
verify_overflow_empty();
return cumNumDirtyCards;
}
// NOTE: preclean_mod_union_table() above and preclean_card_table()
// below are largely identical; if you need to modify
// one of these methods, please check the other method too.
size_t CMSCollector::preclean_card_table(ConcurrentMarkSweepGeneration* old_gen,
ScanMarkedObjectsAgainCarefullyClosure* cl) {
// strategy: it's similar to precleamModUnionTable above, in that
// we accumulate contiguous ranges of dirty cards, mark these cards
// precleaned, then scan the region covered by these cards.
HeapWord* endAddr = (HeapWord*)(old_gen->_virtual_space.high());
HeapWord* startAddr = (HeapWord*)(old_gen->_virtual_space.low());
cl->setFreelistLock(old_gen->freelistLock()); // needed for yielding
size_t numDirtyCards, cumNumDirtyCards;
HeapWord *lastAddr, *nextAddr;
for (cumNumDirtyCards = numDirtyCards = 0,
nextAddr = lastAddr = startAddr;
nextAddr < endAddr;
nextAddr = lastAddr, cumNumDirtyCards += numDirtyCards) {
ResourceMark rm;
HandleMark hm;
MemRegion dirtyRegion;
{
// See comments in "Precleaning notes" above on why we
// do this locking. XXX Could the locking overheads be
// too high when dirty cards are sparse? [I don't think so.]
stopTimer();
CMSTokenSync x(true); // is cms thread
startTimer();
sample_eden();
// Get and clear dirty region from card table
dirtyRegion = _ct->dirty_card_range_after_reset(MemRegion(nextAddr, endAddr),
true,
CardTable::precleaned_card_val());
assert(dirtyRegion.start() >= nextAddr,
"returned region inconsistent?");
}
lastAddr = dirtyRegion.end();
numDirtyCards =
dirtyRegion.word_size()/CardTable::card_size_in_words;
if (!dirtyRegion.is_empty()) {
stopTimer();
CMSTokenSyncWithLocks ts(true, old_gen->freelistLock(), bitMapLock());
startTimer();
sample_eden();
verify_work_stacks_empty();
verify_overflow_empty();
HeapWord* stop_point =
old_gen->cmsSpace()->object_iterate_careful_m(dirtyRegion, cl);
if (stop_point != NULL) {
assert((_collectorState == AbortablePreclean && should_abort_preclean()),
"Should only be AbortablePreclean.");
_ct->invalidate(MemRegion(stop_point, dirtyRegion.end()));
if (should_abort_preclean()) {
break; // out of preclean loop
} else {
// Compute the next address at which preclean should pick up.
lastAddr = next_card_start_after_block(stop_point);
}
}
} else {
break;
}
}
verify_work_stacks_empty();
verify_overflow_empty();
return cumNumDirtyCards;
}
class PrecleanCLDClosure : public CLDClosure {
MetadataVisitingOopsInGenClosure* _cm_closure;
public:
PrecleanCLDClosure(MetadataVisitingOopsInGenClosure* oop_closure) : _cm_closure(oop_closure) {}
void do_cld(ClassLoaderData* cld) {
if (cld->has_accumulated_modified_oops()) {
cld->clear_accumulated_modified_oops();
_cm_closure->do_cld(cld);
}
}
};
// The freelist lock is needed to prevent asserts, is it really needed?
void CMSCollector::preclean_cld(MarkRefsIntoAndScanClosure* cl, Mutex* freelistLock) {
// Needed to walk CLDG
MutexLocker ml(ClassLoaderDataGraph_lock);
cl->set_freelistLock(freelistLock);
CMSTokenSyncWithLocks ts(true, freelistLock, bitMapLock());
// SSS: Add equivalent to ScanMarkedObjectsAgainCarefullyClosure::do_yield_check and should_abort_preclean?
// SSS: We should probably check if precleaning should be aborted, at suitable intervals?
PrecleanCLDClosure preclean_closure(cl);
ClassLoaderDataGraph::cld_do(&preclean_closure);
verify_work_stacks_empty();
verify_overflow_empty();
}
void CMSCollector::checkpointRootsFinal() {
assert(_collectorState == FinalMarking, "incorrect state transition?");
check_correct_thread_executing();
// world is stopped at this checkpoint
assert(SafepointSynchronize::is_at_safepoint(),
"world should be stopped");
TraceCMSMemoryManagerStats tms(_collectorState, CMSHeap::heap()->gc_cause());
verify_work_stacks_empty();
verify_overflow_empty();
log_debug(gc)("YG occupancy: " SIZE_FORMAT " K (" SIZE_FORMAT " K)",
_young_gen->used() / K, _young_gen->capacity() / K);
{
if (CMSScavengeBeforeRemark) {
CMSHeap* heap = CMSHeap::heap();
// Temporarily set flag to false, GCH->do_collection will
// expect it to be false and set to true
FlagSetting fl(heap->_is_gc_active, false);
heap->do_collection(true, // full (i.e. force, see below)
false, // !clear_all_soft_refs
0, // size
false, // is_tlab
GenCollectedHeap::YoungGen // type
);
}
FreelistLocker x(this);
MutexLocker y(bitMapLock(),
Mutex::_no_safepoint_check_flag);
checkpointRootsFinalWork();
_cmsGen->cmsSpace()->recalculate_used_stable();
}
verify_work_stacks_empty();
verify_overflow_empty();
}
void CMSCollector::checkpointRootsFinalWork() {
GCTraceTime(Trace, gc, phases) tm("checkpointRootsFinalWork", _gc_timer_cm);
assert(haveFreelistLocks(), "must have free list locks");
assert_lock_strong(bitMapLock());
ResourceMark rm;
HandleMark hm;
CMSHeap* heap = CMSHeap::heap();
assert(haveFreelistLocks(), "must have free list locks");
assert_lock_strong(bitMapLock());
// We might assume that we need not fill TLAB's when
// CMSScavengeBeforeRemark is set, because we may have just done
// a scavenge which would have filled all TLAB's -- and besides
// Eden would be empty. This however may not always be the case --
// for instance although we asked for a scavenge, it may not have
// happened because of a JNI critical section. We probably need
// a policy for deciding whether we can in that case wait until
// the critical section releases and then do the remark following
// the scavenge, and skip it here. In the absence of that policy,
// or of an indication of whether the scavenge did indeed occur,
// we cannot rely on TLAB's having been filled and must do
// so here just in case a scavenge did not happen.
heap->ensure_parsability(false); // fill TLAB's, but no need to retire them
// Update the saved marks which may affect the root scans.
heap->save_marks();
print_eden_and_survivor_chunk_arrays();
{
#if COMPILER2_OR_JVMCI
DerivedPointerTableDeactivate dpt_deact;
#endif
// Note on the role of the mod union table:
// Since the marker in "markFromRoots" marks concurrently with
// mutators, it is possible for some reachable objects not to have been
// scanned. For instance, an only reference to an object A was
// placed in object B after the marker scanned B. Unless B is rescanned,
// A would be collected. Such updates to references in marked objects
// are detected via the mod union table which is the set of all cards
// dirtied since the first checkpoint in this GC cycle and prior to
// the most recent young generation GC, minus those cleaned up by the
// concurrent precleaning.
if (CMSParallelRemarkEnabled) {
GCTraceTime(Debug, gc, phases) t("Rescan (parallel)", _gc_timer_cm);
do_remark_parallel();
} else {
GCTraceTime(Debug, gc, phases) t("Rescan (non-parallel)", _gc_timer_cm);
do_remark_non_parallel();
}
}
verify_work_stacks_empty();
verify_overflow_empty();
{
GCTraceTime(Trace, gc, phases) ts("refProcessingWork", _gc_timer_cm);
refProcessingWork();
}
verify_work_stacks_empty();
verify_overflow_empty();
if (should_unload_classes()) {
heap->prune_scavengable_nmethods();
}
// If we encountered any (marking stack / work queue) overflow
// events during the current CMS cycle, take appropriate
// remedial measures, where possible, so as to try and avoid
// recurrence of that condition.
assert(_markStack.isEmpty(), "No grey objects");
size_t ser_ovflw = _ser_pmc_remark_ovflw + _ser_pmc_preclean_ovflw +
_ser_kac_ovflw + _ser_kac_preclean_ovflw;
if (ser_ovflw > 0) {
log_trace(gc)("Marking stack overflow (benign) (pmc_pc=" SIZE_FORMAT ", pmc_rm=" SIZE_FORMAT ", kac=" SIZE_FORMAT ", kac_preclean=" SIZE_FORMAT ")",
_ser_pmc_preclean_ovflw, _ser_pmc_remark_ovflw, _ser_kac_ovflw, _ser_kac_preclean_ovflw);
_markStack.expand();
_ser_pmc_remark_ovflw = 0;
_ser_pmc_preclean_ovflw = 0;
_ser_kac_preclean_ovflw = 0;
_ser_kac_ovflw = 0;
}
if (_par_pmc_remark_ovflw > 0 || _par_kac_ovflw > 0) {
log_trace(gc)("Work queue overflow (benign) (pmc_rm=" SIZE_FORMAT ", kac=" SIZE_FORMAT ")",
_par_pmc_remark_ovflw, _par_kac_ovflw);
_par_pmc_remark_ovflw = 0;
_par_kac_ovflw = 0;
}
if (_markStack._hit_limit > 0) {
log_trace(gc)(" (benign) Hit max stack size limit (" SIZE_FORMAT ")",
_markStack._hit_limit);
}
if (_markStack._failed_double > 0) {
log_trace(gc)(" (benign) Failed stack doubling (" SIZE_FORMAT "), current capacity " SIZE_FORMAT,
_markStack._failed_double, _markStack.capacity());
}
_markStack._hit_limit = 0;
_markStack._failed_double = 0;
if ((VerifyAfterGC || VerifyDuringGC) &&
CMSHeap::heap()->total_collections() >= VerifyGCStartAt) {
verify_after_remark();
}
_gc_tracer_cm->report_object_count_after_gc(&_is_alive_closure);
// Change under the freelistLocks.
_collectorState = Sweeping;
// Call isAllClear() under bitMapLock
assert(_modUnionTable.isAllClear(),
"Should be clear by end of the final marking");
assert(_ct->cld_rem_set()->mod_union_is_clear(),
"Should be clear by end of the final marking");
}
void CMSParInitialMarkTask::work(uint worker_id) {
elapsedTimer _timer;
ResourceMark rm;
HandleMark hm;
// ---------- scan from roots --------------
_timer.start();
CMSHeap* heap = CMSHeap::heap();
ParMarkRefsIntoClosure par_mri_cl(_collector->_span, &(_collector->_markBitMap));
// ---------- young gen roots --------------
{
work_on_young_gen_roots(&par_mri_cl);
_timer.stop();
log_trace(gc, task)("Finished young gen initial mark scan work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
}
// ---------- remaining roots --------------
_timer.reset();
_timer.start();
CLDToOopClosure cld_closure(&par_mri_cl, ClassLoaderData::_claim_strong);
heap->cms_process_roots(_strong_roots_scope,
false, // yg was scanned above
GenCollectedHeap::ScanningOption(_collector->CMSCollector::roots_scanning_options()),
_collector->should_unload_classes(),
&par_mri_cl,
&cld_closure);
assert(_collector->should_unload_classes()
|| (_collector->CMSCollector::roots_scanning_options() & GenCollectedHeap::SO_AllCodeCache),
"if we didn't scan the code cache, we have to be ready to drop nmethods with expired weak oops");
_timer.stop();
log_trace(gc, task)("Finished remaining root initial mark scan work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
}
// Parallel remark task
class CMSParRemarkTask: public CMSParMarkTask {
CompactibleFreeListSpace* _cms_space;
// The per-thread work queues, available here for stealing.
OopTaskQueueSet* _task_queues;
TaskTerminator _term;
StrongRootsScope* _strong_roots_scope;
public:
// A value of 0 passed to n_workers will cause the number of
// workers to be taken from the active workers in the work gang.
CMSParRemarkTask(CMSCollector* collector,
CompactibleFreeListSpace* cms_space,
uint n_workers, WorkGang* workers,
OopTaskQueueSet* task_queues,
StrongRootsScope* strong_roots_scope):
CMSParMarkTask("Rescan roots and grey objects in parallel",
collector, n_workers),
_cms_space(cms_space),
_task_queues(task_queues),
_term(n_workers, task_queues),
_strong_roots_scope(strong_roots_scope) { }
OopTaskQueueSet* task_queues() { return _task_queues; }
OopTaskQueue* work_queue(int i) { return task_queues()->queue(i); }
ParallelTaskTerminator* terminator() { return _term.terminator(); }
uint n_workers() { return _n_workers; }
void work(uint worker_id);
private:
// ... of dirty cards in old space
void do_dirty_card_rescan_tasks(CompactibleFreeListSpace* sp, int i,
ParMarkRefsIntoAndScanClosure* cl);
// ... work stealing for the above
void do_work_steal(int i, ParMarkRefsIntoAndScanClosure* cl);
};
class RemarkCLDClosure : public CLDClosure {
CLDToOopClosure _cm_closure;
public:
RemarkCLDClosure(OopClosure* oop_closure) : _cm_closure(oop_closure, ClassLoaderData::_claim_strong) {}
void do_cld(ClassLoaderData* cld) {
// Check if we have modified any oops in the CLD during the concurrent marking.
if (cld->has_accumulated_modified_oops()) {
cld->clear_accumulated_modified_oops();
// We could have transfered the current modified marks to the accumulated marks,
// like we do with the Card Table to Mod Union Table. But it's not really necessary.
} else if (cld->has_modified_oops()) {
// Don't clear anything, this info is needed by the next young collection.
} else {
// No modified oops in the ClassLoaderData.
return;
}
// The klass has modified fields, need to scan the klass.
_cm_closure.do_cld(cld);
}
};
void CMSParMarkTask::work_on_young_gen_roots(OopsInGenClosure* cl) {
ParNewGeneration* young_gen = _collector->_young_gen;
ContiguousSpace* eden_space = young_gen->eden();
ContiguousSpace* from_space = young_gen->from();
ContiguousSpace* to_space = young_gen->to();
HeapWord** eca = _collector->_eden_chunk_array;
size_t ect = _collector->_eden_chunk_index;
HeapWord** sca = _collector->_survivor_chunk_array;
size_t sct = _collector->_survivor_chunk_index;
assert(ect <= _collector->_eden_chunk_capacity, "out of bounds");
assert(sct <= _collector->_survivor_chunk_capacity, "out of bounds");
do_young_space_rescan(cl, to_space, NULL, 0);
do_young_space_rescan(cl, from_space, sca, sct);
do_young_space_rescan(cl, eden_space, eca, ect);
}
// work_queue(i) is passed to the closure
// ParMarkRefsIntoAndScanClosure. The "i" parameter
// also is passed to do_dirty_card_rescan_tasks() and to
// do_work_steal() to select the i-th task_queue.
void CMSParRemarkTask::work(uint worker_id) {
elapsedTimer _timer;
ResourceMark rm;
HandleMark hm;
// ---------- rescan from roots --------------
_timer.start();
CMSHeap* heap = CMSHeap::heap();
ParMarkRefsIntoAndScanClosure par_mrias_cl(_collector,
_collector->_span, _collector->ref_processor(),
&(_collector->_markBitMap),
work_queue(worker_id));
// Rescan young gen roots first since these are likely
// coarsely partitioned and may, on that account, constitute
// the critical path; thus, it's best to start off that
// work first.
// ---------- young gen roots --------------
{
work_on_young_gen_roots(&par_mrias_cl);
_timer.stop();
log_trace(gc, task)("Finished young gen rescan work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
}
// ---------- remaining roots --------------
_timer.reset();
_timer.start();
heap->cms_process_roots(_strong_roots_scope,
false, // yg was scanned above
GenCollectedHeap::ScanningOption(_collector->CMSCollector::roots_scanning_options()),
_collector->should_unload_classes(),
&par_mrias_cl,
NULL); // The dirty klasses will be handled below
assert(_collector->should_unload_classes()
|| (_collector->CMSCollector::roots_scanning_options() & GenCollectedHeap::SO_AllCodeCache),
"if we didn't scan the code cache, we have to be ready to drop nmethods with expired weak oops");
_timer.stop();
log_trace(gc, task)("Finished remaining root rescan work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
// ---------- unhandled CLD scanning ----------
if (worker_id == 0) { // Single threaded at the moment.
_timer.reset();
_timer.start();
// Scan all new class loader data objects and new dependencies that were
// introduced during concurrent marking.
ResourceMark rm;
GrowableArray<ClassLoaderData*>* array = ClassLoaderDataGraph::new_clds();
for (int i = 0; i < array->length(); i++) {
Devirtualizer::do_cld(&par_mrias_cl, array->at(i));
}
// We don't need to keep track of new CLDs anymore.
ClassLoaderDataGraph::remember_new_clds(false);
_timer.stop();
log_trace(gc, task)("Finished unhandled CLD scanning work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
}
// We might have added oops to ClassLoaderData::_handles during the
// concurrent marking phase. These oops do not always point to newly allocated objects
// that are guaranteed to be kept alive. Hence,
// we do have to revisit the _handles block during the remark phase.
// ---------- dirty CLD scanning ----------
if (worker_id == 0) { // Single threaded at the moment.
_timer.reset();
_timer.start();
// Scan all classes that was dirtied during the concurrent marking phase.
RemarkCLDClosure remark_closure(&par_mrias_cl);
ClassLoaderDataGraph::cld_do(&remark_closure);
_timer.stop();
log_trace(gc, task)("Finished dirty CLD scanning work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
}
// ---------- rescan dirty cards ------------
_timer.reset();
_timer.start();
// Do the rescan tasks for each of the two spaces
// (cms_space) in turn.
// "worker_id" is passed to select the task_queue for "worker_id"
do_dirty_card_rescan_tasks(_cms_space, worker_id, &par_mrias_cl);
_timer.stop();
log_trace(gc, task)("Finished dirty card rescan work in %dth thread: %3.3f sec", worker_id, _timer.seconds());
// ---------- steal work from other threads ...
// ---------- ... and drain overflow list.
_timer.reset();
_timer.start();
do_work_steal(worker_id, &par_mrias_cl);
_timer.stop();
log_trace(gc, task)("Finished work stealing in %dth thread: %3.3f sec", worker_id, _timer.seconds());
}
void
CMSParMarkTask::do_young_space_rescan(
OopsInGenClosure* cl, ContiguousSpace* space,
HeapWord** chunk_array, size_t chunk_top) {
// Until all tasks completed:
// . claim an unclaimed task
// . compute region boundaries corresponding to task claimed
// using chunk_array
// . par_oop_iterate(cl) over that region
ResourceMark rm;
HandleMark hm;
SequentialSubTasksDone* pst = space->par_seq_tasks();
uint nth_task = 0;
uint n_tasks = pst->n_tasks();
if (n_tasks > 0) {
assert(pst->valid(), "Uninitialized use?");
HeapWord *start, *end;
while (pst->try_claim_task(/* reference */ nth_task)) {
// We claimed task # nth_task; compute its boundaries.
if (chunk_top == 0) { // no samples were taken
assert(nth_task == 0 && n_tasks == 1, "Can have only 1 eden task");
start = space->bottom();
end = space->top();
} else if (nth_task == 0) {
start = space->bottom();
end = chunk_array[nth_task];
} else if (nth_task < (uint)chunk_top) {
assert(nth_task >= 1, "Control point invariant");
start = chunk_array[nth_task - 1];
end = chunk_array[nth_task];
} else {
assert(nth_task == (uint)chunk_top, "Control point invariant");
start = chunk_array[chunk_top - 1];
end = space->top();
}
MemRegion mr(start, end);
// Verify that mr is in space
assert(mr.is_empty() || space->used_region().contains(mr),
"Should be in space");
// Verify that "start" is an object boundary
assert(mr.is_empty() || oopDesc::is_oop(oop(mr.start())),
"Should be an oop");
space->par_oop_iterate(mr, cl);
}
pst->all_tasks_completed();
}
}
void
CMSParRemarkTask::do_dirty_card_rescan_tasks(
CompactibleFreeListSpace* sp, int i,
ParMarkRefsIntoAndScanClosure* cl) {
// Until all tasks completed:
// . claim an unclaimed task
// . compute region boundaries corresponding to task claimed
// . transfer dirty bits ct->mut for that region
// . apply rescanclosure to dirty mut bits for that region
ResourceMark rm;
HandleMark hm;
OopTaskQueue* work_q = work_queue(i);
ModUnionClosure modUnionClosure(&(_collector->_modUnionTable));
// CAUTION! CAUTION! CAUTION! CAUTION! CAUTION! CAUTION! CAUTION!
// CAUTION: This closure has state that persists across calls to
// the work method dirty_range_iterate_clear() in that it has
// embedded in it a (subtype of) UpwardsObjectClosure. The
// use of that state in the embedded UpwardsObjectClosure instance
// assumes that the cards are always iterated (even if in parallel
// by several threads) in monotonically increasing order per each
// thread. This is true of the implementation below which picks
// card ranges (chunks) in monotonically increasing order globally
// and, a-fortiori, in monotonically increasing order per thread
// (the latter order being a subsequence of the former).
// If the work code below is ever reorganized into a more chaotic
// work-partitioning form than the current "sequential tasks"
// paradigm, the use of that persistent state will have to be
// revisited and modified appropriately. See also related
// bug 4756801 work on which should examine this code to make
// sure that the changes there do not run counter to the
// assumptions made here and necessary for correctness and
// efficiency. Note also that this code might yield inefficient
// behavior in the case of very large objects that span one or
// more work chunks. Such objects would potentially be scanned
// several times redundantly. Work on 4756801 should try and
// address that performance anomaly if at all possible. XXX
MemRegion full_span = _collector->_span;
CMSBitMap* bm = &(_collector->_markBitMap); // shared
MarkFromDirtyCardsClosure
greyRescanClosure(_collector, full_span, // entire span of interest
sp, bm, work_q, cl);
SequentialSubTasksDone* pst = sp->conc_par_seq_tasks();
assert(pst->valid(), "Uninitialized use?");
uint nth_task = 0;
const int alignment = CardTable::card_size * BitsPerWord;
MemRegion span = sp->used_region();
HeapWord* start_addr = span.start();
HeapWord* end_addr = align_up(span.end(), alignment);
const size_t chunk_size = sp->rescan_task_size(); // in HeapWord units
assert(is_aligned(start_addr, alignment), "Check alignment");
assert(is_aligned(chunk_size, alignment), "Check alignment");
while (pst->try_claim_task(/* reference */ nth_task)) {
// Having claimed the nth_task, compute corresponding mem-region,
// which is a-fortiori aligned correctly (i.e. at a MUT boundary).
// The alignment restriction ensures that we do not need any
// synchronization with other gang-workers while setting or
// clearing bits in thus chunk of the MUT.
MemRegion this_span = MemRegion(start_addr + nth_task*chunk_size,
start_addr + (nth_task+1)*chunk_size);
// The last chunk's end might be way beyond end of the
// used region. In that case pull back appropriately.
if (this_span.end() > end_addr) {
this_span.set_end(end_addr);
assert(!this_span.is_empty(), "Program logic (calculation of n_tasks)");
}
// Iterate over the dirty cards covering this chunk, marking them
// precleaned, and setting the corresponding bits in the mod union
// table. Since we have been careful to partition at Card and MUT-word
// boundaries no synchronization is needed between parallel threads.
_collector->_ct->dirty_card_iterate(this_span,
&modUnionClosure);
// Having transferred these marks into the modUnionTable,
// rescan the marked objects on the dirty cards in the modUnionTable.
// Even if this is at a synchronous collection, the initial marking
// may have been done during an asynchronous collection so there
// may be dirty bits in the mod-union table.
_collector->_modUnionTable.dirty_range_iterate_clear(
this_span, &greyRescanClosure);
_collector->_modUnionTable.verifyNoOneBitsInRange(
this_span.start(),
this_span.end());
}
pst->all_tasks_completed(); // declare that i am done
}
// . see if we can share work_queues with ParNew? XXX
void
CMSParRemarkTask::do_work_steal(int i, ParMarkRefsIntoAndScanClosure* cl) {
OopTaskQueue* work_q = work_queue(i);
NOT_PRODUCT(int num_steals = 0;)
oop obj_to_scan;
CMSBitMap* bm = &(_collector->_markBitMap);
while (true) {
// Completely finish any left over work from (an) earlier round(s)
cl->trim_queue(0);
size_t num_from_overflow_list = MIN2((size_t)(work_q->max_elems() - work_q->size())/4,
(size_t)ParGCDesiredObjsFromOverflowList);
// Now check if there's any work in the overflow list
// Passing ParallelGCThreads as the third parameter, no_of_gc_threads,
// only affects the number of attempts made to get work from the
// overflow list and does not affect the number of workers. Just
// pass ParallelGCThreads so this behavior is unchanged.
if (_collector->par_take_from_overflow_list(num_from_overflow_list,
work_q,
ParallelGCThreads)) {
// found something in global overflow list;
// not yet ready to go stealing work from others.
// We'd like to assert(work_q->size() != 0, ...)
// because we just took work from the overflow list,
// but of course we can't since all of that could have
// been already stolen from us.
// "He giveth and He taketh away."
continue;
}
// Verify that we have no work before we resort to stealing
assert(work_q->size() == 0, "Have work, shouldn't steal");
// Try to steal from other queues that have work
if (task_queues()->steal(i, /* reference */ obj_to_scan)) {
NOT_PRODUCT(num_steals++;)
assert(oopDesc::is_oop(obj_to_scan), "Oops, not an oop!");
assert(bm->isMarked((HeapWord*)obj_to_scan), "Stole an unmarked oop?");
// Do scanning work
obj_to_scan->oop_iterate(cl);
// Loop around, finish this work, and try to steal some more
} else if (terminator()->offer_termination()) {
break; // nirvana from the infinite cycle
}
}
log_develop_trace(gc, task)("\t(%d: stole %d oops)", i, num_steals);
assert(work_q->size() == 0 && _collector->overflow_list_is_empty(),
"Else our work is not yet done");
}
// Record object boundaries in _eden_chunk_array by sampling the eden
// top in the slow-path eden object allocation code path and record
// the boundaries, if CMSEdenChunksRecordAlways is true. If
// CMSEdenChunksRecordAlways is false, we use the other asynchronous
// sampling in sample_eden() that activates during the part of the
// preclean phase.
void CMSCollector::sample_eden_chunk() {
if (CMSEdenChunksRecordAlways && _eden_chunk_array != NULL) {
if (_eden_chunk_lock->try_lock()) {
// Record a sample. This is the critical section. The contents
// of the _eden_chunk_array have to be non-decreasing in the
// address order.
_eden_chunk_array[_eden_chunk_index] = *_top_addr;
assert(_eden_chunk_array[_eden_chunk_index] <= *_end_addr,
"Unexpected state of Eden");
if (_eden_chunk_index == 0 ||
((_eden_chunk_array[_eden_chunk_index] > _eden_chunk_array[_eden_chunk_index-1]) &&
(pointer_delta(_eden_chunk_array[_eden_chunk_index],
_eden_chunk_array[_eden_chunk_index-1]) >= CMSSamplingGrain))) {
_eden_chunk_index++; // commit sample
}
_eden_chunk_lock->unlock();
}
}
}
// Return a thread-local PLAB recording array, as appropriate.
void* CMSCollector::get_data_recorder(int thr_num) {
if (_survivor_plab_array != NULL &&
(CMSPLABRecordAlways ||
(_collectorState > Marking && _collectorState < FinalMarking))) {
assert(thr_num < (int)ParallelGCThreads, "thr_num is out of bounds");
ChunkArray* ca = &_survivor_plab_array[thr_num];
ca->reset(); // clear it so that fresh data is recorded
return (void*) ca;
} else {
return NULL;
}
}
// Reset all the thread-local PLAB recording arrays
void CMSCollector::reset_survivor_plab_arrays() {
for (uint i = 0; i < ParallelGCThreads; i++) {
_survivor_plab_array[i].reset();
}
}
// Merge the per-thread plab arrays into the global survivor chunk
// array which will provide the partitioning of the survivor space
// for CMS initial scan and rescan.
void CMSCollector::merge_survivor_plab_arrays(ContiguousSpace* surv,
int no_of_gc_threads) {
assert(_survivor_plab_array != NULL, "Error");
assert(_survivor_chunk_array != NULL, "Error");
assert(_collectorState == FinalMarking ||
(CMSParallelInitialMarkEnabled && _collectorState == InitialMarking), "Error");
for (int j = 0; j < no_of_gc_threads; j++) {
_cursor[j] = 0;
}
HeapWord* top = surv->top();
size_t i;
for (i = 0; i < _survivor_chunk_capacity; i++) { // all sca entries
HeapWord* min_val = top; // Higher than any PLAB address
uint min_tid = 0; // position of min_val this round
for (int j = 0; j < no_of_gc_threads; j++) {
ChunkArray* cur_sca = &_survivor_plab_array[j];
if (_cursor[j] == cur_sca->end()) {
continue;
}
assert(_cursor[j] < cur_sca->end(), "ctl pt invariant");
HeapWord* cur_val = cur_sca->nth(_cursor[j]);
assert(surv->used_region().contains(cur_val), "Out of bounds value");
if (cur_val < min_val) {
min_tid = j;
min_val = cur_val;
} else {
assert(cur_val < top, "All recorded addresses should be less");
}
}
// At this point min_val and min_tid are respectively
// the least address in _survivor_plab_array[j]->nth(_cursor[j])
// and the thread (j) that witnesses that address.
// We record this address in the _survivor_chunk_array[i]
// and increment _cursor[min_tid] prior to the next round i.
if (min_val == top) {
break;
}
_survivor_chunk_array[i] = min_val;
_cursor[min_tid]++;
}
// We are all done; record the size of the _survivor_chunk_array
_survivor_chunk_index = i; // exclusive: [0, i)
log_trace(gc, survivor)(" (Survivor:" SIZE_FORMAT "chunks) ", i);
// Verify that we used up all the recorded entries
#ifdef ASSERT
size_t total = 0;
for (int j = 0; j < no_of_gc_threads; j++) {
assert(_cursor[j] == _survivor_plab_array[j].end(), "Ctl pt invariant");
total += _cursor[j];
}
assert(total == _survivor_chunk_index, "Ctl Pt Invariant");
// Check that the merged array is in sorted order
if (total > 0) {
for (size_t i = 0; i < total - 1; i++) {
log_develop_trace(gc, survivor)(" (chunk" SIZE_FORMAT ":" INTPTR_FORMAT ") ",
i, p2i(_survivor_chunk_array[i]));
assert(_survivor_chunk_array[i] < _survivor_chunk_array[i+1],
"Not sorted");
}
}
#endif // ASSERT
}
// Set up the space's par_seq_tasks structure for work claiming
// for parallel initial scan and rescan of young gen.
// See ParRescanTask where this is currently used.
void
CMSCollector::
initialize_sequential_subtasks_for_young_gen_rescan(int n_threads) {
assert(n_threads > 0, "Unexpected n_threads argument");
// Eden space
if (!_young_gen->eden()->is_empty()) {
SequentialSubTasksDone* pst = _young_gen->eden()->par_seq_tasks();
assert(!pst->valid(), "Clobbering existing data?");
// Each valid entry in [0, _eden_chunk_index) represents a task.
size_t n_tasks = _eden_chunk_index + 1;
assert(n_tasks == 1 || _eden_chunk_array != NULL, "Error");
// Sets the condition for completion of the subtask (how many threads
// need to finish in order to be done).
pst->set_n_threads(n_threads);
pst->set_n_tasks((int)n_tasks);
}
// Merge the survivor plab arrays into _survivor_chunk_array
if (_survivor_plab_array != NULL) {
merge_survivor_plab_arrays(_young_gen->from(), n_threads);
} else {
assert(_survivor_chunk_index == 0, "Error");
}
// To space
{
SequentialSubTasksDone* pst = _young_gen->to()->par_seq_tasks();
assert(!pst->valid(), "Clobbering existing data?");
// Sets the condition for completion of the subtask (how many threads
// need to finish in order to be done).
pst->set_n_threads(n_threads);
pst->set_n_tasks(1);
assert(pst->valid(), "Error");
}
// From space
{
SequentialSubTasksDone* pst = _young_gen->from()->par_seq_tasks();
assert(!pst->valid(), "Clobbering existing data?");
size_t n_tasks = _survivor_chunk_index + 1;
assert(n_tasks == 1 || _survivor_chunk_array != NULL, "Error");
// Sets the condition for completion of the subtask (how many threads
// need to finish in order to be done).
pst->set_n_threads(n_threads);
pst->set_n_tasks((int)n_tasks);
assert(pst->valid(), "Error");
}
}
// Parallel version of remark
void CMSCollector::do_remark_parallel() {
CMSHeap* heap = CMSHeap::heap();
WorkGang* workers = heap->workers();
assert(workers != NULL, "Need parallel worker threads.");
// Choose to use the number of GC workers most recently set
// into "active_workers".
uint n_workers = workers->active_workers();
CompactibleFreeListSpace* cms_space = _cmsGen->cmsSpace();
StrongRootsScope srs(n_workers);
CMSParRemarkTask tsk(this, cms_space, n_workers, workers, task_queues(), &srs);
// We won't be iterating over the cards in the card table updating
// the younger_gen cards, so we shouldn't call the following else
// the verification code as well as subsequent younger_refs_iterate
// code would get confused. XXX
// heap->rem_set()->prepare_for_younger_refs_iterate(true); // parallel
// The young gen rescan work will not be done as part of
// process_roots (which currently doesn't know how to
// parallelize such a scan), but rather will be broken up into
// a set of parallel tasks (via the sampling that the [abortable]
// preclean phase did of eden, plus the [two] tasks of
// scanning the [two] survivor spaces. Further fine-grain
// parallelization of the scanning of the survivor spaces
// themselves, and of precleaning of the young gen itself
// is deferred to the future.
initialize_sequential_subtasks_for_young_gen_rescan(n_workers);
// The dirty card rescan work is broken up into a "sequence"
// of parallel tasks (per constituent space) that are dynamically
// claimed by the parallel threads.
cms_space->initialize_sequential_subtasks_for_rescan(n_workers);
// It turns out that even when we're using 1 thread, doing the work in a
// separate thread causes wide variance in run times. We can't help this
// in the multi-threaded case, but we special-case n=1 here to get
// repeatable measurements of the 1-thread overhead of the parallel code.
if (n_workers > 1) {
// Make refs discovery MT-safe, if it isn't already: it may not
// necessarily be so, since it's possible that we are doing
// ST marking.
ReferenceProcessorMTDiscoveryMutator mt(ref_processor(), true);
workers->run_task(&tsk);
} else {
ReferenceProcessorMTDiscoveryMutator mt(ref_processor(), false);
tsk.work(0);
}
// restore, single-threaded for now, any preserved marks
// as a result of work_q overflow
restore_preserved_marks_if_any();
}
// Non-parallel version of remark
void CMSCollector::do_remark_non_parallel() {
ResourceMark rm;
HandleMark hm;
CMSHeap* heap = CMSHeap::heap();
ReferenceProcessorMTDiscoveryMutator mt(ref_processor(), false);
MarkRefsIntoAndScanClosure
mrias_cl(_span, ref_processor(), &_markBitMap, NULL /* not precleaning */,
&_markStack, this,
false /* should_yield */, false /* not precleaning */);
MarkFromDirtyCardsClosure
markFromDirtyCardsClosure(this, _span,
NULL, // space is set further below
&_markBitMap, &_markStack, &mrias_cl);
{
GCTraceTime(Trace, gc, phases) t("Grey Object Rescan", _gc_timer_cm);
// Iterate over the dirty cards, setting the corresponding bits in the
// mod union table.
{
ModUnionClosure modUnionClosure(&_modUnionTable);
_ct->dirty_card_iterate(_cmsGen->used_region(),
&modUnionClosure);
}
// Having transferred these marks into the modUnionTable, we just need
// to rescan the marked objects on the dirty cards in the modUnionTable.
// The initial marking may have been done during an asynchronous
// collection so there may be dirty bits in the mod-union table.
const int alignment = CardTable::card_size * BitsPerWord;
{
// ... First handle dirty cards in CMS gen
markFromDirtyCardsClosure.set_space(_cmsGen->cmsSpace());
MemRegion ur = _cmsGen->used_region();
HeapWord* lb = ur.start();
HeapWord* ub = align_up(ur.end(), alignment);
MemRegion cms_span(lb, ub);
_modUnionTable.dirty_range_iterate_clear(cms_span,
&markFromDirtyCardsClosure);
verify_work_stacks_empty();
log_trace(gc)(" (re-scanned " SIZE_FORMAT " dirty cards in cms gen) ", markFromDirtyCardsClosure.num_dirty_cards());
}
}
if (VerifyDuringGC &&
CMSHeap::heap()->total_collections() >= VerifyGCStartAt) {
HandleMark hm; // Discard invalid handles created during verification
Universe::verify();
}
{
GCTraceTime(Trace, gc, phases) t("Root Rescan", _gc_timer_cm);
verify_work_stacks_empty();
heap->rem_set()->prepare_for_younger_refs_iterate(false); // Not parallel.
StrongRootsScope srs(1);
heap->cms_process_roots(&srs,
true, // young gen as roots
GenCollectedHeap::ScanningOption(roots_scanning_options()),
should_unload_classes(),
&mrias_cl,
NULL); // The dirty klasses will be handled below
assert(should_unload_classes()
|| (roots_scanning_options() & GenCollectedHeap::SO_AllCodeCache),
"if we didn't scan the code cache, we have to be ready to drop nmethods with expired weak oops");
}
{
GCTraceTime(Trace, gc, phases) t("Visit Unhandled CLDs", _gc_timer_cm);
verify_work_stacks_empty();
// Scan all class loader data objects that might have been introduced
// during concurrent marking.
ResourceMark rm;
GrowableArray<ClassLoaderData*>* array = ClassLoaderDataGraph::new_clds();
for (int i = 0; i < array->length(); i++) {
Devirtualizer::do_cld(&mrias_cl, array->at(i));
}
// We don't need to keep track of new CLDs anymore.
ClassLoaderDataGraph::remember_new_clds(false);
verify_work_stacks_empty();
}
// We might have added oops to ClassLoaderData::_handles during the
// concurrent marking phase. These oops do not point to newly allocated objects
// that are guaranteed to be kept alive. Hence,
// we do have to revisit the _handles block during the remark phase.
{
GCTraceTime(Trace, gc, phases) t("Dirty CLD Scan", _gc_timer_cm);
verify_work_stacks_empty();
RemarkCLDClosure remark_closure(&mrias_cl);
ClassLoaderDataGraph::cld_do(&remark_closure);
verify_work_stacks_empty();
}
verify_work_stacks_empty();
// Restore evacuated mark words, if any, used for overflow list links
restore_preserved_marks_if_any();
verify_overflow_empty();
}
////////////////////////////////////////////////////////
// Parallel Reference Processing Task Proxy Class
////////////////////////////////////////////////////////
class AbstractGangTaskWOopQueues : public AbstractGangTask {
OopTaskQueueSet* _queues;
TaskTerminator _terminator;
public:
AbstractGangTaskWOopQueues(const char* name, OopTaskQueueSet* queues, uint n_threads) :
AbstractGangTask(name), _queues(queues), _terminator(n_threads, _queues) {}
ParallelTaskTerminator* terminator() { return _terminator.terminator(); }
OopTaskQueueSet* queues() { return _queues; }
};
class CMSRefProcTaskProxy: public AbstractGangTaskWOopQueues {
typedef AbstractRefProcTaskExecutor::ProcessTask ProcessTask;
CMSCollector* _collector;
CMSBitMap* _mark_bit_map;
const MemRegion _span;
ProcessTask& _task;
public:
CMSRefProcTaskProxy(ProcessTask& task,
CMSCollector* collector,
const MemRegion& span,
CMSBitMap* mark_bit_map,
AbstractWorkGang* workers,
OopTaskQueueSet* task_queues):
AbstractGangTaskWOopQueues("Process referents by policy in parallel",
task_queues,
workers->active_workers()),
_collector(collector),
_mark_bit_map(mark_bit_map),
_span(span),
_task(task)
{
assert(_collector->_span.equals(_span) && !_span.is_empty(),
"Inconsistency in _span");
}
OopTaskQueueSet* task_queues() { return queues(); }
OopTaskQueue* work_queue(int i) { return task_queues()->queue(i); }
void do_work_steal(int i,
CMSParDrainMarkingStackClosure* drain,
CMSParKeepAliveClosure* keep_alive);
virtual void work(uint worker_id);
};
void CMSRefProcTaskProxy::work(uint worker_id) {
ResourceMark rm;
HandleMark hm;
assert(_collector->_span.equals(_span), "Inconsistency in _span");
CMSParKeepAliveClosure par_keep_alive(_collector, _span,
_mark_bit_map,
work_queue(worker_id));
CMSParDrainMarkingStackClosure par_drain_stack(_collector, _span,
_mark_bit_map,
work_queue(worker_id));
CMSIsAliveClosure is_alive_closure(_span, _mark_bit_map);
_task.work(worker_id, is_alive_closure, par_keep_alive, par_drain_stack);
if (_task.marks_oops_alive()) {
do_work_steal(worker_id, &par_drain_stack, &par_keep_alive);
}
assert(work_queue(worker_id)->size() == 0, "work_queue should be empty");
assert(_collector->_overflow_list == NULL, "non-empty _overflow_list");
}
CMSParKeepAliveClosure::CMSParKeepAliveClosure(CMSCollector* collector,
MemRegion span, CMSBitMap* bit_map, OopTaskQueue* work_queue):
_span(span),
_work_queue(work_queue),
_bit_map(bit_map),
_mark_and_push(collector, span, bit_map, work_queue),
_low_water_mark(MIN2((work_queue->max_elems()/4),
((uint)CMSWorkQueueDrainThreshold * ParallelGCThreads)))
{ }
// . see if we can share work_queues with ParNew? XXX
void CMSRefProcTaskProxy::do_work_steal(int i,
CMSParDrainMarkingStackClosure* drain,
CMSParKeepAliveClosure* keep_alive) {
OopTaskQueue* work_q = work_queue(i);
NOT_PRODUCT(int num_steals = 0;)
oop obj_to_scan;
while (true) {
// Completely finish any left over work from (an) earlier round(s)
drain->trim_queue(0);
size_t num_from_overflow_list = MIN2((size_t)(work_q->max_elems() - work_q->size())/4,
(size_t)ParGCDesiredObjsFromOverflowList);
// Now check if there's any work in the overflow list
// Passing ParallelGCThreads as the third parameter, no_of_gc_threads,
// only affects the number of attempts made to get work from the
// overflow list and does not affect the number of workers. Just
// pass ParallelGCThreads so this behavior is unchanged.
if (_collector->par_take_from_overflow_list(num_from_overflow_list,
work_q,
ParallelGCThreads)) {
// Found something in global overflow list;
// not yet ready to go stealing work from others.
// We'd like to assert(work_q->size() != 0, ...)
// because we just took work from the overflow list,
// but of course we can't, since all of that might have
// been already stolen from us.
continue;
}
// Verify that we have no work before we resort to stealing
assert(work_q->size() == 0, "Have work, shouldn't steal");
// Try to steal from other queues that have work
if (task_queues()->steal(i, /* reference */ obj_to_scan)) {
NOT_PRODUCT(num_steals++;)
assert(oopDesc::is_oop(obj_to_scan), "Oops, not an oop!");
assert(_mark_bit_map->isMarked((HeapWord*)obj_to_scan), "Stole an unmarked oop?");
// Do scanning work
obj_to_scan->oop_iterate(keep_alive);
// Loop around, finish this work, and try to steal some more
} else if (terminator()->offer_termination()) {
break; // nirvana from the infinite cycle
}
}
log_develop_trace(gc, task)("\t(%d: stole %d oops)", i, num_steals);
}
void CMSRefProcTaskExecutor::execute(ProcessTask& task, uint ergo_workers) {
CMSHeap* heap = CMSHeap::heap();
WorkGang* workers = heap->workers();
assert(workers != NULL, "Need parallel worker threads.");
assert(workers->active_workers() == ergo_workers,
"Ergonomically chosen workers (%u) must be equal to active workers (%u)",
ergo_workers, workers->active_workers());
CMSRefProcTaskProxy rp_task(task, &_collector,
_collector.ref_processor_span(),
_collector.markBitMap(),
workers, _collector.task_queues());
workers->run_task(&rp_task, workers->active_workers());
}
void CMSCollector::refProcessingWork() {
ResourceMark rm;
HandleMark hm;
ReferenceProcessor* rp = ref_processor();
assert(_span_based_discoverer.span().equals(_span), "Spans should be equal");
assert(!rp->enqueuing_is_done(), "Enqueuing should not be complete");
// Process weak references.
rp->setup_policy(false);
verify_work_stacks_empty();
ReferenceProcessorPhaseTimes pt(_gc_timer_cm, rp->max_num_queues());
{
GCTraceTime(Debug, gc, phases) t("Reference Processing", _gc_timer_cm);
// Setup keep_alive and complete closures.
CMSKeepAliveClosure cmsKeepAliveClosure(this, _span, &_markBitMap,
&_markStack, false /* !preclean */);
CMSDrainMarkingStackClosure cmsDrainMarkingStackClosure(this,
_span, &_markBitMap, &_markStack,
&cmsKeepAliveClosure, false /* !preclean */);
ReferenceProcessorStats stats;
if (rp->processing_is_mt()) {
// Set the degree of MT here. If the discovery is done MT, there
// may have been a different number of threads doing the discovery
// and a different number of discovered lists may have Ref objects.
// That is OK as long as the Reference lists are balanced (see
// balance_all_queues() and balance_queues()).
CMSHeap* heap = CMSHeap::heap();
uint active_workers = ParallelGCThreads;
WorkGang* workers = heap->workers();
if (workers != NULL) {
active_workers = workers->active_workers();
// The expectation is that active_workers will have already
// been set to a reasonable value. If it has not been set,
// investigate.
assert(active_workers > 0, "Should have been set during scavenge");
}
rp->set_active_mt_degree(active_workers);
CMSRefProcTaskExecutor task_executor(*this);
stats = rp->process_discovered_references(&_is_alive_closure,
&cmsKeepAliveClosure,
&cmsDrainMarkingStackClosure,
&task_executor,
&pt);
} else {
stats = rp->process_discovered_references(&_is_alive_closure,
&cmsKeepAliveClosure,
&cmsDrainMarkingStackClosure,
NULL,
&pt);
}
_gc_tracer_cm->report_gc_reference_stats(stats);
pt.print_all_references();
}
// This is the point where the entire marking should have completed.
verify_work_stacks_empty();
{
GCTraceTime(Debug, gc, phases) t("Weak Processing", _gc_timer_cm);
WeakProcessor::weak_oops_do(&_is_alive_closure, &do_nothing_cl);
}
if (should_unload_classes()) {
{
GCTraceTime(Debug, gc, phases) t("Class Unloading", _gc_timer_cm);
// Unload classes and purge the SystemDictionary.
bool purged_class = SystemDictionary::do_unloading(_gc_timer_cm);
// Unload nmethods.
CodeCache::do_unloading(&_is_alive_closure, purged_class);
// Prune dead klasses from subklass/sibling/implementor lists.
Klass::clean_weak_klass_links(purged_class);
// Clean JVMCI metadata handles.
JVMCI_ONLY(JVMCI::do_unloading(purged_class));
}
}
// Restore any preserved marks as a result of mark stack or
// work queue overflow
restore_preserved_marks_if_any(); // done single-threaded for now
rp->set_enqueuing_is_done(true);
rp->verify_no_references_recorded();
}
#ifndef PRODUCT
void CMSCollector::check_correct_thread_executing() {
Thread* t = Thread::current();
// Only the VM thread or the CMS thread should be here.
assert(t->is_ConcurrentGC_thread() || t->is_VM_thread(),
"Unexpected thread type");
// If this is the vm thread, the foreground process
// should not be waiting. Note that _foregroundGCIsActive is
// true while the foreground collector is waiting.
if (_foregroundGCShouldWait) {
// We cannot be the VM thread
assert(t->is_ConcurrentGC_thread(),
"Should be CMS thread");
} else {
// We can be the CMS thread only if we are in a stop-world
// phase of CMS collection.
if (t->is_ConcurrentGC_thread()) {
assert(_collectorState == InitialMarking ||
_collectorState == FinalMarking,
"Should be a stop-world phase");
// The CMS thread should be holding the CMS_token.
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"Potential interference with concurrently "
"executing VM thread");
}
}
}
#endif
void CMSCollector::sweep() {
assert(_collectorState == Sweeping, "just checking");
check_correct_thread_executing();
verify_work_stacks_empty();
verify_overflow_empty();
increment_sweep_count();
TraceCMSMemoryManagerStats tms(_collectorState, CMSHeap::heap()->gc_cause());
_inter_sweep_timer.stop();
_inter_sweep_estimate.sample(_inter_sweep_timer.seconds());
assert(!_intra_sweep_timer.is_active(), "Should not be active");
_intra_sweep_timer.reset();
_intra_sweep_timer.start();
{
GCTraceCPUTime tcpu;
CMSPhaseAccounting pa(this, "Concurrent Sweep");
// First sweep the old gen
{
CMSTokenSyncWithLocks ts(true, _cmsGen->freelistLock(),
bitMapLock());
sweepWork(_cmsGen);
}
// Update Universe::_heap_*_at_gc figures.
// We need all the free list locks to make the abstract state
// transition from Sweeping to Resetting. See detailed note
// further below.
{
CMSTokenSyncWithLocks ts(true, _cmsGen->freelistLock());
// Update heap occupancy information which is used as
// input to soft ref clearing policy at the next gc.
Universe::update_heap_info_at_gc();
// recalculate CMS used space after CMS collection
_cmsGen->cmsSpace()->recalculate_used_stable();
_collectorState = Resizing;
}
}
verify_work_stacks_empty();
verify_overflow_empty();
if (should_unload_classes()) {
// Delay purge to the beginning of the next safepoint. Metaspace::contains
// requires that the virtual spaces are stable and not deleted.
ClassLoaderDataGraph::set_should_purge(true);
}
_intra_sweep_timer.stop();
_intra_sweep_estimate.sample(_intra_sweep_timer.seconds());
_inter_sweep_timer.reset();
_inter_sweep_timer.start();
// We need to use a monotonically non-decreasing time in ms
// or we will see time-warp warnings and os::javaTimeMillis()
// does not guarantee monotonicity.
jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
update_time_of_last_gc(now);
// NOTE on abstract state transitions:
// Mutators allocate-live and/or mark the mod-union table dirty
// based on the state of the collection. The former is done in
// the interval [Marking, Sweeping] and the latter in the interval
// [Marking, Sweeping). Thus the transitions into the Marking state
// and out of the Sweeping state must be synchronously visible
// globally to the mutators.
// The transition into the Marking state happens with the world
// stopped so the mutators will globally see it. Sweeping is
// done asynchronously by the background collector so the transition
// from the Sweeping state to the Resizing state must be done
// under the freelistLock (as is the check for whether to
// allocate-live and whether to dirty the mod-union table).
assert(_collectorState == Resizing, "Change of collector state to"
" Resizing must be done under the freelistLocks (plural)");
// Now that sweeping has been completed, we clear
// the incremental_collection_failed flag,
// thus inviting a younger gen collection to promote into
// this generation. If such a promotion may still fail,
// the flag will be set again when a young collection is
// attempted.
CMSHeap* heap = CMSHeap::heap();
heap->clear_incremental_collection_failed(); // Worth retrying as fresh space may have been freed up
heap->update_full_collections_completed(_collection_count_start);
}
// FIX ME!!! Looks like this belongs in CFLSpace, with
// CMSGen merely delegating to it.
void ConcurrentMarkSweepGeneration::setNearLargestChunk() {
double nearLargestPercent = FLSLargestBlockCoalesceProximity;
HeapWord* minAddr = _cmsSpace->bottom();
HeapWord* largestAddr =
(HeapWord*) _cmsSpace->dictionary()->find_largest_dict();
if (largestAddr == NULL) {
// The dictionary appears to be empty. In this case
// try to coalesce at the end of the heap.
largestAddr = _cmsSpace->end();
}
size_t largestOffset = pointer_delta(largestAddr, minAddr);
size_t nearLargestOffset =
(size_t)((double)largestOffset * nearLargestPercent) - MinChunkSize;
log_debug(gc, freelist)("CMS: Large Block: " PTR_FORMAT "; Proximity: " PTR_FORMAT " -> " PTR_FORMAT,
p2i(largestAddr), p2i(_cmsSpace->nearLargestChunk()), p2i(minAddr + nearLargestOffset));
_cmsSpace->set_nearLargestChunk(minAddr + nearLargestOffset);
}
bool ConcurrentMarkSweepGeneration::isNearLargestChunk(HeapWord* addr) {
return addr >= _cmsSpace->nearLargestChunk();
}
FreeChunk* ConcurrentMarkSweepGeneration::find_chunk_at_end() {
return _cmsSpace->find_chunk_at_end();
}
void ConcurrentMarkSweepGeneration::update_gc_stats(Generation* current_generation,
bool full) {
// If the young generation has been collected, gather any statistics
// that are of interest at this point.
bool current_is_young = CMSHeap::heap()->is_young_gen(current_generation);
if (!full && current_is_young) {
// Gather statistics on the young generation collection.
collector()->stats().record_gc0_end(used());
}
_cmsSpace->recalculate_used_stable();
}
void CMSCollector::sweepWork(ConcurrentMarkSweepGeneration* old_gen) {
// We iterate over the space(s) underlying this generation,
// checking the mark bit map to see if the bits corresponding
// to specific blocks are marked or not. Blocks that are
// marked are live and are not swept up. All remaining blocks
// are swept up, with coalescing on-the-fly as we sweep up
// contiguous free and/or garbage blocks:
// We need to ensure that the sweeper synchronizes with allocators
// and stop-the-world collectors. In particular, the following
// locks are used:
// . CMS token: if this is held, a stop the world collection cannot occur
// . freelistLock: if this is held no allocation can occur from this
// generation by another thread
// . bitMapLock: if this is held, no other thread can access or update
//
// Note that we need to hold the freelistLock if we use
// block iterate below; else the iterator might go awry if
// a mutator (or promotion) causes block contents to change
// (for instance if the allocator divvies up a block).
// If we hold the free list lock, for all practical purposes
// young generation GC's can't occur (they'll usually need to
// promote), so we might as well prevent all young generation
// GC's while we do a sweeping step. For the same reason, we might
// as well take the bit map lock for the entire duration
// check that we hold the requisite locks
assert(have_cms_token(), "Should hold cms token");
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(), "Should possess CMS token to sweep");
assert_lock_strong(old_gen->freelistLock());
assert_lock_strong(bitMapLock());
assert(!_inter_sweep_timer.is_active(), "Was switched off in an outer context");
assert(_intra_sweep_timer.is_active(), "Was switched on in an outer context");
old_gen->cmsSpace()->beginSweepFLCensus((float)(_inter_sweep_timer.seconds()),
_inter_sweep_estimate.padded_average(),
_intra_sweep_estimate.padded_average());
old_gen->setNearLargestChunk();
{
SweepClosure sweepClosure(this, old_gen, &_markBitMap, CMSYield);
old_gen->cmsSpace()->blk_iterate_careful(&sweepClosure);
// We need to free-up/coalesce garbage/blocks from a
// co-terminal free run. This is done in the SweepClosure
// destructor; so, do not remove this scope, else the
// end-of-sweep-census below will be off by a little bit.
}
old_gen->cmsSpace()->sweep_completed();
old_gen->cmsSpace()->endSweepFLCensus(sweep_count());
if (should_unload_classes()) { // unloaded classes this cycle,
_concurrent_cycles_since_last_unload = 0; // ... reset count
} else { // did not unload classes,
_concurrent_cycles_since_last_unload++; // ... increment count
}
}
// Reset CMS data structures (for now just the marking bit map)
// preparatory for the next cycle.
void CMSCollector::reset_concurrent() {
CMSTokenSyncWithLocks ts(true, bitMapLock());
// If the state is not "Resetting", the foreground thread
// has done a collection and the resetting.
if (_collectorState != Resetting) {
assert(_collectorState == Idling, "The state should only change"
" because the foreground collector has finished the collection");
return;
}
{
// Clear the mark bitmap (no grey objects to start with)
// for the next cycle.
GCTraceCPUTime tcpu;
CMSPhaseAccounting cmspa(this, "Concurrent Reset");
HeapWord* curAddr = _markBitMap.startWord();
while (curAddr < _markBitMap.endWord()) {
size_t remaining = pointer_delta(_markBitMap.endWord(), curAddr);
MemRegion chunk(curAddr, MIN2(CMSBitMapYieldQuantum, remaining));
_markBitMap.clear_large_range(chunk);
if (ConcurrentMarkSweepThread::should_yield() &&
!foregroundGCIsActive() &&
CMSYield) {
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
assert_lock_strong(bitMapLock());
bitMapLock()->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
stopTimer();
incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0; i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
bitMapLock()->lock_without_safepoint_check();
startTimer();
}
curAddr = chunk.end();
}
// A successful mostly concurrent collection has been done.
// Because only the full (i.e., concurrent mode failure) collections
// are being measured for gc overhead limits, clean the "near" flag
// and count.
size_policy()->reset_gc_overhead_limit_count();
_collectorState = Idling;
}
register_gc_end();
}
// Same as above but for STW paths
void CMSCollector::reset_stw() {
// already have the lock
assert(_collectorState == Resetting, "just checking");
assert_lock_strong(bitMapLock());
GCIdMark gc_id_mark(_cmsThread->gc_id());
_markBitMap.clear_all();
_collectorState = Idling;
register_gc_end();
}
void CMSCollector::do_CMS_operation(CMS_op_type op, GCCause::Cause gc_cause) {
GCTraceCPUTime tcpu;
TraceCollectorStats tcs_cgc(cgc_counters());
switch (op) {
case CMS_op_checkpointRootsInitial: {
GCTraceTime(Info, gc) t("Pause Initial Mark", NULL, GCCause::_no_gc, true);
SvcGCMarker sgcm(SvcGCMarker::CONCURRENT);
checkpointRootsInitial();
break;
}
case CMS_op_checkpointRootsFinal: {
GCTraceTime(Info, gc) t("Pause Remark", NULL, GCCause::_no_gc, true);
SvcGCMarker sgcm(SvcGCMarker::CONCURRENT);
checkpointRootsFinal();
break;
}
default:
fatal("No such CMS_op");
}
}
#ifndef PRODUCT
size_t const CMSCollector::skip_header_HeapWords() {
return FreeChunk::header_size();
}
// Try and collect here conditions that should hold when
// CMS thread is exiting. The idea is that the foreground GC
// thread should not be blocked if it wants to terminate
// the CMS thread and yet continue to run the VM for a while
// after that.
void CMSCollector::verify_ok_to_terminate() const {
assert(Thread::current()->is_ConcurrentGC_thread(),
"should be called by CMS thread");
assert(!_foregroundGCShouldWait, "should be false");
// We could check here that all the various low-level locks
// are not held by the CMS thread, but that is overkill; see
// also CMSThread::verify_ok_to_terminate() where the CGC_lock
// is checked.
}
#endif
size_t CMSCollector::block_size_using_printezis_bits(HeapWord* addr) const {
assert(_markBitMap.isMarked(addr) && _markBitMap.isMarked(addr + 1),
"missing Printezis mark?");
HeapWord* nextOneAddr = _markBitMap.getNextMarkedWordAddress(addr + 2);
size_t size = pointer_delta(nextOneAddr + 1, addr);
assert(size == CompactibleFreeListSpace::adjustObjectSize(size),
"alignment problem");
assert(size >= 3, "Necessary for Printezis marks to work");
return size;
}
// A variant of the above (block_size_using_printezis_bits()) except
// that we return 0 if the P-bits are not yet set.
size_t CMSCollector::block_size_if_printezis_bits(HeapWord* addr) const {
if (_markBitMap.isMarked(addr + 1)) {
assert(_markBitMap.isMarked(addr), "P-bit can be set only for marked objects");
HeapWord* nextOneAddr = _markBitMap.getNextMarkedWordAddress(addr + 2);
size_t size = pointer_delta(nextOneAddr + 1, addr);
assert(size == CompactibleFreeListSpace::adjustObjectSize(size),
"alignment problem");
assert(size >= 3, "Necessary for Printezis marks to work");
return size;
}
return 0;
}
HeapWord* CMSCollector::next_card_start_after_block(HeapWord* addr) const {
size_t sz = 0;
oop p = (oop)addr;
if (p->klass_or_null_acquire() != NULL) {
sz = CompactibleFreeListSpace::adjustObjectSize(p->size());
} else {
sz = block_size_using_printezis_bits(addr);
}
assert(sz > 0, "size must be nonzero");
HeapWord* next_block = addr + sz;
HeapWord* next_card = align_up(next_block, CardTable::card_size);
assert(align_down((uintptr_t)addr, CardTable::card_size) <
align_down((uintptr_t)next_card, CardTable::card_size),
"must be different cards");
return next_card;
}
// CMS Bit Map Wrapper /////////////////////////////////////////
// Construct a CMS bit map infrastructure, but don't create the
// bit vector itself. That is done by a separate call CMSBitMap::allocate()
// further below.
CMSBitMap::CMSBitMap(int shifter, int mutex_rank, const char* mutex_name):
_shifter(shifter),
_bm(),
_lock(mutex_rank >= 0 ? new Mutex(mutex_rank, mutex_name, true,
Monitor::_safepoint_check_never) : NULL)
{
_bmStartWord = 0;
_bmWordSize = 0;
}
bool CMSBitMap::allocate(MemRegion mr) {
_bmStartWord = mr.start();
_bmWordSize = mr.word_size();
ReservedSpace brs(ReservedSpace::allocation_align_size_up(
(_bmWordSize >> (_shifter + LogBitsPerByte)) + 1));
if (!brs.is_reserved()) {
log_warning(gc)("CMS bit map allocation failure");
return false;
}
// For now we'll just commit all of the bit map up front.
// Later on we'll try to be more parsimonious with swap.
if (!_virtual_space.initialize(brs, brs.size())) {
log_warning(gc)("CMS bit map backing store failure");
return false;
}
assert(_virtual_space.committed_size() == brs.size(),
"didn't reserve backing store for all of CMS bit map?");
assert(_virtual_space.committed_size() << (_shifter + LogBitsPerByte) >=
_bmWordSize, "inconsistency in bit map sizing");
_bm = BitMapView((BitMap::bm_word_t*)_virtual_space.low(), _bmWordSize >> _shifter);
// bm.clear(); // can we rely on getting zero'd memory? verify below
assert(isAllClear(),
"Expected zero'd memory from ReservedSpace constructor");
assert(_bm.size() == heapWordDiffToOffsetDiff(sizeInWords()),
"consistency check");
return true;
}
void CMSBitMap::dirty_range_iterate_clear(MemRegion mr, MemRegionClosure* cl) {
HeapWord *next_addr, *end_addr, *last_addr;
assert_locked();
assert(covers(mr), "out-of-range error");
// XXX assert that start and end are appropriately aligned
for (next_addr = mr.start(), end_addr = mr.end();
next_addr < end_addr; next_addr = last_addr) {
MemRegion dirty_region = getAndClearMarkedRegion(next_addr, end_addr);
last_addr = dirty_region.end();
if (!dirty_region.is_empty()) {
cl->do_MemRegion(dirty_region);
} else {
assert(last_addr == end_addr, "program logic");
return;
}
}
}
void CMSBitMap::print_on_error(outputStream* st, const char* prefix) const {
_bm.print_on_error(st, prefix);
}
#ifndef PRODUCT
void CMSBitMap::assert_locked() const {
CMSLockVerifier::assert_locked(lock());
}
bool CMSBitMap::covers(MemRegion mr) const {
// assert(_bm.map() == _virtual_space.low(), "map inconsistency");
assert((size_t)_bm.size() == (_bmWordSize >> _shifter),
"size inconsistency");
return (mr.start() >= _bmStartWord) &&
(mr.end() <= endWord());
}
bool CMSBitMap::covers(HeapWord* start, size_t size) const {
return (start >= _bmStartWord && (start + size) <= endWord());
}
void CMSBitMap::verifyNoOneBitsInRange(HeapWord* left, HeapWord* right) {
// verify that there are no 1 bits in the interval [left, right)
FalseBitMapClosure falseBitMapClosure;
iterate(&falseBitMapClosure, left, right);
}
void CMSBitMap::region_invariant(MemRegion mr)
{
assert_locked();
// mr = mr.intersection(MemRegion(_bmStartWord, _bmWordSize));
assert(!mr.is_empty(), "unexpected empty region");
assert(covers(mr), "mr should be covered by bit map");
// convert address range into offset range
size_t start_ofs = heapWordToOffset(mr.start());
// Make sure that end() is appropriately aligned
assert(mr.end() == align_up(mr.end(), (1 << (_shifter+LogHeapWordSize))),
"Misaligned mr.end()");
size_t end_ofs = heapWordToOffset(mr.end());
assert(end_ofs > start_ofs, "Should mark at least one bit");
}
#endif
bool CMSMarkStack::allocate(size_t size) {
// allocate a stack of the requisite depth
ReservedSpace rs(ReservedSpace::allocation_align_size_up(
size * sizeof(oop)));
if (!rs.is_reserved()) {
log_warning(gc)("CMSMarkStack allocation failure");
return false;
}
if (!_virtual_space.initialize(rs, rs.size())) {
log_warning(gc)("CMSMarkStack backing store failure");
return false;
}
assert(_virtual_space.committed_size() == rs.size(),
"didn't reserve backing store for all of CMS stack?");
_base = (oop*)(_virtual_space.low());
_index = 0;
_capacity = size;
NOT_PRODUCT(_max_depth = 0);
return true;
}
// XXX FIX ME !!! In the MT case we come in here holding a
// leaf lock. For printing we need to take a further lock
// which has lower rank. We need to recalibrate the two
// lock-ranks involved in order to be able to print the
// messages below. (Or defer the printing to the caller.
// For now we take the expedient path of just disabling the
// messages for the problematic case.)
void CMSMarkStack::expand() {
assert(_capacity <= MarkStackSizeMax, "stack bigger than permitted");
if (_capacity == MarkStackSizeMax) {
if (_hit_limit++ == 0 && !CMSConcurrentMTEnabled) {
// We print a warning message only once per CMS cycle.
log_debug(gc)(" (benign) Hit CMSMarkStack max size limit");
}
return;
}
// Double capacity if possible
size_t new_capacity = MIN2(_capacity*2, MarkStackSizeMax);
// Do not give up existing stack until we have managed to
// get the double capacity that we desired.
ReservedSpace rs(ReservedSpace::allocation_align_size_up(
new_capacity * sizeof(oop)));
if (rs.is_reserved()) {
// Release the backing store associated with old stack
_virtual_space.release();
// Reinitialize virtual space for new stack
if (!_virtual_space.initialize(rs, rs.size())) {
fatal("Not enough swap for expanded marking stack");
}
_base = (oop*)(_virtual_space.low());
_index = 0;
_capacity = new_capacity;
} else if (_failed_double++ == 0 && !CMSConcurrentMTEnabled) {
// Failed to double capacity, continue;
// we print a detail message only once per CMS cycle.
log_debug(gc)(" (benign) Failed to expand marking stack from " SIZE_FORMAT "K to " SIZE_FORMAT "K",
_capacity / K, new_capacity / K);
}
}
// Closures
// XXX: there seems to be a lot of code duplication here;
// should refactor and consolidate common code.
// This closure is used to mark refs into the CMS generation in
// the CMS bit map. Called at the first checkpoint. This closure
// assumes that we do not need to re-mark dirty cards; if the CMS
// generation on which this is used is not an oldest
// generation then this will lose younger_gen cards!
MarkRefsIntoClosure::MarkRefsIntoClosure(
MemRegion span, CMSBitMap* bitMap):
_span(span),
_bitMap(bitMap)
{
assert(ref_discoverer() == NULL, "deliberately left NULL");
assert(_bitMap->covers(_span), "_bitMap/_span mismatch");
}
void MarkRefsIntoClosure::do_oop(oop obj) {
// if p points into _span, then mark corresponding bit in _markBitMap
assert(oopDesc::is_oop(obj), "expected an oop");
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr)) {
// this should be made more efficient
_bitMap->mark(addr);
}
}
ParMarkRefsIntoClosure::ParMarkRefsIntoClosure(
MemRegion span, CMSBitMap* bitMap):
_span(span),
_bitMap(bitMap)
{
assert(ref_discoverer() == NULL, "deliberately left NULL");
assert(_bitMap->covers(_span), "_bitMap/_span mismatch");
}
void ParMarkRefsIntoClosure::do_oop(oop obj) {
// if p points into _span, then mark corresponding bit in _markBitMap
assert(oopDesc::is_oop(obj), "expected an oop");
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr)) {
// this should be made more efficient
_bitMap->par_mark(addr);
}
}
// A variant of the above, used for CMS marking verification.
MarkRefsIntoVerifyClosure::MarkRefsIntoVerifyClosure(
MemRegion span, CMSBitMap* verification_bm, CMSBitMap* cms_bm):
_span(span),
_verification_bm(verification_bm),
_cms_bm(cms_bm)
{
assert(ref_discoverer() == NULL, "deliberately left NULL");
assert(_verification_bm->covers(_span), "_verification_bm/_span mismatch");
}
void MarkRefsIntoVerifyClosure::do_oop(oop obj) {
// if p points into _span, then mark corresponding bit in _markBitMap
assert(oopDesc::is_oop(obj), "expected an oop");
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr)) {
_verification_bm->mark(addr);
if (!_cms_bm->isMarked(addr)) {
Log(gc, verify) log;
ResourceMark rm;
LogStream ls(log.error());
oop(addr)->print_on(&ls);
log.error(" (" INTPTR_FORMAT " should have been marked)", p2i(addr));
fatal("... aborting");
}
}
}
//////////////////////////////////////////////////
// MarkRefsIntoAndScanClosure
//////////////////////////////////////////////////
MarkRefsIntoAndScanClosure::MarkRefsIntoAndScanClosure(MemRegion span,
ReferenceDiscoverer* rd,
CMSBitMap* bit_map,
CMSBitMap* mod_union_table,
CMSMarkStack* mark_stack,
CMSCollector* collector,
bool should_yield,
bool concurrent_precleaning):
_span(span),
_bit_map(bit_map),
_mark_stack(mark_stack),
_pushAndMarkClosure(collector, span, rd, bit_map, mod_union_table,
mark_stack, concurrent_precleaning),
_collector(collector),
_freelistLock(NULL),
_yield(should_yield),
_concurrent_precleaning(concurrent_precleaning)
{
// FIXME: Should initialize in base class constructor.
assert(rd != NULL, "ref_discoverer shouldn't be NULL");
set_ref_discoverer_internal(rd);
}
// This closure is used to mark refs into the CMS generation at the
// second (final) checkpoint, and to scan and transitively follow
// the unmarked oops. It is also used during the concurrent precleaning
// phase while scanning objects on dirty cards in the CMS generation.
// The marks are made in the marking bit map and the marking stack is
// used for keeping the (newly) grey objects during the scan.
// The parallel version (Par_...) appears further below.
void MarkRefsIntoAndScanClosure::do_oop(oop obj) {
if (obj != NULL) {
assert(oopDesc::is_oop(obj), "expected an oop");
HeapWord* addr = (HeapWord*)obj;
assert(_mark_stack->isEmpty(), "pre-condition (eager drainage)");
assert(_collector->overflow_list_is_empty(),
"overflow list should be empty");
if (_span.contains(addr) &&
!_bit_map->isMarked(addr)) {
// mark bit map (object is now grey)
_bit_map->mark(addr);
// push on marking stack (stack should be empty), and drain the
// stack by applying this closure to the oops in the oops popped
// from the stack (i.e. blacken the grey objects)
bool res = _mark_stack->push(obj);
assert(res, "Should have space to push on empty stack");
do {
oop new_oop = _mark_stack->pop();
assert(new_oop != NULL && oopDesc::is_oop(new_oop), "Expected an oop");
assert(_bit_map->isMarked((HeapWord*)new_oop),
"only grey objects on this stack");
// iterate over the oops in this oop, marking and pushing
// the ones in CMS heap (i.e. in _span).
new_oop->oop_iterate(&_pushAndMarkClosure);
// check if it's time to yield
do_yield_check();
} while (!_mark_stack->isEmpty() ||
(!_concurrent_precleaning && take_from_overflow_list()));
// if marking stack is empty, and we are not doing this
// during precleaning, then check the overflow list
}
assert(_mark_stack->isEmpty(), "post-condition (eager drainage)");
assert(_collector->overflow_list_is_empty(),
"overflow list was drained above");
assert(_collector->no_preserved_marks(),
"All preserved marks should have been restored above");
}
}
void MarkRefsIntoAndScanClosure::do_yield_work() {
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
assert_lock_strong(_freelistLock);
assert_lock_strong(_bit_map->lock());
// relinquish the free_list_lock and bitMaplock()
_bit_map->lock()->unlock();
_freelistLock->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0;
i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive();
++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
_freelistLock->lock_without_safepoint_check();
_bit_map->lock()->lock_without_safepoint_check();
_collector->startTimer();
}
///////////////////////////////////////////////////////////
// ParMarkRefsIntoAndScanClosure: a parallel version of
// MarkRefsIntoAndScanClosure
///////////////////////////////////////////////////////////
ParMarkRefsIntoAndScanClosure::ParMarkRefsIntoAndScanClosure(
CMSCollector* collector, MemRegion span, ReferenceDiscoverer* rd,
CMSBitMap* bit_map, OopTaskQueue* work_queue):
_span(span),
_bit_map(bit_map),
_work_queue(work_queue),
_low_water_mark(MIN2((work_queue->max_elems()/4),
((uint)CMSWorkQueueDrainThreshold * ParallelGCThreads))),
_parPushAndMarkClosure(collector, span, rd, bit_map, work_queue)
{
// FIXME: Should initialize in base class constructor.
assert(rd != NULL, "ref_discoverer shouldn't be NULL");
set_ref_discoverer_internal(rd);
}
// This closure is used to mark refs into the CMS generation at the
// second (final) checkpoint, and to scan and transitively follow
// the unmarked oops. The marks are made in the marking bit map and
// the work_queue is used for keeping the (newly) grey objects during
// the scan phase whence they are also available for stealing by parallel
// threads. Since the marking bit map is shared, updates are
// synchronized (via CAS).
void ParMarkRefsIntoAndScanClosure::do_oop(oop obj) {
if (obj != NULL) {
// Ignore mark word because this could be an already marked oop
// that may be chained at the end of the overflow list.
assert(oopDesc::is_oop(obj, true), "expected an oop");
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr) &&
!_bit_map->isMarked(addr)) {
// mark bit map (object will become grey):
// It is possible for several threads to be
// trying to "claim" this object concurrently;
// the unique thread that succeeds in marking the
// object first will do the subsequent push on
// to the work queue (or overflow list).
if (_bit_map->par_mark(addr)) {
// push on work_queue (which may not be empty), and trim the
// queue to an appropriate length by applying this closure to
// the oops in the oops popped from the stack (i.e. blacken the
// grey objects)
bool res = _work_queue->push(obj);
assert(res, "Low water mark should be less than capacity?");
trim_queue(_low_water_mark);
} // Else, another thread claimed the object
}
}
}
// This closure is used to rescan the marked objects on the dirty cards
// in the mod union table and the card table proper.
size_t ScanMarkedObjectsAgainCarefullyClosure::do_object_careful_m(
oop p, MemRegion mr) {
size_t size = 0;
HeapWord* addr = (HeapWord*)p;
DEBUG_ONLY(_collector->verify_work_stacks_empty();)
assert(_span.contains(addr), "we are scanning the CMS generation");
// check if it's time to yield
if (do_yield_check()) {
// We yielded for some foreground stop-world work,
// and we have been asked to abort this ongoing preclean cycle.
return 0;
}
if (_bitMap->isMarked(addr)) {
// it's marked; is it potentially uninitialized?
if (p->klass_or_null_acquire() != NULL) {
// an initialized object; ignore mark word in verification below
// since we are running concurrent with mutators
assert(oopDesc::is_oop(p, true), "should be an oop");
if (p->is_objArray()) {
// objArrays are precisely marked; restrict scanning
// to dirty cards only.
size = CompactibleFreeListSpace::adjustObjectSize(
p->oop_iterate_size(_scanningClosure, mr));
} else {
// A non-array may have been imprecisely marked; we need
// to scan object in its entirety.
size = CompactibleFreeListSpace::adjustObjectSize(
p->oop_iterate_size(_scanningClosure));
}
#ifdef ASSERT
size_t direct_size =
CompactibleFreeListSpace::adjustObjectSize(p->size());
assert(size == direct_size, "Inconsistency in size");
assert(size >= 3, "Necessary for Printezis marks to work");
HeapWord* start_pbit = addr + 1;
HeapWord* end_pbit = addr + size - 1;
assert(_bitMap->isMarked(start_pbit) == _bitMap->isMarked(end_pbit),
"inconsistent Printezis mark");
// Verify inner mark bits (between Printezis bits) are clear,
// but don't repeat if there are multiple dirty regions for
// the same object, to avoid potential O(N^2) performance.
if (addr != _last_scanned_object) {
_bitMap->verifyNoOneBitsInRange(start_pbit + 1, end_pbit);
_last_scanned_object = addr;
}
#endif // ASSERT
} else {
// An uninitialized object.
assert(_bitMap->isMarked(addr+1), "missing Printezis mark?");
HeapWord* nextOneAddr = _bitMap->getNextMarkedWordAddress(addr + 2);
size = pointer_delta(nextOneAddr + 1, addr);
assert(size == CompactibleFreeListSpace::adjustObjectSize(size),
"alignment problem");
// Note that pre-cleaning needn't redirty the card. OopDesc::set_klass()
// will dirty the card when the klass pointer is installed in the
// object (signaling the completion of initialization).
}
} else {
// Either a not yet marked object or an uninitialized object
if (p->klass_or_null_acquire() == NULL) {
// An uninitialized object, skip to the next card, since
// we may not be able to read its P-bits yet.
assert(size == 0, "Initial value");
} else {
// An object not (yet) reached by marking: we merely need to
// compute its size so as to go look at the next block.
assert(oopDesc::is_oop(p, true), "should be an oop");
size = CompactibleFreeListSpace::adjustObjectSize(p->size());
}
}
DEBUG_ONLY(_collector->verify_work_stacks_empty();)
return size;
}
void ScanMarkedObjectsAgainCarefullyClosure::do_yield_work() {
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
assert_lock_strong(_freelistLock);
assert_lock_strong(_bitMap->lock());
// relinquish the free_list_lock and bitMaplock()
_bitMap->lock()->unlock();
_freelistLock->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0; i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
_freelistLock->lock_without_safepoint_check();
_bitMap->lock()->lock_without_safepoint_check();
_collector->startTimer();
}
//////////////////////////////////////////////////////////////////
// SurvivorSpacePrecleanClosure
//////////////////////////////////////////////////////////////////
// This (single-threaded) closure is used to preclean the oops in
// the survivor spaces.
size_t SurvivorSpacePrecleanClosure::do_object_careful(oop p) {
HeapWord* addr = (HeapWord*)p;
DEBUG_ONLY(_collector->verify_work_stacks_empty();)
assert(!_span.contains(addr), "we are scanning the survivor spaces");
assert(p->klass_or_null() != NULL, "object should be initialized");
// an initialized object; ignore mark word in verification below
// since we are running concurrent with mutators
assert(oopDesc::is_oop(p, true), "should be an oop");
// Note that we do not yield while we iterate over
// the interior oops of p, pushing the relevant ones
// on our marking stack.
size_t size = p->oop_iterate_size(_scanning_closure);
do_yield_check();
// Observe that below, we do not abandon the preclean
// phase as soon as we should; rather we empty the
// marking stack before returning. This is to satisfy
// some existing assertions. In general, it may be a
// good idea to abort immediately and complete the marking
// from the grey objects at a later time.
while (!_mark_stack->isEmpty()) {
oop new_oop = _mark_stack->pop();
assert(new_oop != NULL && oopDesc::is_oop(new_oop), "Expected an oop");
assert(_bit_map->isMarked((HeapWord*)new_oop),
"only grey objects on this stack");
// iterate over the oops in this oop, marking and pushing
// the ones in CMS heap (i.e. in _span).
new_oop->oop_iterate(_scanning_closure);
// check if it's time to yield
do_yield_check();
}
unsigned int after_count =
CMSHeap::heap()->total_collections();
bool abort = (_before_count != after_count) ||
_collector->should_abort_preclean();
return abort ? 0 : size;
}
void SurvivorSpacePrecleanClosure::do_yield_work() {
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
assert_lock_strong(_bit_map->lock());
// Relinquish the bit map lock
_bit_map->lock()->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0; i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
_bit_map->lock()->lock_without_safepoint_check();
_collector->startTimer();
}
// This closure is used to rescan the marked objects on the dirty cards
// in the mod union table and the card table proper. In the parallel
// case, although the bitMap is shared, we do a single read so the
// isMarked() query is "safe".
bool ScanMarkedObjectsAgainClosure::do_object_bm(oop p, MemRegion mr) {
// Ignore mark word because we are running concurrent with mutators
assert(oopDesc::is_oop_or_null(p, true), "Expected an oop or NULL at " PTR_FORMAT, p2i(p));
HeapWord* addr = (HeapWord*)p;
assert(_span.contains(addr), "we are scanning the CMS generation");
bool is_obj_array = false;
#ifdef ASSERT
if (!_parallel) {
assert(_mark_stack->isEmpty(), "pre-condition (eager drainage)");
assert(_collector->overflow_list_is_empty(),
"overflow list should be empty");
}
#endif // ASSERT
if (_bit_map->isMarked(addr)) {
// Obj arrays are precisely marked, non-arrays are not;
// so we scan objArrays precisely and non-arrays in their
// entirety.
if (p->is_objArray()) {
is_obj_array = true;
if (_parallel) {
p->oop_iterate(_par_scan_closure, mr);
} else {
p->oop_iterate(_scan_closure, mr);
}
} else {
if (_parallel) {
p->oop_iterate(_par_scan_closure);
} else {
p->oop_iterate(_scan_closure);
}
}
}
#ifdef ASSERT
if (!_parallel) {
assert(_mark_stack->isEmpty(), "post-condition (eager drainage)");
assert(_collector->overflow_list_is_empty(),
"overflow list should be empty");
}
#endif // ASSERT
return is_obj_array;
}
MarkFromRootsClosure::MarkFromRootsClosure(CMSCollector* collector,
MemRegion span,
CMSBitMap* bitMap, CMSMarkStack* markStack,
bool should_yield, bool verifying):
_collector(collector),
_span(span),
_bitMap(bitMap),
_mut(&collector->_modUnionTable),
_markStack(markStack),
_yield(should_yield),
_skipBits(0)
{
assert(_markStack->isEmpty(), "stack should be empty");
_finger = _bitMap->startWord();
_threshold = _finger;
assert(_collector->_restart_addr == NULL, "Sanity check");
assert(_span.contains(_finger), "Out of bounds _finger?");
DEBUG_ONLY(_verifying = verifying;)
}
void MarkFromRootsClosure::reset(HeapWord* addr) {
assert(_markStack->isEmpty(), "would cause duplicates on stack");
assert(_span.contains(addr), "Out of bounds _finger?");
_finger = addr;
_threshold = align_up(_finger, CardTable::card_size);
}
// Should revisit to see if this should be restructured for
// greater efficiency.
bool MarkFromRootsClosure::do_bit(size_t offset) {
if (_skipBits > 0) {
_skipBits--;
return true;
}
// convert offset into a HeapWord*
HeapWord* addr = _bitMap->startWord() + offset;
assert(_bitMap->endWord() && addr < _bitMap->endWord(),
"address out of range");
assert(_bitMap->isMarked(addr), "tautology");
if (_bitMap->isMarked(addr+1)) {
// this is an allocated but not yet initialized object
assert(_skipBits == 0, "tautology");
_skipBits = 2; // skip next two marked bits ("Printezis-marks")
oop p = oop(addr);
if (p->klass_or_null_acquire() == NULL) {
DEBUG_ONLY(if (!_verifying) {)
// We re-dirty the cards on which this object lies and increase
// the _threshold so that we'll come back to scan this object
// during the preclean or remark phase. (CMSCleanOnEnter)
if (CMSCleanOnEnter) {
size_t sz = _collector->block_size_using_printezis_bits(addr);
HeapWord* end_card_addr = align_up(addr + sz, CardTable::card_size);
MemRegion redirty_range = MemRegion(addr, end_card_addr);
assert(!redirty_range.is_empty(), "Arithmetical tautology");
// Bump _threshold to end_card_addr; note that
// _threshold cannot possibly exceed end_card_addr, anyhow.
// This prevents future clearing of the card as the scan proceeds
// to the right.
assert(_threshold <= end_card_addr,
"Because we are just scanning into this object");
if (_threshold < end_card_addr) {
_threshold = end_card_addr;
}
if (p->klass_or_null_acquire() != NULL) {
// Redirty the range of cards...
_mut->mark_range(redirty_range);
} // ...else the setting of klass will dirty the card anyway.
}
DEBUG_ONLY(})
return true;
}
}
scanOopsInOop(addr);
return true;
}
// We take a break if we've been at this for a while,
// so as to avoid monopolizing the locks involved.
void MarkFromRootsClosure::do_yield_work() {
// First give up the locks, then yield, then re-lock
// We should probably use a constructor/destructor idiom to
// do this unlock/lock or modify the MutexUnlocker class to
// serve our purpose. XXX
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
assert_lock_strong(_bitMap->lock());
_bitMap->lock()->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0; i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
_bitMap->lock()->lock_without_safepoint_check();
_collector->startTimer();
}
void MarkFromRootsClosure::scanOopsInOop(HeapWord* ptr) {
assert(_bitMap->isMarked(ptr), "expected bit to be set");
assert(_markStack->isEmpty(),
"should drain stack to limit stack usage");
// convert ptr to an oop preparatory to scanning
oop obj = oop(ptr);
// Ignore mark word in verification below, since we
// may be running concurrent with mutators.
assert(oopDesc::is_oop(obj, true), "should be an oop");
assert(_finger <= ptr, "_finger runneth ahead");
// advance the finger to right end of this object
_finger = ptr + obj->size();
assert(_finger > ptr, "we just incremented it above");
// On large heaps, it may take us some time to get through
// the marking phase. During
// this time it's possible that a lot of mutations have
// accumulated in the card table and the mod union table --
// these mutation records are redundant until we have
// actually traced into the corresponding card.
// Here, we check whether advancing the finger would make
// us cross into a new card, and if so clear corresponding
// cards in the MUT (preclean them in the card-table in the
// future).
DEBUG_ONLY(if (!_verifying) {)
// The clean-on-enter optimization is disabled by default,
// until we fix 6178663.
if (CMSCleanOnEnter && (_finger > _threshold)) {
// [_threshold, _finger) represents the interval
// of cards to be cleared in MUT (or precleaned in card table).
// The set of cards to be cleared is all those that overlap
// with the interval [_threshold, _finger); note that
// _threshold is always kept card-aligned but _finger isn't
// always card-aligned.
HeapWord* old_threshold = _threshold;
assert(is_aligned(old_threshold, CardTable::card_size),
"_threshold should always be card-aligned");
_threshold = align_up(_finger, CardTable::card_size);
MemRegion mr(old_threshold, _threshold);
assert(!mr.is_empty(), "Control point invariant");
assert(_span.contains(mr), "Should clear within span");
_mut->clear_range(mr);
}
DEBUG_ONLY(})
// Note: the finger doesn't advance while we drain
// the stack below.
PushOrMarkClosure pushOrMarkClosure(_collector,
_span, _bitMap, _markStack,
_finger, this);
bool res = _markStack->push(obj);
assert(res, "Empty non-zero size stack should have space for single push");
while (!_markStack->isEmpty()) {
oop new_oop = _markStack->pop();
// Skip verifying header mark word below because we are
// running concurrent with mutators.
assert(oopDesc::is_oop(new_oop, true), "Oops! expected to pop an oop");
// now scan this oop's oops
new_oop->oop_iterate(&pushOrMarkClosure);
do_yield_check();
}
assert(_markStack->isEmpty(), "tautology, emphasizing post-condition");
}
ParMarkFromRootsClosure::ParMarkFromRootsClosure(CMSConcMarkingTask* task,
CMSCollector* collector, MemRegion span,
CMSBitMap* bit_map,
OopTaskQueue* work_queue,
CMSMarkStack* overflow_stack):
_collector(collector),
_whole_span(collector->_span),
_span(span),
_bit_map(bit_map),
_mut(&collector->_modUnionTable),
_work_queue(work_queue),
_overflow_stack(overflow_stack),
_skip_bits(0),
_task(task)
{
assert(_work_queue->size() == 0, "work_queue should be empty");
_finger = span.start();
_threshold = _finger; // XXX Defer clear-on-enter optimization for now
assert(_span.contains(_finger), "Out of bounds _finger?");
}
// Should revisit to see if this should be restructured for
// greater efficiency.
bool ParMarkFromRootsClosure::do_bit(size_t offset) {
if (_skip_bits > 0) {
_skip_bits--;
return true;
}
// convert offset into a HeapWord*
HeapWord* addr = _bit_map->startWord() + offset;
assert(_bit_map->endWord() && addr < _bit_map->endWord(),
"address out of range");
assert(_bit_map->isMarked(addr), "tautology");
if (_bit_map->isMarked(addr+1)) {
// this is an allocated object that might not yet be initialized
assert(_skip_bits == 0, "tautology");
_skip_bits = 2; // skip next two marked bits ("Printezis-marks")
oop p = oop(addr);
if (p->klass_or_null_acquire() == NULL) {
// in the case of Clean-on-Enter optimization, redirty card
// and avoid clearing card by increasing the threshold.
return true;
}
}
scan_oops_in_oop(addr);
return true;
}
void ParMarkFromRootsClosure::scan_oops_in_oop(HeapWord* ptr) {
assert(_bit_map->isMarked(ptr), "expected bit to be set");
// Should we assert that our work queue is empty or
// below some drain limit?
assert(_work_queue->size() == 0,
"should drain stack to limit stack usage");
// convert ptr to an oop preparatory to scanning
oop obj = oop(ptr);
// Ignore mark word in verification below, since we
// may be running concurrent with mutators.
assert(oopDesc::is_oop(obj, true), "should be an oop");
assert(_finger <= ptr, "_finger runneth ahead");
// advance the finger to right end of this object
_finger = ptr + obj->size();
assert(_finger > ptr, "we just incremented it above");
// On large heaps, it may take us some time to get through
// the marking phase. During
// this time it's possible that a lot of mutations have
// accumulated in the card table and the mod union table --
// these mutation records are redundant until we have
// actually traced into the corresponding card.
// Here, we check whether advancing the finger would make
// us cross into a new card, and if so clear corresponding
// cards in the MUT (preclean them in the card-table in the
// future).
// The clean-on-enter optimization is disabled by default,
// until we fix 6178663.
if (CMSCleanOnEnter && (_finger > _threshold)) {
// [_threshold, _finger) represents the interval
// of cards to be cleared in MUT (or precleaned in card table).
// The set of cards to be cleared is all those that overlap
// with the interval [_threshold, _finger); note that
// _threshold is always kept card-aligned but _finger isn't
// always card-aligned.
HeapWord* old_threshold = _threshold;
assert(is_aligned(old_threshold, CardTable::card_size),
"_threshold should always be card-aligned");
_threshold = align_up(_finger, CardTable::card_size);
MemRegion mr(old_threshold, _threshold);
assert(!mr.is_empty(), "Control point invariant");
assert(_span.contains(mr), "Should clear within span"); // _whole_span ??
_mut->clear_range(mr);
}
// Note: the local finger doesn't advance while we drain
// the stack below, but the global finger sure can and will.
HeapWord* volatile* gfa = _task->global_finger_addr();
ParPushOrMarkClosure pushOrMarkClosure(_collector,
_span, _bit_map,
_work_queue,
_overflow_stack,
_finger,
gfa, this);
bool res = _work_queue->push(obj); // overflow could occur here
assert(res, "Will hold once we use workqueues");
while (true) {
oop new_oop;
if (!_work_queue->pop_local(new_oop)) {
// We emptied our work_queue; check if there's stuff that can
// be gotten from the overflow stack.
if (CMSConcMarkingTask::get_work_from_overflow_stack(
_overflow_stack, _work_queue)) {
do_yield_check();
continue;
} else { // done
break;
}
}
// Skip verifying header mark word below because we are
// running concurrent with mutators.
assert(oopDesc::is_oop(new_oop, true), "Oops! expected to pop an oop");
// now scan this oop's oops
new_oop->oop_iterate(&pushOrMarkClosure);
do_yield_check();
}
assert(_work_queue->size() == 0, "tautology, emphasizing post-condition");
}
// Yield in response to a request from VM Thread or
// from mutators.
void ParMarkFromRootsClosure::do_yield_work() {
assert(_task != NULL, "sanity");
_task->yield();
}
// A variant of the above used for verifying CMS marking work.
MarkFromRootsVerifyClosure::MarkFromRootsVerifyClosure(CMSCollector* collector,
MemRegion span,
CMSBitMap* verification_bm, CMSBitMap* cms_bm,
CMSMarkStack* mark_stack):
_collector(collector),
_span(span),
_verification_bm(verification_bm),
_cms_bm(cms_bm),
_mark_stack(mark_stack),
_pam_verify_closure(collector, span, verification_bm, cms_bm,
mark_stack)
{
assert(_mark_stack->isEmpty(), "stack should be empty");
_finger = _verification_bm->startWord();
assert(_collector->_restart_addr == NULL, "Sanity check");
assert(_span.contains(_finger), "Out of bounds _finger?");
}
void MarkFromRootsVerifyClosure::reset(HeapWord* addr) {
assert(_mark_stack->isEmpty(), "would cause duplicates on stack");
assert(_span.contains(addr), "Out of bounds _finger?");
_finger = addr;
}
// Should revisit to see if this should be restructured for
// greater efficiency.
bool MarkFromRootsVerifyClosure::do_bit(size_t offset) {
// convert offset into a HeapWord*
HeapWord* addr = _verification_bm->startWord() + offset;
assert(_verification_bm->endWord() && addr < _verification_bm->endWord(),
"address out of range");
assert(_verification_bm->isMarked(addr), "tautology");
assert(_cms_bm->isMarked(addr), "tautology");
assert(_mark_stack->isEmpty(),
"should drain stack to limit stack usage");
// convert addr to an oop preparatory to scanning
oop obj = oop(addr);
assert(oopDesc::is_oop(obj), "should be an oop");
assert(_finger <= addr, "_finger runneth ahead");
// advance the finger to right end of this object
_finger = addr + obj->size();
assert(_finger > addr, "we just incremented it above");
// Note: the finger doesn't advance while we drain
// the stack below.
bool res = _mark_stack->push(obj);
assert(res, "Empty non-zero size stack should have space for single push");
while (!_mark_stack->isEmpty()) {
oop new_oop = _mark_stack->pop();
assert(oopDesc::is_oop(new_oop), "Oops! expected to pop an oop");
// now scan this oop's oops
new_oop->oop_iterate(&_pam_verify_closure);
}
assert(_mark_stack->isEmpty(), "tautology, emphasizing post-condition");
return true;
}
PushAndMarkVerifyClosure::PushAndMarkVerifyClosure(
CMSCollector* collector, MemRegion span,
CMSBitMap* verification_bm, CMSBitMap* cms_bm,
CMSMarkStack* mark_stack):
MetadataVisitingOopIterateClosure(collector->ref_processor()),
_collector(collector),
_span(span),
_verification_bm(verification_bm),
_cms_bm(cms_bm),
_mark_stack(mark_stack)
{ }
template <class T> void PushAndMarkVerifyClosure::do_oop_work(T *p) {
oop obj = RawAccess<>::oop_load(p);
do_oop(obj);
}
void PushAndMarkVerifyClosure::do_oop(oop* p) { PushAndMarkVerifyClosure::do_oop_work(p); }
void PushAndMarkVerifyClosure::do_oop(narrowOop* p) { PushAndMarkVerifyClosure::do_oop_work(p); }
// Upon stack overflow, we discard (part of) the stack,
// remembering the least address amongst those discarded
// in CMSCollector's _restart_address.
void PushAndMarkVerifyClosure::handle_stack_overflow(HeapWord* lost) {
// Remember the least grey address discarded
HeapWord* ra = (HeapWord*)_mark_stack->least_value(lost);
_collector->lower_restart_addr(ra);
_mark_stack->reset(); // discard stack contents
_mark_stack->expand(); // expand the stack if possible
}
void PushAndMarkVerifyClosure::do_oop(oop obj) {
assert(oopDesc::is_oop_or_null(obj), "Expected an oop or NULL at " PTR_FORMAT, p2i(obj));
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr) && !_verification_bm->isMarked(addr)) {
// Oop lies in _span and isn't yet grey or black
_verification_bm->mark(addr); // now grey
if (!_cms_bm->isMarked(addr)) {
Log(gc, verify) log;
ResourceMark rm;
LogStream ls(log.error());
oop(addr)->print_on(&ls);
log.error(" (" INTPTR_FORMAT " should have been marked)", p2i(addr));
fatal("... aborting");
}
if (!_mark_stack->push(obj)) { // stack overflow
log_trace(gc)("CMS marking stack overflow (benign) at " SIZE_FORMAT, _mark_stack->capacity());
assert(_mark_stack->isFull(), "Else push should have succeeded");
handle_stack_overflow(addr);
}
// anything including and to the right of _finger
// will be scanned as we iterate over the remainder of the
// bit map
}
}
PushOrMarkClosure::PushOrMarkClosure(CMSCollector* collector,
MemRegion span,
CMSBitMap* bitMap, CMSMarkStack* markStack,
HeapWord* finger, MarkFromRootsClosure* parent) :
MetadataVisitingOopIterateClosure(collector->ref_processor()),
_collector(collector),
_span(span),
_bitMap(bitMap),
_markStack(markStack),
_finger(finger),
_parent(parent)
{ }
ParPushOrMarkClosure::ParPushOrMarkClosure(CMSCollector* collector,
MemRegion span,
CMSBitMap* bit_map,
OopTaskQueue* work_queue,
CMSMarkStack* overflow_stack,
HeapWord* finger,
HeapWord* volatile* global_finger_addr,
ParMarkFromRootsClosure* parent) :
MetadataVisitingOopIterateClosure(collector->ref_processor()),
_collector(collector),
_whole_span(collector->_span),
_span(span),
_bit_map(bit_map),
_work_queue(work_queue),
_overflow_stack(overflow_stack),
_finger(finger),
_global_finger_addr(global_finger_addr),
_parent(parent)
{ }
// Assumes thread-safe access by callers, who are
// responsible for mutual exclusion.
void CMSCollector::lower_restart_addr(HeapWord* low) {
assert(_span.contains(low), "Out of bounds addr");
if (_restart_addr == NULL) {
_restart_addr = low;
} else {
_restart_addr = MIN2(_restart_addr, low);
}
}
// Upon stack overflow, we discard (part of) the stack,
// remembering the least address amongst those discarded
// in CMSCollector's _restart_address.
void PushOrMarkClosure::handle_stack_overflow(HeapWord* lost) {
// Remember the least grey address discarded
HeapWord* ra = (HeapWord*)_markStack->least_value(lost);
_collector->lower_restart_addr(ra);
_markStack->reset(); // discard stack contents
_markStack->expand(); // expand the stack if possible
}
// Upon stack overflow, we discard (part of) the stack,
// remembering the least address amongst those discarded
// in CMSCollector's _restart_address.
void ParPushOrMarkClosure::handle_stack_overflow(HeapWord* lost) {
// We need to do this under a mutex to prevent other
// workers from interfering with the work done below.
MutexLocker ml(_overflow_stack->par_lock(),
Mutex::_no_safepoint_check_flag);
// Remember the least grey address discarded
HeapWord* ra = (HeapWord*)_overflow_stack->least_value(lost);
_collector->lower_restart_addr(ra);
_overflow_stack->reset(); // discard stack contents
_overflow_stack->expand(); // expand the stack if possible
}
void PushOrMarkClosure::do_oop(oop obj) {
// Ignore mark word because we are running concurrent with mutators.
assert(oopDesc::is_oop_or_null(obj, true), "Expected an oop or NULL at " PTR_FORMAT, p2i(obj));
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr) && !_bitMap->isMarked(addr)) {
// Oop lies in _span and isn't yet grey or black
_bitMap->mark(addr); // now grey
if (addr < _finger) {
// the bit map iteration has already either passed, or
// sampled, this bit in the bit map; we'll need to
// use the marking stack to scan this oop's oops.
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow || !_markStack->push(obj)) { // stack overflow
log_trace(gc)("CMS marking stack overflow (benign) at " SIZE_FORMAT, _markStack->capacity());
assert(simulate_overflow || _markStack->isFull(), "Else push should have succeeded");
handle_stack_overflow(addr);
}
}
// anything including and to the right of _finger
// will be scanned as we iterate over the remainder of the
// bit map
do_yield_check();
}
}
void ParPushOrMarkClosure::do_oop(oop obj) {
// Ignore mark word because we are running concurrent with mutators.
assert(oopDesc::is_oop_or_null(obj, true), "Expected an oop or NULL at " PTR_FORMAT, p2i(obj));
HeapWord* addr = (HeapWord*)obj;
if (_whole_span.contains(addr) && !_bit_map->isMarked(addr)) {
// Oop lies in _span and isn't yet grey or black
// We read the global_finger (volatile read) strictly after marking oop
bool res = _bit_map->par_mark(addr); // now grey
volatile HeapWord** gfa = (volatile HeapWord**)_global_finger_addr;
// Should we push this marked oop on our stack?
// -- if someone else marked it, nothing to do
// -- if target oop is above global finger nothing to do
// -- if target oop is in chunk and above local finger
// then nothing to do
// -- else push on work queue
if ( !res // someone else marked it, they will deal with it
|| (addr >= *gfa) // will be scanned in a later task
|| (_span.contains(addr) && addr >= _finger)) { // later in this chunk
return;
}
// the bit map iteration has already either passed, or
// sampled, this bit in the bit map; we'll need to
// use the marking stack to scan this oop's oops.
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow ||
!(_work_queue->push(obj) || _overflow_stack->par_push(obj))) {
// stack overflow
log_trace(gc)("CMS marking stack overflow (benign) at " SIZE_FORMAT, _overflow_stack->capacity());
// We cannot assert that the overflow stack is full because
// it may have been emptied since.
assert(simulate_overflow ||
_work_queue->size() == _work_queue->max_elems(),
"Else push should have succeeded");
handle_stack_overflow(addr);
}
do_yield_check();
}
}
PushAndMarkClosure::PushAndMarkClosure(CMSCollector* collector,
MemRegion span,
ReferenceDiscoverer* rd,
CMSBitMap* bit_map,
CMSBitMap* mod_union_table,
CMSMarkStack* mark_stack,
bool concurrent_precleaning):
MetadataVisitingOopIterateClosure(rd),
_collector(collector),
_span(span),
_bit_map(bit_map),
_mod_union_table(mod_union_table),
_mark_stack(mark_stack),
_concurrent_precleaning(concurrent_precleaning)
{
assert(ref_discoverer() != NULL, "ref_discoverer shouldn't be NULL");
}
// Grey object rescan during pre-cleaning and second checkpoint phases --
// the non-parallel version (the parallel version appears further below.)
void PushAndMarkClosure::do_oop(oop obj) {
// Ignore mark word verification. If during concurrent precleaning,
// the object monitor may be locked. If during the checkpoint
// phases, the object may already have been reached by a different
// path and may be at the end of the global overflow list (so
// the mark word may be NULL).
assert(oopDesc::is_oop_or_null(obj, true /* ignore mark word */),
"Expected an oop or NULL at " PTR_FORMAT, p2i(obj));
HeapWord* addr = (HeapWord*)obj;
// Check if oop points into the CMS generation
// and is not marked
if (_span.contains(addr) && !_bit_map->isMarked(addr)) {
// a white object ...
_bit_map->mark(addr); // ... now grey
// push on the marking stack (grey set)
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow || !_mark_stack->push(obj)) {
if (_concurrent_precleaning) {
// During precleaning we can just dirty the appropriate card(s)
// in the mod union table, thus ensuring that the object remains
// in the grey set and continue. In the case of object arrays
// we need to dirty all of the cards that the object spans,
// since the rescan of object arrays will be limited to the
// dirty cards.
// Note that no one can be interfering with us in this action
// of dirtying the mod union table, so no locking or atomics
// are required.
if (obj->is_objArray()) {
size_t sz = obj->size();
HeapWord* end_card_addr = align_up(addr + sz, CardTable::card_size);
MemRegion redirty_range = MemRegion(addr, end_card_addr);
assert(!redirty_range.is_empty(), "Arithmetical tautology");
_mod_union_table->mark_range(redirty_range);
} else {
_mod_union_table->mark(addr);
}
_collector->_ser_pmc_preclean_ovflw++;
} else {
// During the remark phase, we need to remember this oop
// in the overflow list.
_collector->push_on_overflow_list(obj);
_collector->_ser_pmc_remark_ovflw++;
}
}
}
}
ParPushAndMarkClosure::ParPushAndMarkClosure(CMSCollector* collector,
MemRegion span,
ReferenceDiscoverer* rd,
CMSBitMap* bit_map,
OopTaskQueue* work_queue):
MetadataVisitingOopIterateClosure(rd),
_collector(collector),
_span(span),
_bit_map(bit_map),
_work_queue(work_queue)
{
assert(ref_discoverer() != NULL, "ref_discoverer shouldn't be NULL");
}
// Grey object rescan during second checkpoint phase --
// the parallel version.
void ParPushAndMarkClosure::do_oop(oop obj) {
// In the assert below, we ignore the mark word because
// this oop may point to an already visited object that is
// on the overflow stack (in which case the mark word has
// been hijacked for chaining into the overflow stack --
// if this is the last object in the overflow stack then
// its mark word will be NULL). Because this object may
// have been subsequently popped off the global overflow
// stack, and the mark word possibly restored to the prototypical
// value, by the time we get to examined this failing assert in
// the debugger, is_oop_or_null(false) may subsequently start
// to hold.
assert(oopDesc::is_oop_or_null(obj, true),
"Expected an oop or NULL at " PTR_FORMAT, p2i(obj));
HeapWord* addr = (HeapWord*)obj;
// Check if oop points into the CMS generation
// and is not marked
if (_span.contains(addr) && !_bit_map->isMarked(addr)) {
// a white object ...
// If we manage to "claim" the object, by being the
// first thread to mark it, then we push it on our
// marking stack
if (_bit_map->par_mark(addr)) { // ... now grey
// push on work queue (grey set)
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->par_simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow || !_work_queue->push(obj)) {
_collector->par_push_on_overflow_list(obj);
_collector->_par_pmc_remark_ovflw++; // imprecise OK: no need to CAS
}
} // Else, some other thread got there first
}
}
void CMSPrecleanRefsYieldClosure::do_yield_work() {
Mutex* bml = _collector->bitMapLock();
assert_lock_strong(bml);
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
bml->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0; i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
bml->lock_without_safepoint_check();
_collector->startTimer();
}
bool CMSPrecleanRefsYieldClosure::should_return() {
if (ConcurrentMarkSweepThread::should_yield()) {
do_yield_work();
}
return _collector->foregroundGCIsActive();
}
void MarkFromDirtyCardsClosure::do_MemRegion(MemRegion mr) {
assert(((size_t)mr.start())%CardTable::card_size_in_words == 0,
"mr should be aligned to start at a card boundary");
// We'd like to assert:
// assert(mr.word_size()%CardTable::card_size_in_words == 0,
// "mr should be a range of cards");
// However, that would be too strong in one case -- the last
// partition ends at _unallocated_block which, in general, can be
// an arbitrary boundary, not necessarily card aligned.
_num_dirty_cards += mr.word_size()/CardTable::card_size_in_words;
_space->object_iterate_mem(mr, &_scan_cl);
}
SweepClosure::SweepClosure(CMSCollector* collector,
ConcurrentMarkSweepGeneration* g,
CMSBitMap* bitMap, bool should_yield) :
_collector(collector),
_g(g),
_sp(g->cmsSpace()),
_limit(_sp->sweep_limit()),
_freelistLock(_sp->freelistLock()),
_bitMap(bitMap),
_inFreeRange(false), // No free range at beginning of sweep
_freeRangeInFreeLists(false), // No free range at beginning of sweep
_lastFreeRangeCoalesced(false),
_yield(should_yield),
_freeFinger(g->used_region().start())
{
NOT_PRODUCT(
_numObjectsFreed = 0;
_numWordsFreed = 0;
_numObjectsLive = 0;
_numWordsLive = 0;
_numObjectsAlreadyFree = 0;
_numWordsAlreadyFree = 0;
_last_fc = NULL;
_sp->initializeIndexedFreeListArrayReturnedBytes();
_sp->dictionary()->initialize_dict_returned_bytes();
)
assert(_limit >= _sp->bottom() && _limit <= _sp->end(),
"sweep _limit out of bounds");
log_develop_trace(gc, sweep)("====================");
log_develop_trace(gc, sweep)("Starting new sweep with limit " PTR_FORMAT, p2i(_limit));
}
void SweepClosure::print_on(outputStream* st) const {
st->print_cr("_sp = [" PTR_FORMAT "," PTR_FORMAT ")",
p2i(_sp->bottom()), p2i(_sp->end()));
st->print_cr("_limit = " PTR_FORMAT, p2i(_limit));
st->print_cr("_freeFinger = " PTR_FORMAT, p2i(_freeFinger));
NOT_PRODUCT(st->print_cr("_last_fc = " PTR_FORMAT, p2i(_last_fc));)
st->print_cr("_inFreeRange = %d, _freeRangeInFreeLists = %d, _lastFreeRangeCoalesced = %d",
_inFreeRange, _freeRangeInFreeLists, _lastFreeRangeCoalesced);
}
#ifndef PRODUCT
// Assertion checking only: no useful work in product mode --
// however, if any of the flags below become product flags,
// you may need to review this code to see if it needs to be
// enabled in product mode.
SweepClosure::~SweepClosure() {
assert_lock_strong(_freelistLock);
assert(_limit >= _sp->bottom() && _limit <= _sp->end(),
"sweep _limit out of bounds");
if (inFreeRange()) {
Log(gc, sweep) log;
log.error("inFreeRange() should have been reset; dumping state of SweepClosure");
ResourceMark rm;
LogStream ls(log.error());
print_on(&ls);
ShouldNotReachHere();
}
if (log_is_enabled(Debug, gc, sweep)) {
log_debug(gc, sweep)("Collected " SIZE_FORMAT " objects, " SIZE_FORMAT " bytes",
_numObjectsFreed, _numWordsFreed*sizeof(HeapWord));
log_debug(gc, sweep)("Live " SIZE_FORMAT " objects, " SIZE_FORMAT " bytes Already free " SIZE_FORMAT " objects, " SIZE_FORMAT " bytes",
_numObjectsLive, _numWordsLive*sizeof(HeapWord), _numObjectsAlreadyFree, _numWordsAlreadyFree*sizeof(HeapWord));
size_t totalBytes = (_numWordsFreed + _numWordsLive + _numWordsAlreadyFree) * sizeof(HeapWord);
log_debug(gc, sweep)("Total sweep: " SIZE_FORMAT " bytes", totalBytes);
}
if (log_is_enabled(Trace, gc, sweep) && CMSVerifyReturnedBytes) {
size_t indexListReturnedBytes = _sp->sumIndexedFreeListArrayReturnedBytes();
size_t dict_returned_bytes = _sp->dictionary()->sum_dict_returned_bytes();
size_t returned_bytes = indexListReturnedBytes + dict_returned_bytes;
log_trace(gc, sweep)("Returned " SIZE_FORMAT " bytes Indexed List Returned " SIZE_FORMAT " bytes Dictionary Returned " SIZE_FORMAT " bytes",
returned_bytes, indexListReturnedBytes, dict_returned_bytes);
}
log_develop_trace(gc, sweep)("end of sweep with _limit = " PTR_FORMAT, p2i(_limit));
log_develop_trace(gc, sweep)("================");
}
#endif // PRODUCT
void SweepClosure::initialize_free_range(HeapWord* freeFinger,
bool freeRangeInFreeLists) {
log_develop_trace(gc, sweep)("---- Start free range at " PTR_FORMAT " with free block (%d)",
p2i(freeFinger), freeRangeInFreeLists);
assert(!inFreeRange(), "Trampling existing free range");
set_inFreeRange(true);
set_lastFreeRangeCoalesced(false);
set_freeFinger(freeFinger);
set_freeRangeInFreeLists(freeRangeInFreeLists);
if (CMSTestInFreeList) {
if (freeRangeInFreeLists) {
FreeChunk* fc = (FreeChunk*) freeFinger;
assert(fc->is_free(), "A chunk on the free list should be free.");
assert(fc->size() > 0, "Free range should have a size");
assert(_sp->verify_chunk_in_free_list(fc), "Chunk is not in free lists");
}
}
}
// Note that the sweeper runs concurrently with mutators. Thus,
// it is possible for direct allocation in this generation to happen
// in the middle of the sweep. Note that the sweeper also coalesces
// contiguous free blocks. Thus, unless the sweeper and the allocator
// synchronize appropriately freshly allocated blocks may get swept up.
// This is accomplished by the sweeper locking the free lists while
// it is sweeping. Thus blocks that are determined to be free are
// indeed free. There is however one additional complication:
// blocks that have been allocated since the final checkpoint and
// mark, will not have been marked and so would be treated as
// unreachable and swept up. To prevent this, the allocator marks
// the bit map when allocating during the sweep phase. This leads,
// however, to a further complication -- objects may have been allocated
// but not yet initialized -- in the sense that the header isn't yet
// installed. The sweeper can not then determine the size of the block
// in order to skip over it. To deal with this case, we use a technique
// (due to Printezis) to encode such uninitialized block sizes in the
// bit map. Since the bit map uses a bit per every HeapWord, but the
// CMS generation has a minimum object size of 3 HeapWords, it follows
// that "normal marks" won't be adjacent in the bit map (there will
// always be at least two 0 bits between successive 1 bits). We make use
// of these "unused" bits to represent uninitialized blocks -- the bit
// corresponding to the start of the uninitialized object and the next
// bit are both set. Finally, a 1 bit marks the end of the object that
// started with the two consecutive 1 bits to indicate its potentially
// uninitialized state.
size_t SweepClosure::do_blk_careful(HeapWord* addr) {
FreeChunk* fc = (FreeChunk*)addr;
size_t res;
// Check if we are done sweeping. Below we check "addr >= _limit" rather
// than "addr == _limit" because although _limit was a block boundary when
// we started the sweep, it may no longer be one because heap expansion
// may have caused us to coalesce the block ending at the address _limit
// with a newly expanded chunk (this happens when _limit was set to the
// previous _end of the space), so we may have stepped past _limit:
// see the following Zeno-like trail of CRs 6977970, 7008136, 7042740.
if (addr >= _limit) { // we have swept up to or past the limit: finish up
assert(_limit >= _sp->bottom() && _limit <= _sp->end(),
"sweep _limit out of bounds");
assert(addr < _sp->end(), "addr out of bounds");
// Flush any free range we might be holding as a single
// coalesced chunk to the appropriate free list.
if (inFreeRange()) {
assert(freeFinger() >= _sp->bottom() && freeFinger() < _limit,
"freeFinger() " PTR_FORMAT " is out of bounds", p2i(freeFinger()));
flush_cur_free_chunk(freeFinger(),
pointer_delta(addr, freeFinger()));
log_develop_trace(gc, sweep)("Sweep: last chunk: put_free_blk " PTR_FORMAT " (" SIZE_FORMAT ") [coalesced:%d]",
p2i(freeFinger()), pointer_delta(addr, freeFinger()),
lastFreeRangeCoalesced() ? 1 : 0);
}
// help the iterator loop finish
return pointer_delta(_sp->end(), addr);
}
assert(addr < _limit, "sweep invariant");
// check if we should yield
do_yield_check(addr);
if (fc->is_free()) {
// Chunk that is already free
res = fc->size();
do_already_free_chunk(fc);
debug_only(_sp->verifyFreeLists());
// If we flush the chunk at hand in lookahead_and_flush()
// and it's coalesced with a preceding chunk, then the
// process of "mangling" the payload of the coalesced block
// will cause erasure of the size information from the
// (erstwhile) header of all the coalesced blocks but the
// first, so the first disjunct in the assert will not hold
// in that specific case (in which case the second disjunct
// will hold).
assert(res == fc->size() || ((HeapWord*)fc) + res >= _limit,
"Otherwise the size info doesn't change at this step");
NOT_PRODUCT(
_numObjectsAlreadyFree++;
_numWordsAlreadyFree += res;
)
NOT_PRODUCT(_last_fc = fc;)
} else if (!_bitMap->isMarked(addr)) {
// Chunk is fresh garbage
res = do_garbage_chunk(fc);
debug_only(_sp->verifyFreeLists());
NOT_PRODUCT(
_numObjectsFreed++;
_numWordsFreed += res;
)
} else {
// Chunk that is alive.
res = do_live_chunk(fc);
debug_only(_sp->verifyFreeLists());
NOT_PRODUCT(
_numObjectsLive++;
_numWordsLive += res;
)
}
return res;
}
// For the smart allocation, record following
// split deaths - a free chunk is removed from its free list because
// it is being split into two or more chunks.
// split birth - a free chunk is being added to its free list because
// a larger free chunk has been split and resulted in this free chunk.
// coal death - a free chunk is being removed from its free list because
// it is being coalesced into a large free chunk.
// coal birth - a free chunk is being added to its free list because
// it was created when two or more free chunks where coalesced into
// this free chunk.
//
// These statistics are used to determine the desired number of free
// chunks of a given size. The desired number is chosen to be relative
// to the end of a CMS sweep. The desired number at the end of a sweep
// is the
// count-at-end-of-previous-sweep (an amount that was enough)
// - count-at-beginning-of-current-sweep (the excess)
// + split-births (gains in this size during interval)
// - split-deaths (demands on this size during interval)
// where the interval is from the end of one sweep to the end of the
// next.
//
// When sweeping the sweeper maintains an accumulated chunk which is
// the chunk that is made up of chunks that have been coalesced. That
// will be termed the left-hand chunk. A new chunk of garbage that
// is being considered for coalescing will be referred to as the
// right-hand chunk.
//
// When making a decision on whether to coalesce a right-hand chunk with
// the current left-hand chunk, the current count vs. the desired count
// of the left-hand chunk is considered. Also if the right-hand chunk
// is near the large chunk at the end of the heap (see
// ConcurrentMarkSweepGeneration::isNearLargestChunk()), then the
// left-hand chunk is coalesced.
//
// When making a decision about whether to split a chunk, the desired count
// vs. the current count of the candidate to be split is also considered.
// If the candidate is underpopulated (currently fewer chunks than desired)
// a chunk of an overpopulated (currently more chunks than desired) size may
// be chosen. The "hint" associated with a free list, if non-null, points
// to a free list which may be overpopulated.
//
void SweepClosure::do_already_free_chunk(FreeChunk* fc) {
const size_t size = fc->size();
// Chunks that cannot be coalesced are not in the
// free lists.
if (CMSTestInFreeList && !fc->cantCoalesce()) {
assert(_sp->verify_chunk_in_free_list(fc),
"free chunk should be in free lists");
}
// a chunk that is already free, should not have been
// marked in the bit map
HeapWord* const addr = (HeapWord*) fc;
assert(!_bitMap->isMarked(addr), "free chunk should be unmarked");
// Verify that the bit map has no bits marked between
// addr and purported end of this block.
_bitMap->verifyNoOneBitsInRange(addr + 1, addr + size);
// Some chunks cannot be coalesced under any circumstances.
// See the definition of cantCoalesce().
if (!fc->cantCoalesce()) {
// This chunk can potentially be coalesced.
// All the work is done in
do_post_free_or_garbage_chunk(fc, size);
// Note that if the chunk is not coalescable (the else arm
// below), we unconditionally flush, without needing to do
// a "lookahead," as we do below.
if (inFreeRange()) lookahead_and_flush(fc, size);
} else {
// Code path common to both original and adaptive free lists.
// cant coalesce with previous block; this should be treated
// as the end of a free run if any
if (inFreeRange()) {
// we kicked some butt; time to pick up the garbage
assert(freeFinger() < addr, "freeFinger points too high");
flush_cur_free_chunk(freeFinger(), pointer_delta(addr, freeFinger()));
}
// else, nothing to do, just continue
}
}
size_t SweepClosure::do_garbage_chunk(FreeChunk* fc) {
// This is a chunk of garbage. It is not in any free list.
// Add it to a free list or let it possibly be coalesced into
// a larger chunk.
HeapWord* const addr = (HeapWord*) fc;
const size_t size = CompactibleFreeListSpace::adjustObjectSize(oop(addr)->size());
// Verify that the bit map has no bits marked between
// addr and purported end of just dead object.
_bitMap->verifyNoOneBitsInRange(addr + 1, addr + size);
do_post_free_or_garbage_chunk(fc, size);
assert(_limit >= addr + size,
"A freshly garbage chunk can't possibly straddle over _limit");
if (inFreeRange()) lookahead_and_flush(fc, size);
return size;
}
size_t SweepClosure::do_live_chunk(FreeChunk* fc) {
HeapWord* addr = (HeapWord*) fc;
// The sweeper has just found a live object. Return any accumulated
// left hand chunk to the free lists.
if (inFreeRange()) {
assert(freeFinger() < addr, "freeFinger points too high");
flush_cur_free_chunk(freeFinger(), pointer_delta(addr, freeFinger()));
}
// This object is live: we'd normally expect this to be
// an oop, and like to assert the following:
// assert(oopDesc::is_oop(oop(addr)), "live block should be an oop");
// However, as we commented above, this may be an object whose
// header hasn't yet been initialized.
size_t size;
assert(_bitMap->isMarked(addr), "Tautology for this control point");
if (_bitMap->isMarked(addr + 1)) {
// Determine the size from the bit map, rather than trying to
// compute it from the object header.
HeapWord* nextOneAddr = _bitMap->getNextMarkedWordAddress(addr + 2);
size = pointer_delta(nextOneAddr + 1, addr);
assert(size == CompactibleFreeListSpace::adjustObjectSize(size),
"alignment problem");
#ifdef ASSERT
if (oop(addr)->klass_or_null_acquire() != NULL) {
// Ignore mark word because we are running concurrent with mutators
assert(oopDesc::is_oop(oop(addr), true), "live block should be an oop");
assert(size ==
CompactibleFreeListSpace::adjustObjectSize(oop(addr)->size()),
"P-mark and computed size do not agree");
}
#endif
} else {
// This should be an initialized object that's alive.
assert(oop(addr)->klass_or_null_acquire() != NULL,
"Should be an initialized object");
// Ignore mark word because we are running concurrent with mutators
assert(oopDesc::is_oop(oop(addr), true), "live block should be an oop");
// Verify that the bit map has no bits marked between
// addr and purported end of this block.
size = CompactibleFreeListSpace::adjustObjectSize(oop(addr)->size());
assert(size >= 3, "Necessary for Printezis marks to work");
assert(!_bitMap->isMarked(addr+1), "Tautology for this control point");
DEBUG_ONLY(_bitMap->verifyNoOneBitsInRange(addr+2, addr+size);)
}
return size;
}
void SweepClosure::do_post_free_or_garbage_chunk(FreeChunk* fc,
size_t chunkSize) {
// do_post_free_or_garbage_chunk() should only be called in the case
// of the adaptive free list allocator.
const bool fcInFreeLists = fc->is_free();
assert((HeapWord*)fc <= _limit, "sweep invariant");
if (CMSTestInFreeList && fcInFreeLists) {
assert(_sp->verify_chunk_in_free_list(fc), "free chunk is not in free lists");
}
log_develop_trace(gc, sweep)(" -- pick up another chunk at " PTR_FORMAT " (" SIZE_FORMAT ")", p2i(fc), chunkSize);
HeapWord* const fc_addr = (HeapWord*) fc;
bool coalesce = false;
const size_t left = pointer_delta(fc_addr, freeFinger());
const size_t right = chunkSize;
switch (FLSCoalescePolicy) {
// numeric value forms a coalition aggressiveness metric
case 0: { // never coalesce
coalesce = false;
break;
}
case 1: { // coalesce if left & right chunks on overpopulated lists
coalesce = _sp->coalOverPopulated(left) &&
_sp->coalOverPopulated(right);
break;
}
case 2: { // coalesce if left chunk on overpopulated list (default)
coalesce = _sp->coalOverPopulated(left);
break;
}
case 3: { // coalesce if left OR right chunk on overpopulated list
coalesce = _sp->coalOverPopulated(left) ||
_sp->coalOverPopulated(right);
break;
}
case 4: { // always coalesce
coalesce = true;
break;
}
default:
ShouldNotReachHere();
}
// Should the current free range be coalesced?
// If the chunk is in a free range and either we decided to coalesce above
// or the chunk is near the large block at the end of the heap
// (isNearLargestChunk() returns true), then coalesce this chunk.
const bool doCoalesce = inFreeRange()
&& (coalesce || _g->isNearLargestChunk(fc_addr));
if (doCoalesce) {
// Coalesce the current free range on the left with the new
// chunk on the right. If either is on a free list,
// it must be removed from the list and stashed in the closure.
if (freeRangeInFreeLists()) {
FreeChunk* const ffc = (FreeChunk*)freeFinger();
assert(ffc->size() == pointer_delta(fc_addr, freeFinger()),
"Size of free range is inconsistent with chunk size.");
if (CMSTestInFreeList) {
assert(_sp->verify_chunk_in_free_list(ffc),
"Chunk is not in free lists");
}
_sp->coalDeath(ffc->size());
_sp->removeFreeChunkFromFreeLists(ffc);
set_freeRangeInFreeLists(false);
}
if (fcInFreeLists) {
_sp->coalDeath(chunkSize);
assert(fc->size() == chunkSize,
"The chunk has the wrong size or is not in the free lists");
_sp->removeFreeChunkFromFreeLists(fc);
}
set_lastFreeRangeCoalesced(true);
print_free_block_coalesced(fc);
} else { // not in a free range and/or should not coalesce
// Return the current free range and start a new one.
if (inFreeRange()) {
// In a free range but cannot coalesce with the right hand chunk.
// Put the current free range into the free lists.
flush_cur_free_chunk(freeFinger(),
pointer_delta(fc_addr, freeFinger()));
}
// Set up for new free range. Pass along whether the right hand
// chunk is in the free lists.
initialize_free_range((HeapWord*)fc, fcInFreeLists);
}
}
// Lookahead flush:
// If we are tracking a free range, and this is the last chunk that
// we'll look at because its end crosses past _limit, we'll preemptively
// flush it along with any free range we may be holding on to. Note that
// this can be the case only for an already free or freshly garbage
// chunk. If this block is an object, it can never straddle
// over _limit. The "straddling" occurs when _limit is set at
// the previous end of the space when this cycle started, and
// a subsequent heap expansion caused the previously co-terminal
// free block to be coalesced with the newly expanded portion,
// thus rendering _limit a non-block-boundary making it dangerous
// for the sweeper to step over and examine.
void SweepClosure::lookahead_and_flush(FreeChunk* fc, size_t chunk_size) {
assert(inFreeRange(), "Should only be called if currently in a free range.");
HeapWord* const eob = ((HeapWord*)fc) + chunk_size;
assert(_sp->used_region().contains(eob - 1),
"eob = " PTR_FORMAT " eob-1 = " PTR_FORMAT " _limit = " PTR_FORMAT
" out of bounds wrt _sp = [" PTR_FORMAT "," PTR_FORMAT ")"
" when examining fc = " PTR_FORMAT "(" SIZE_FORMAT ")",
p2i(eob), p2i(eob-1), p2i(_limit), p2i(_sp->bottom()), p2i(_sp->end()), p2i(fc), chunk_size);
if (eob >= _limit) {
assert(eob == _limit || fc->is_free(), "Only a free chunk should allow us to cross over the limit");
log_develop_trace(gc, sweep)("_limit " PTR_FORMAT " reached or crossed by block "
"[" PTR_FORMAT "," PTR_FORMAT ") in space "
"[" PTR_FORMAT "," PTR_FORMAT ")",
p2i(_limit), p2i(fc), p2i(eob), p2i(_sp->bottom()), p2i(_sp->end()));
// Return the storage we are tracking back into the free lists.
log_develop_trace(gc, sweep)("Flushing ... ");
assert(freeFinger() < eob, "Error");
flush_cur_free_chunk( freeFinger(), pointer_delta(eob, freeFinger()));
}
}
void SweepClosure::flush_cur_free_chunk(HeapWord* chunk, size_t size) {
assert(inFreeRange(), "Should only be called if currently in a free range.");
assert(size > 0,
"A zero sized chunk cannot be added to the free lists.");
if (!freeRangeInFreeLists()) {
if (CMSTestInFreeList) {
FreeChunk* fc = (FreeChunk*) chunk;
fc->set_size(size);
assert(!_sp->verify_chunk_in_free_list(fc),
"chunk should not be in free lists yet");
}
log_develop_trace(gc, sweep)(" -- add free block " PTR_FORMAT " (" SIZE_FORMAT ") to free lists", p2i(chunk), size);
// A new free range is going to be starting. The current
// free range has not been added to the free lists yet or
// was removed so add it back.
// If the current free range was coalesced, then the death
// of the free range was recorded. Record a birth now.
if (lastFreeRangeCoalesced()) {
_sp->coalBirth(size);
}
_sp->addChunkAndRepairOffsetTable(chunk, size,
lastFreeRangeCoalesced());
} else {
log_develop_trace(gc, sweep)("Already in free list: nothing to flush");
}
set_inFreeRange(false);
set_freeRangeInFreeLists(false);
}
// We take a break if we've been at this for a while,
// so as to avoid monopolizing the locks involved.
void SweepClosure::do_yield_work(HeapWord* addr) {
// Return current free chunk being used for coalescing (if any)
// to the appropriate freelist. After yielding, the next
// free block encountered will start a coalescing range of
// free blocks. If the next free block is adjacent to the
// chunk just flushed, they will need to wait for the next
// sweep to be coalesced.
if (inFreeRange()) {
flush_cur_free_chunk(freeFinger(), pointer_delta(addr, freeFinger()));
}
// First give up the locks, then yield, then re-lock.
// We should probably use a constructor/destructor idiom to
// do this unlock/lock or modify the MutexUnlocker class to
// serve our purpose. XXX
assert_lock_strong(_bitMap->lock());
assert_lock_strong(_freelistLock);
assert(ConcurrentMarkSweepThread::cms_thread_has_cms_token(),
"CMS thread should hold CMS token");
_bitMap->lock()->unlock();
_freelistLock->unlock();
ConcurrentMarkSweepThread::desynchronize(true);
_collector->stopTimer();
_collector->incrementYields();
// See the comment in coordinator_yield()
for (unsigned i = 0; i < CMSYieldSleepCount &&
ConcurrentMarkSweepThread::should_yield() &&
!CMSCollector::foregroundGCIsActive(); ++i) {
os::naked_short_sleep(1);
}
ConcurrentMarkSweepThread::synchronize(true);
_freelistLock->lock_without_safepoint_check();
_bitMap->lock()->lock_without_safepoint_check();
_collector->startTimer();
}
#ifndef PRODUCT
// This is actually very useful in a product build if it can
// be called from the debugger. Compile it into the product
// as needed.
bool debug_verify_chunk_in_free_list(FreeChunk* fc) {
return debug_cms_space->verify_chunk_in_free_list(fc);
}
#endif
void SweepClosure::print_free_block_coalesced(FreeChunk* fc) const {
log_develop_trace(gc, sweep)("Sweep:coal_free_blk " PTR_FORMAT " (" SIZE_FORMAT ")",
p2i(fc), fc->size());
}
// CMSIsAliveClosure
bool CMSIsAliveClosure::do_object_b(oop obj) {
HeapWord* addr = (HeapWord*)obj;
return addr != NULL &&
(!_span.contains(addr) || _bit_map->isMarked(addr));
}
CMSKeepAliveClosure::CMSKeepAliveClosure( CMSCollector* collector,
MemRegion span,
CMSBitMap* bit_map, CMSMarkStack* mark_stack,
bool cpc):
_collector(collector),
_span(span),
_mark_stack(mark_stack),
_bit_map(bit_map),
_concurrent_precleaning(cpc) {
assert(!_span.is_empty(), "Empty span could spell trouble");
}
// CMSKeepAliveClosure: the serial version
void CMSKeepAliveClosure::do_oop(oop obj) {
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr) &&
!_bit_map->isMarked(addr)) {
_bit_map->mark(addr);
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow || !_mark_stack->push(obj)) {
if (_concurrent_precleaning) {
// We dirty the overflown object and let the remark
// phase deal with it.
assert(_collector->overflow_list_is_empty(), "Error");
// In the case of object arrays, we need to dirty all of
// the cards that the object spans. No locking or atomics
// are needed since no one else can be mutating the mod union
// table.
if (obj->is_objArray()) {
size_t sz = obj->size();
HeapWord* end_card_addr = align_up(addr + sz, CardTable::card_size);
MemRegion redirty_range = MemRegion(addr, end_card_addr);
assert(!redirty_range.is_empty(), "Arithmetical tautology");
_collector->_modUnionTable.mark_range(redirty_range);
} else {
_collector->_modUnionTable.mark(addr);
}
_collector->_ser_kac_preclean_ovflw++;
} else {
_collector->push_on_overflow_list(obj);
_collector->_ser_kac_ovflw++;
}
}
}
}
// CMSParKeepAliveClosure: a parallel version of the above.
// The work queues are private to each closure (thread),
// but (may be) available for stealing by other threads.
void CMSParKeepAliveClosure::do_oop(oop obj) {
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr) &&
!_bit_map->isMarked(addr)) {
// In general, during recursive tracing, several threads
// may be concurrently getting here; the first one to
// "tag" it, claims it.
if (_bit_map->par_mark(addr)) {
bool res = _work_queue->push(obj);
assert(res, "Low water mark should be much less than capacity");
// Do a recursive trim in the hope that this will keep
// stack usage lower, but leave some oops for potential stealers
trim_queue(_low_water_mark);
} // Else, another thread got there first
}
}
void CMSParKeepAliveClosure::trim_queue(uint max) {
while (_work_queue->size() > max) {
oop new_oop;
if (_work_queue->pop_local(new_oop)) {
assert(new_oop != NULL && oopDesc::is_oop(new_oop), "Expected an oop");
assert(_bit_map->isMarked((HeapWord*)new_oop),
"no white objects on this stack!");
assert(_span.contains((HeapWord*)new_oop), "Out of bounds oop");
// iterate over the oops in this oop, marking and pushing
// the ones in CMS heap (i.e. in _span).
new_oop->oop_iterate(&_mark_and_push);
}
}
}
CMSInnerParMarkAndPushClosure::CMSInnerParMarkAndPushClosure(
CMSCollector* collector,
MemRegion span, CMSBitMap* bit_map,
OopTaskQueue* work_queue):
_collector(collector),
_span(span),
_work_queue(work_queue),
_bit_map(bit_map) { }
void CMSInnerParMarkAndPushClosure::do_oop(oop obj) {
HeapWord* addr = (HeapWord*)obj;
if (_span.contains(addr) &&
!_bit_map->isMarked(addr)) {
if (_bit_map->par_mark(addr)) {
bool simulate_overflow = false;
NOT_PRODUCT(
if (CMSMarkStackOverflowALot &&
_collector->par_simulate_overflow()) {
// simulate a stack overflow
simulate_overflow = true;
}
)
if (simulate_overflow || !_work_queue->push(obj)) {
_collector->par_push_on_overflow_list(obj);
_collector->_par_kac_ovflw++;
}
} // Else another thread got there already
}
}
//////////////////////////////////////////////////////////////////
// CMSExpansionCause /////////////////////////////
//////////////////////////////////////////////////////////////////
const char* CMSExpansionCause::to_string(CMSExpansionCause::Cause cause) {
switch (cause) {
case _no_expansion:
return "No expansion";
case _satisfy_free_ratio:
return "Free ratio";
case _satisfy_promotion:
return "Satisfy promotion";
case _satisfy_allocation:
return "allocation";
case _allocate_par_lab:
return "Par LAB";
case _allocate_par_spooling_space:
return "Par Spooling Space";
case _adaptive_size_policy:
return "Ergonomics";
default:
return "unknown";
}
}
void CMSDrainMarkingStackClosure::do_void() {
// the max number to take from overflow list at a time
const size_t num = _mark_stack->capacity()/4;
assert(!_concurrent_precleaning || _collector->overflow_list_is_empty(),
"Overflow list should be NULL during concurrent phases");
while (!_mark_stack->isEmpty() ||
// if stack is empty, check the overflow list
_collector->take_from_overflow_list(num, _mark_stack)) {
oop obj = _mark_stack->pop();
HeapWord* addr = (HeapWord*)obj;
assert(_span.contains(addr), "Should be within span");
assert(_bit_map->isMarked(addr), "Should be marked");
assert(oopDesc::is_oop(obj), "Should be an oop");
obj->oop_iterate(_keep_alive);
}
}
void CMSParDrainMarkingStackClosure::do_void() {
// drain queue
trim_queue(0);
}
// Trim our work_queue so its length is below max at return
void CMSParDrainMarkingStackClosure::trim_queue(uint max) {
while (_work_queue->size() > max) {
oop new_oop;
if (_work_queue->pop_local(new_oop)) {
assert(oopDesc::is_oop(new_oop), "Expected an oop");
assert(_bit_map->isMarked((HeapWord*)new_oop),
"no white objects on this stack!");
assert(_span.contains((HeapWord*)new_oop), "Out of bounds oop");
// iterate over the oops in this oop, marking and pushing
// the ones in CMS heap (i.e. in _span).
new_oop->oop_iterate(&_mark_and_push);
}
}
}
////////////////////////////////////////////////////////////////////
// Support for Marking Stack Overflow list handling and related code
////////////////////////////////////////////////////////////////////
// Much of the following code is similar in shape and spirit to the
// code used in ParNewGC. We should try and share that code
// as much as possible in the future.
#ifndef PRODUCT
// Debugging support for CMSStackOverflowALot
// It's OK to call this multi-threaded; the worst thing
// that can happen is that we'll get a bunch of closely
// spaced simulated overflows, but that's OK, in fact
// probably good as it would exercise the overflow code
// under contention.
bool CMSCollector::simulate_overflow() {
if (_overflow_counter-- <= 0) { // just being defensive
_overflow_counter = CMSMarkStackOverflowInterval;
return true;
} else {
return false;
}
}
bool CMSCollector::par_simulate_overflow() {
return simulate_overflow();
}
#endif
// Single-threaded
bool CMSCollector::take_from_overflow_list(size_t num, CMSMarkStack* stack) {
assert(stack->isEmpty(), "Expected precondition");
assert(stack->capacity() > num, "Shouldn't bite more than can chew");
size_t i = num;
oop cur = _overflow_list;
const markWord proto = markWord::prototype();
NOT_PRODUCT(ssize_t n = 0;)
for (oop next; i > 0 && cur != NULL; cur = next, i--) {
next = oop(cur->mark_raw().to_pointer());
cur->set_mark_raw(proto); // until proven otherwise
assert(oopDesc::is_oop(cur), "Should be an oop");
bool res = stack->push(cur);
assert(res, "Bit off more than can chew?");
NOT_PRODUCT(n++;)
}
_overflow_list = cur;
#ifndef PRODUCT
assert(_num_par_pushes >= n, "Too many pops?");
_num_par_pushes -=n;
#endif
return !stack->isEmpty();
}
#define BUSY (cast_to_oop<intptr_t>(0x1aff1aff))
// (MT-safe) Get a prefix of at most "num" from the list.
// The overflow list is chained through the mark word of
// each object in the list. We fetch the entire list,
// break off a prefix of the right size and return the
// remainder. If other threads try to take objects from
// the overflow list at that time, they will wait for
// some time to see if data becomes available. If (and
// only if) another thread places one or more object(s)
// on the global list before we have returned the suffix
// to the global list, we will walk down our local list
// to find its end and append the global list to
// our suffix before returning it. This suffix walk can
// prove to be expensive (quadratic in the amount of traffic)
// when there are many objects in the overflow list and
// there is much producer-consumer contention on the list.
// *NOTE*: The overflow list manipulation code here and
// in ParNewGeneration:: are very similar in shape,
// except that in the ParNew case we use the old (from/eden)
// copy of the object to thread the list via its klass word.
// Because of the common code, if you make any changes in
// the code below, please check the ParNew version to see if
// similar changes might be needed.
// CR 6797058 has been filed to consolidate the common code.
bool CMSCollector::par_take_from_overflow_list(size_t num,
OopTaskQueue* work_q,
int no_of_gc_threads) {
assert(work_q->size() == 0, "First empty local work queue");
assert(num < work_q->max_elems(), "Can't bite more than we can chew");
if (_overflow_list == NULL) {
return false;
}
// Grab the entire list; we'll put back a suffix
oop prefix = cast_to_oop(Atomic::xchg((oopDesc*)BUSY, &_overflow_list));
// Before "no_of_gc_threads" was introduced CMSOverflowSpinCount was
// set to ParallelGCThreads.
size_t CMSOverflowSpinCount = (size_t) no_of_gc_threads; // was ParallelGCThreads;
size_t sleep_time_millis = MAX2((size_t)1, num/100);
// If the list is busy, we spin for a short while,
// sleeping between attempts to get the list.
for (size_t spin = 0; prefix == BUSY && spin < CMSOverflowSpinCount; spin++) {
os::naked_sleep(sleep_time_millis);
if (_overflow_list == NULL) {
// Nothing left to take
return false;
} else if (_overflow_list != BUSY) {
// Try and grab the prefix
prefix = cast_to_oop(Atomic::xchg((oopDesc*)BUSY, &_overflow_list));
}
}
// If the list was found to be empty, or we spun long
// enough, we give up and return empty-handed. If we leave
// the list in the BUSY state below, it must be the case that
// some other thread holds the overflow list and will set it
// to a non-BUSY state in the future.
if (prefix == NULL || prefix == BUSY) {
// Nothing to take or waited long enough
if (prefix == NULL) {
// Write back the NULL in case we overwrote it with BUSY above
// and it is still the same value.
Atomic::cmpxchg((oopDesc*)NULL, &_overflow_list, (oopDesc*)BUSY);
}
return false;
}
assert(prefix != NULL && prefix != BUSY, "Error");
size_t i = num;
oop cur = prefix;
// Walk down the first "num" objects, unless we reach the end.
for (; i > 1 && cur->mark_raw().to_pointer() != NULL; cur = oop(cur->mark_raw().to_pointer()), i--);
if (cur->mark_raw().to_pointer() == NULL) {
// We have "num" or fewer elements in the list, so there
// is nothing to return to the global list.
// Write back the NULL in lieu of the BUSY we wrote
// above, if it is still the same value.
if (_overflow_list == BUSY) {
Atomic::cmpxchg((oopDesc*)NULL, &_overflow_list, (oopDesc*)BUSY);
}
} else {
// Chop off the suffix and return it to the global list.
assert(cur->mark_raw().to_pointer() != (void*)BUSY, "Error");
oop suffix_head = oop(cur->mark_raw().to_pointer()); // suffix will be put back on global list
cur->set_mark_raw(markWord::from_pointer(NULL)); // break off suffix
// It's possible that the list is still in the empty(busy) state
// we left it in a short while ago; in that case we may be
// able to place back the suffix without incurring the cost
// of a walk down the list.
oop observed_overflow_list = _overflow_list;
oop cur_overflow_list = observed_overflow_list;
bool attached = false;
while (observed_overflow_list == BUSY || observed_overflow_list == NULL) {
observed_overflow_list =
Atomic::cmpxchg((oopDesc*)suffix_head, &_overflow_list, (oopDesc*)cur_overflow_list);
if (cur_overflow_list == observed_overflow_list) {
attached = true;
break;
} else cur_overflow_list = observed_overflow_list;
}
if (!attached) {
// Too bad, someone else sneaked in (at least) an element; we'll need
// to do a splice. Find tail of suffix so we can prepend suffix to global
// list.
for (cur = suffix_head; cur->mark_raw().to_pointer() != NULL; cur = (oop)(cur->mark_raw().to_pointer()));
oop suffix_tail = cur;
assert(suffix_tail != NULL && suffix_tail->mark_raw().to_pointer() == NULL,
"Tautology");
observed_overflow_list = _overflow_list;
do {
cur_overflow_list = observed_overflow_list;
if (cur_overflow_list != BUSY) {
// Do the splice ...
suffix_tail->set_mark_raw(markWord::from_pointer((void*)cur_overflow_list));
} else { // cur_overflow_list == BUSY
suffix_tail->set_mark_raw(markWord::from_pointer(NULL));
}
// ... and try to place spliced list back on overflow_list ...
observed_overflow_list =
Atomic::cmpxchg((oopDesc*)suffix_head, &_overflow_list, (oopDesc*)cur_overflow_list);
} while (cur_overflow_list != observed_overflow_list);
// ... until we have succeeded in doing so.
}
}
// Push the prefix elements on work_q
assert(prefix != NULL, "control point invariant");
const markWord proto = markWord::prototype();
oop next;
NOT_PRODUCT(ssize_t n = 0;)
for (cur = prefix; cur != NULL; cur = next) {
next = oop(cur->mark_raw().to_pointer());
cur->set_mark_raw(proto); // until proven otherwise
assert(oopDesc::is_oop(cur), "Should be an oop");
bool res = work_q->push(cur);
assert(res, "Bit off more than we can chew?");
NOT_PRODUCT(n++;)
}
#ifndef PRODUCT
assert(_num_par_pushes >= n, "Too many pops?");
Atomic::sub(n, &_num_par_pushes);
#endif
return true;
}
// Single-threaded
void CMSCollector::push_on_overflow_list(oop p) {
NOT_PRODUCT(_num_par_pushes++;)
assert(oopDesc::is_oop(p), "Not an oop");
preserve_mark_if_necessary(p);
p->set_mark_raw(markWord::from_pointer(_overflow_list));
_overflow_list = p;
}
// Multi-threaded; use CAS to prepend to overflow list
void CMSCollector::par_push_on_overflow_list(oop p) {
NOT_PRODUCT(Atomic::inc(&_num_par_pushes);)
assert(oopDesc::is_oop(p), "Not an oop");
par_preserve_mark_if_necessary(p);
oop observed_overflow_list = _overflow_list;
oop cur_overflow_list;
do {
cur_overflow_list = observed_overflow_list;
if (cur_overflow_list != BUSY) {
p->set_mark_raw(markWord::from_pointer((void*)cur_overflow_list));
} else {
p->set_mark_raw(markWord::from_pointer(NULL));
}
observed_overflow_list =
Atomic::cmpxchg((oopDesc*)p, &_overflow_list, (oopDesc*)cur_overflow_list);
} while (cur_overflow_list != observed_overflow_list);
}
#undef BUSY
// Single threaded
// General Note on GrowableArray: pushes may silently fail
// because we are (temporarily) out of C-heap for expanding
// the stack. The problem is quite ubiquitous and affects
// a lot of code in the JVM. The prudent thing for GrowableArray
// to do (for now) is to exit with an error. However, that may
// be too draconian in some cases because the caller may be
// able to recover without much harm. For such cases, we
// should probably introduce a "soft_push" method which returns
// an indication of success or failure with the assumption that
// the caller may be able to recover from a failure; code in
// the VM can then be changed, incrementally, to deal with such
// failures where possible, thus, incrementally hardening the VM
// in such low resource situations.
void CMSCollector::preserve_mark_work(oop p, markWord m) {
_preserved_oop_stack.push(p);
_preserved_mark_stack.push(m);
assert(m == p->mark_raw(), "Mark word changed");
assert(_preserved_oop_stack.size() == _preserved_mark_stack.size(),
"bijection");
}
// Single threaded
void CMSCollector::preserve_mark_if_necessary(oop p) {
markWord m = p->mark_raw();
if (p->mark_must_be_preserved(m)) {
preserve_mark_work(p, m);
}
}
void CMSCollector::par_preserve_mark_if_necessary(oop p) {
markWord m = p->mark_raw();
if (p->mark_must_be_preserved(m)) {
MutexLocker x(ParGCRareEvent_lock, Mutex::_no_safepoint_check_flag);
// Even though we read the mark word without holding
// the lock, we are assured that it will not change
// because we "own" this oop, so no other thread can
// be trying to push it on the overflow list; see
// the assertion in preserve_mark_work() that checks
// that m == p->mark_raw().
preserve_mark_work(p, m);
}
}
// We should be able to do this multi-threaded,
// a chunk of stack being a task (this is
// correct because each oop only ever appears
// once in the overflow list. However, it's
// not very easy to completely overlap this with
// other operations, so will generally not be done
// until all work's been completed. Because we
// expect the preserved oop stack (set) to be small,
// it's probably fine to do this single-threaded.
// We can explore cleverer concurrent/overlapped/parallel
// processing of preserved marks if we feel the
// need for this in the future. Stack overflow should
// be so rare in practice and, when it happens, its
// effect on performance so great that this will
// likely just be in the noise anyway.
void CMSCollector::restore_preserved_marks_if_any() {
assert(SafepointSynchronize::is_at_safepoint(),
"world should be stopped");
assert(Thread::current()->is_ConcurrentGC_thread() ||
Thread::current()->is_VM_thread(),
"should be single-threaded");
assert(_preserved_oop_stack.size() == _preserved_mark_stack.size(),
"bijection");
while (!_preserved_oop_stack.is_empty()) {
oop p = _preserved_oop_stack.pop();
assert(oopDesc::is_oop(p), "Should be an oop");
assert(_span.contains(p), "oop should be in _span");
assert(p->mark_raw() == markWord::prototype(),
"Set when taken from overflow list");
markWord m = _preserved_mark_stack.pop();
p->set_mark_raw(m);
}
assert(_preserved_mark_stack.is_empty() && _preserved_oop_stack.is_empty(),
"stacks were cleared above");
}
#ifndef PRODUCT
bool CMSCollector::no_preserved_marks() const {
return _preserved_mark_stack.is_empty() && _preserved_oop_stack.is_empty();
}
#endif
// Transfer some number of overflown objects to usual marking
// stack. Return true if some objects were transferred.
bool MarkRefsIntoAndScanClosure::take_from_overflow_list() {
size_t num = MIN2((size_t)(_mark_stack->capacity() - _mark_stack->length())/4,
(size_t)ParGCDesiredObjsFromOverflowList);
bool res = _collector->take_from_overflow_list(num, _mark_stack);
assert(_collector->overflow_list_is_empty() || res,
"If list is not empty, we should have taken something");
assert(!res || !_mark_stack->isEmpty(),
"If we took something, it should now be on our stack");
return res;
}
size_t MarkDeadObjectsClosure::do_blk(HeapWord* addr) {
size_t res = _sp->block_size_no_stall(addr, _collector);
if (_sp->block_is_obj(addr)) {
if (_live_bit_map->isMarked(addr)) {
// It can't have been dead in a previous cycle
guarantee(!_dead_bit_map->isMarked(addr), "No resurrection!");
} else {
_dead_bit_map->mark(addr); // mark the dead object
}
}
// Could be 0, if the block size could not be computed without stalling.
return res;
}
TraceCMSMemoryManagerStats::TraceCMSMemoryManagerStats(CMSCollector::CollectorState phase, GCCause::Cause cause): TraceMemoryManagerStats() {
GCMemoryManager* manager = CMSHeap::heap()->old_manager();
switch (phase) {
case CMSCollector::InitialMarking:
initialize(manager /* GC manager */ ,
cause /* cause of the GC */,
true /* allMemoryPoolsAffected */,
true /* recordGCBeginTime */,
true /* recordPreGCUsage */,
false /* recordPeakUsage */,
false /* recordPostGCusage */,
true /* recordAccumulatedGCTime */,
false /* recordGCEndTime */,
false /* countCollection */ );
break;
case CMSCollector::FinalMarking:
initialize(manager /* GC manager */ ,
cause /* cause of the GC */,
true /* allMemoryPoolsAffected */,
false /* recordGCBeginTime */,
false /* recordPreGCUsage */,
false /* recordPeakUsage */,
false /* recordPostGCusage */,
true /* recordAccumulatedGCTime */,
false /* recordGCEndTime */,
false /* countCollection */ );
break;
case CMSCollector::Sweeping:
initialize(manager /* GC manager */ ,
cause /* cause of the GC */,
true /* allMemoryPoolsAffected */,
false /* recordGCBeginTime */,
false /* recordPreGCUsage */,
true /* recordPeakUsage */,
true /* recordPostGCusage */,
false /* recordAccumulatedGCTime */,
true /* recordGCEndTime */,
true /* countCollection */ );
break;
default:
ShouldNotReachHere();
}
}