8202017: Merge Reference Enqueuing phase with phase 3 of Reference processing
Summary: Do reference enqueuing work directly in phase 3 after every Reference.
Reviewed-by: kbarrett, sangheki
/*
* Copyright (c) 2001, 2018, 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.
*
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* 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
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*/
#include "precompiled.hpp"
#include "classfile/javaClasses.inline.hpp"
#include "classfile/systemDictionary.hpp"
#include "gc/shared/collectedHeap.hpp"
#include "gc/shared/collectedHeap.inline.hpp"
#include "gc/shared/gcTimer.hpp"
#include "gc/shared/gcTraceTime.inline.hpp"
#include "gc/shared/referencePolicy.hpp"
#include "gc/shared/referenceProcessor.inline.hpp"
#include "logging/log.hpp"
#include "memory/allocation.inline.hpp"
#include "memory/resourceArea.hpp"
#include "oops/access.inline.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/java.hpp"
ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
void referenceProcessor_init() {
ReferenceProcessor::init_statics();
}
void ReferenceProcessor::init_statics() {
// We need a monotonically non-decreasing time in ms but
// os::javaTimeMillis() does not guarantee monotonicity.
jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
// Initialize the soft ref timestamp clock.
_soft_ref_timestamp_clock = now;
// Also update the soft ref clock in j.l.r.SoftReference
java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
_always_clear_soft_ref_policy = new AlwaysClearPolicy();
if (is_server_compilation_mode_vm()) {
_default_soft_ref_policy = new LRUMaxHeapPolicy();
} else {
_default_soft_ref_policy = new LRUCurrentHeapPolicy();
}
if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
vm_exit_during_initialization("Could not allocate reference policy object");
}
guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
RefDiscoveryPolicy == ReferentBasedDiscovery,
"Unrecognized RefDiscoveryPolicy");
}
void ReferenceProcessor::enable_discovery(bool check_no_refs) {
#ifdef ASSERT
// Verify that we're not currently discovering refs
assert(!_discovering_refs, "nested call?");
if (check_no_refs) {
// Verify that the discovered lists are empty
verify_no_references_recorded();
}
#endif // ASSERT
// Someone could have modified the value of the static
// field in the j.l.r.SoftReference class that holds the
// soft reference timestamp clock using reflection or
// Unsafe between GCs. Unconditionally update the static
// field in ReferenceProcessor here so that we use the new
// value during reference discovery.
_soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
_discovering_refs = true;
}
ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
bool mt_processing,
uint mt_processing_degree,
bool mt_discovery,
uint mt_discovery_degree,
bool atomic_discovery,
BoolObjectClosure* is_alive_non_header) :
_is_subject_to_discovery(is_subject_to_discovery),
_discovering_refs(false),
_enqueuing_is_done(false),
_is_alive_non_header(is_alive_non_header),
_processing_is_mt(mt_processing),
_next_id(0)
{
assert(is_subject_to_discovery != NULL, "must be set");
_discovery_is_atomic = atomic_discovery;
_discovery_is_mt = mt_discovery;
_num_queues = MAX2(1U, mt_processing_degree);
_max_num_queues = MAX2(_num_queues, mt_discovery_degree);
_discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
_max_num_queues * number_of_subclasses_of_ref(), mtGC);
if (_discovered_refs == NULL) {
vm_exit_during_initialization("Could not allocated RefProc Array");
}
_discoveredSoftRefs = &_discovered_refs[0];
_discoveredWeakRefs = &_discoveredSoftRefs[_max_num_queues];
_discoveredFinalRefs = &_discoveredWeakRefs[_max_num_queues];
_discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues];
// Initialize all entries to NULL
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
_discovered_refs[i].set_head(NULL);
_discovered_refs[i].set_length(0);
}
setup_policy(false /* default soft ref policy */);
}
#ifndef PRODUCT
void ReferenceProcessor::verify_no_references_recorded() {
guarantee(!_discovering_refs, "Discovering refs?");
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
guarantee(_discovered_refs[i].is_empty(),
"Found non-empty discovered list at %u", i);
}
}
#endif
void ReferenceProcessor::weak_oops_do(OopClosure* f) {
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
if (UseCompressedOops) {
f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
} else {
f->do_oop((oop*)_discovered_refs[i].adr_head());
}
}
}
void ReferenceProcessor::update_soft_ref_master_clock() {
// Update (advance) the soft ref master clock field. This must be done
// after processing the soft ref list.
// We need a monotonically non-decreasing time in ms but
// os::javaTimeMillis() does not guarantee monotonicity.
jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
NOT_PRODUCT(
if (now < _soft_ref_timestamp_clock) {
log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
_soft_ref_timestamp_clock, now);
}
)
// The values of now and _soft_ref_timestamp_clock are set using
// javaTimeNanos(), which is guaranteed to be monotonically
// non-decreasing provided the underlying platform provides such
// a time source (and it is bug free).
// In product mode, however, protect ourselves from non-monotonicity.
if (now > _soft_ref_timestamp_clock) {
_soft_ref_timestamp_clock = now;
java_lang_ref_SoftReference::set_clock(now);
}
// Else leave clock stalled at its old value until time progresses
// past clock value.
}
size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
size_t total = 0;
for (uint i = 0; i < _max_num_queues; ++i) {
total += lists[i].length();
}
return total;
}
ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc,
AbstractRefProcTaskExecutor* task_executor,
ReferenceProcessorPhaseTimes* phase_times) {
double start_time = os::elapsedTime();
assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
// Stop treating discovered references specially.
disable_discovery();
// If discovery was concurrent, someone could have modified
// the value of the static field in the j.l.r.SoftReference
// class that holds the soft reference timestamp clock using
// reflection or Unsafe between when discovery was enabled and
// now. Unconditionally update the static field in ReferenceProcessor
// here so that we use the new value during processing of the
// discovered soft refs.
_soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
total_count(_discoveredWeakRefs),
total_count(_discoveredFinalRefs),
total_count(_discoveredPhantomRefs));
// Soft references
{
RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this);
process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
is_alive, keep_alive, complete_gc, task_executor, phase_times);
}
update_soft_ref_master_clock();
// Weak references
{
RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this);
process_discovered_reflist(_discoveredWeakRefs, NULL, true,
is_alive, keep_alive, complete_gc, task_executor, phase_times);
}
// Final references
{
RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this);
process_discovered_reflist(_discoveredFinalRefs, NULL, false,
is_alive, keep_alive, complete_gc, task_executor, phase_times);
}
// Phantom references
{
RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this);
process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
is_alive, keep_alive, complete_gc, task_executor, phase_times);
}
if (task_executor != NULL) {
// Record the work done by the parallel workers.
task_executor->set_single_threaded_mode();
}
phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
return stats;
}
void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
_current_discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_current_discovered);
oop discovered = java_lang_ref_Reference::discovered(_current_discovered);
assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
"Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
_next_discovered = discovered;
_referent_addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
_referent = java_lang_ref_Reference::referent(_current_discovered);
assert(Universe::heap()->is_in_reserved_or_null(_referent),
"Wrong oop found in java.lang.Reference object");
assert(allow_null_referent ?
oopDesc::is_oop_or_null(_referent)
: oopDesc::is_oop(_referent),
"Expected an oop%s for referent field at " PTR_FORMAT,
(allow_null_referent ? " or NULL" : ""),
p2i(_referent));
}
void DiscoveredListIterator::remove() {
assert(oopDesc::is_oop(_current_discovered), "Dropping a bad reference");
RawAccess<>::oop_store(_current_discovered_addr, oop(NULL));
// First _prev_next ref actually points into DiscoveredList (gross).
oop new_next;
if (_next_discovered == _current_discovered) {
// At the end of the list, we should make _prev point to itself.
// If _ref is the first ref, then _prev_next will be in the DiscoveredList,
// and _prev will be NULL.
new_next = _prev_discovered;
} else {
new_next = _next_discovered;
}
// Remove Reference object from discovered list. Note that G1 does not need a
// pre-barrier here because we know the Reference has already been found/marked,
// that's how it ended up in the discovered list in the first place.
RawAccess<>::oop_store(_prev_discovered_addr, new_next);
NOT_PRODUCT(_removed++);
_refs_list.dec_length(1);
}
void DiscoveredListIterator::clear_referent() {
RawAccess<>::oop_store(_referent_addr, oop(NULL));
}
void DiscoveredListIterator::enqueue() {
// Self-loop next, so as to make Ref not active.
java_lang_ref_Reference::set_next_raw(_current_discovered, _current_discovered);
HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_current_discovered,
java_lang_ref_Reference::discovered_offset,
_next_discovered);
}
void DiscoveredListIterator::complete_enqeue() {
if (_prev_discovered != NULL) {
// This is the last object.
// Swap refs_list into pending list and set obj's
// discovered to what we read from the pending list.
oop old = Universe::swap_reference_pending_list(_refs_list.head());
HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_prev_discovered, java_lang_ref_Reference::discovered_offset, old);
}
}
// NOTE: process_phase*() are largely similar, and at a high level
// merely iterate over the extant list applying a predicate to
// each of its elements and possibly removing that element from the
// list and applying some further closures to that element.
// We should consider the possibility of replacing these
// process_phase*() methods by abstracting them into
// a single general iterator invocation that receives appropriate
// closures that accomplish this work.
// (SoftReferences only) Traverse the list and remove any SoftReferences whose
// referents are not alive, but that should be kept alive for policy reasons.
// Keep alive the transitive closure of all such referents.
void
ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
ReferencePolicy* policy,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc) {
assert(policy != NULL, "Must have a non-NULL policy");
DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
// Decide which softly reachable refs should be kept alive.
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
if (referent_is_dead &&
!policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
p2i(iter.obj()), iter.obj()->klass()->internal_name());
// Remove Reference object from list
iter.remove();
// keep the referent around
iter.make_referent_alive();
iter.move_to_next();
} else {
iter.next();
}
}
// Close the reachable set
complete_gc->do_void();
log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
iter.removed(), iter.processed(), p2i(&refs_list));
}
void ReferenceProcessor::process_phase2(DiscoveredList& refs_list,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc) {
if (discovery_is_atomic()) {
// complete_gc is ignored in this case for this phase
pp2_work(refs_list, is_alive, keep_alive);
} else {
assert(complete_gc != NULL, "Error");
pp2_work_concurrent_discovery(refs_list, is_alive,
keep_alive, complete_gc);
}
}
// Traverse the list and remove any Refs that are not active, or
// whose referents are either alive or NULL.
void
ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
BoolObjectClosure* is_alive,
OopClosure* keep_alive) {
assert(discovery_is_atomic(), "Error");
DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
assert(next == NULL, "Should not discover inactive Reference");
if (iter.is_referent_alive()) {
log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
p2i(iter.obj()), iter.obj()->klass()->internal_name());
// The referent is reachable after all.
// Remove Reference object from list.
iter.remove();
// Update the referent pointer as necessary: Note that this
// should not entail any recursive marking because the
// referent must already have been traversed.
iter.make_referent_alive();
iter.move_to_next();
} else {
iter.next();
}
}
NOT_PRODUCT(
if (iter.processed() > 0) {
log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
" Refs in discovered list " INTPTR_FORMAT,
iter.removed(), iter.processed(), p2i(&refs_list));
}
)
}
void
ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc) {
assert(!discovery_is_atomic(), "Error");
DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
HeapWord* next_addr = java_lang_ref_Reference::next_addr_raw(iter.obj());
oop next = java_lang_ref_Reference::next(iter.obj());
if ((iter.referent() == NULL || iter.is_referent_alive() ||
next != NULL)) {
assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
// Remove Reference object from list
iter.remove();
// Trace the cohorts
iter.make_referent_alive();
if (UseCompressedOops) {
keep_alive->do_oop((narrowOop*)next_addr);
} else {
keep_alive->do_oop((oop*)next_addr);
}
iter.move_to_next();
} else {
iter.next();
}
}
// Now close the newly reachable set
complete_gc->do_void();
NOT_PRODUCT(
if (iter.processed() > 0) {
log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
" Refs in discovered list " INTPTR_FORMAT,
iter.removed(), iter.processed(), p2i(&refs_list));
}
)
}
void ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
bool clear_referent,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc) {
ResourceMark rm;
DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
if (clear_referent) {
// NULL out referent pointer
iter.clear_referent();
} else {
// keep the referent around
iter.make_referent_alive();
}
iter.enqueue();
log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
iter.next();
}
iter.complete_enqeue();
// Close the reachable set
complete_gc->do_void();
// Clear the list.
refs_list.set_head(NULL);
refs_list.set_length(0);
}
void
ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
oop obj = NULL;
oop next = refs_list.head();
while (next != obj) {
obj = next;
next = java_lang_ref_Reference::discovered(obj);
java_lang_ref_Reference::set_discovered_raw(obj, NULL);
}
refs_list.set_head(NULL);
refs_list.set_length(0);
}
void ReferenceProcessor::abandon_partial_discovery() {
// loop over the lists
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
if ((i % _max_num_queues) == 0) {
log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
}
clear_discovered_references(_discovered_refs[i]);
}
}
size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
DiscoveredList* list = NULL;
switch (type) {
case REF_SOFT:
list = _discoveredSoftRefs;
break;
case REF_WEAK:
list = _discoveredWeakRefs;
break;
case REF_FINAL:
list = _discoveredFinalRefs;
break;
case REF_PHANTOM:
list = _discoveredPhantomRefs;
break;
case REF_OTHER:
case REF_NONE:
default:
ShouldNotReachHere();
}
return total_count(list);
}
class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
public:
RefProcPhase1Task(ReferenceProcessor& ref_processor,
DiscoveredList refs_lists[],
ReferencePolicy* policy,
bool marks_oops_alive,
ReferenceProcessorPhaseTimes* phase_times)
: ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
_policy(policy)
{ }
virtual void work(unsigned int i, BoolObjectClosure& is_alive,
OopClosure& keep_alive,
VoidClosure& complete_gc)
{
RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
_ref_processor.process_phase1(_refs_lists[i], _policy,
&is_alive, &keep_alive, &complete_gc);
}
private:
ReferencePolicy* _policy;
};
class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
public:
RefProcPhase2Task(ReferenceProcessor& ref_processor,
DiscoveredList refs_lists[],
bool marks_oops_alive,
ReferenceProcessorPhaseTimes* phase_times)
: ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
{ }
virtual void work(unsigned int i, BoolObjectClosure& is_alive,
OopClosure& keep_alive,
VoidClosure& complete_gc)
{
RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
_ref_processor.process_phase2(_refs_lists[i],
&is_alive, &keep_alive, &complete_gc);
}
};
class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
public:
RefProcPhase3Task(ReferenceProcessor& ref_processor,
DiscoveredList refs_lists[],
bool clear_referent,
bool marks_oops_alive,
ReferenceProcessorPhaseTimes* phase_times)
: ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
_clear_referent(clear_referent)
{ }
virtual void work(unsigned int i, BoolObjectClosure& is_alive,
OopClosure& keep_alive,
VoidClosure& complete_gc)
{
RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
_ref_processor.process_phase3(_refs_lists[i], _clear_referent,
&is_alive, &keep_alive, &complete_gc);
}
private:
bool _clear_referent;
};
#ifndef PRODUCT
void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
if (!log_is_enabled(Trace, gc, ref)) {
return;
}
stringStream st;
for (uint i = 0; i < active_length; ++i) {
st.print(SIZE_FORMAT " ", ref_lists[i].length());
}
log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
#ifdef ASSERT
for (uint i = active_length; i < _max_num_queues; i++) {
assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
ref_lists[i].length(), i);
}
#endif
}
#endif
void ReferenceProcessor::set_active_mt_degree(uint v) {
_num_queues = v;
_next_id = 0;
}
// Balances reference queues.
// Move entries from all queues[0, 1, ..., _max_num_q-1] to
// queues[0, 1, ..., _num_q-1] because only the first _num_q
// corresponding to the active workers will be processed.
void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
{
// calculate total length
size_t total_refs = 0;
log_develop_trace(gc, ref)("Balance ref_lists ");
for (uint i = 0; i < _max_num_queues; ++i) {
total_refs += ref_lists[i].length();
}
log_reflist_counts(ref_lists, _max_num_queues, total_refs);
size_t avg_refs = total_refs / _num_queues + 1;
uint to_idx = 0;
for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
bool move_all = false;
if (from_idx >= _num_queues) {
move_all = ref_lists[from_idx].length() > 0;
}
while ((ref_lists[from_idx].length() > avg_refs) ||
move_all) {
assert(to_idx < _num_queues, "Sanity Check!");
if (ref_lists[to_idx].length() < avg_refs) {
// move superfluous refs
size_t refs_to_move;
// Move all the Ref's if the from queue will not be processed.
if (move_all) {
refs_to_move = MIN2(ref_lists[from_idx].length(),
avg_refs - ref_lists[to_idx].length());
} else {
refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
avg_refs - ref_lists[to_idx].length());
}
assert(refs_to_move > 0, "otherwise the code below will fail");
oop move_head = ref_lists[from_idx].head();
oop move_tail = move_head;
oop new_head = move_head;
// find an element to split the list on
for (size_t j = 0; j < refs_to_move; ++j) {
move_tail = new_head;
new_head = java_lang_ref_Reference::discovered(new_head);
}
// Add the chain to the to list.
if (ref_lists[to_idx].head() == NULL) {
// to list is empty. Make a loop at the end.
java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
} else {
java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
}
ref_lists[to_idx].set_head(move_head);
ref_lists[to_idx].inc_length(refs_to_move);
// Remove the chain from the from list.
if (move_tail == new_head) {
// We found the end of the from list.
ref_lists[from_idx].set_head(NULL);
} else {
ref_lists[from_idx].set_head(new_head);
}
ref_lists[from_idx].dec_length(refs_to_move);
if (ref_lists[from_idx].length() == 0) {
break;
}
} else {
to_idx = (to_idx + 1) % _num_queues;
}
}
}
#ifdef ASSERT
size_t balanced_total_refs = 0;
for (uint i = 0; i < _num_queues; ++i) {
balanced_total_refs += ref_lists[i].length();
}
log_reflist_counts(ref_lists, _num_queues, balanced_total_refs);
assert(total_refs == balanced_total_refs, "Balancing was incomplete");
#endif
}
void ReferenceProcessor::process_discovered_reflist(
DiscoveredList refs_lists[],
ReferencePolicy* policy,
bool clear_referent,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc,
AbstractRefProcTaskExecutor* task_executor,
ReferenceProcessorPhaseTimes* phase_times)
{
bool mt_processing = task_executor != NULL && _processing_is_mt;
phase_times->set_processing_is_mt(mt_processing);
if (mt_processing && ParallelRefProcBalancingEnabled) {
RefProcBalanceQueuesTimeTracker tt(phase_times);
balance_queues(refs_lists);
}
// Phase 1 (soft refs only):
// . Traverse the list and remove any SoftReferences whose
// referents are not alive, but that should be kept alive for
// policy reasons. Keep alive the transitive closure of all
// such referents.
if (policy != NULL) {
RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
if (mt_processing) {
RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
task_executor->execute(phase1);
} else {
for (uint i = 0; i < _max_num_queues; i++) {
process_phase1(refs_lists[i], policy,
is_alive, keep_alive, complete_gc);
}
}
} else { // policy == NULL
assert(refs_lists != _discoveredSoftRefs,
"Policy must be specified for soft references.");
}
// Phase 2:
// . Traverse the list and remove any refs whose referents are alive.
{
RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
if (mt_processing) {
RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
task_executor->execute(phase2);
} else {
for (uint i = 0; i < _max_num_queues; i++) {
process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
}
}
}
// Phase 3:
// . Traverse the list and process referents as appropriate.
{
RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
if (mt_processing) {
RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
task_executor->execute(phase3);
} else {
for (uint i = 0; i < _max_num_queues; i++) {
process_phase3(refs_lists[i], clear_referent,
is_alive, keep_alive, complete_gc);
}
}
}
}
inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
uint id = 0;
// Determine the queue index to use for this object.
if (_discovery_is_mt) {
// During a multi-threaded discovery phase,
// each thread saves to its "own" list.
Thread* thr = Thread::current();
id = thr->as_Worker_thread()->id();
} else {
// single-threaded discovery, we save in round-robin
// fashion to each of the lists.
if (_processing_is_mt) {
id = next_id();
}
}
assert(id < _max_num_queues, "Id is out-of-bounds id %u and max id %u)", id, _max_num_queues);
// Get the discovered queue to which we will add
DiscoveredList* list = NULL;
switch (rt) {
case REF_OTHER:
// Unknown reference type, no special treatment
break;
case REF_SOFT:
list = &_discoveredSoftRefs[id];
break;
case REF_WEAK:
list = &_discoveredWeakRefs[id];
break;
case REF_FINAL:
list = &_discoveredFinalRefs[id];
break;
case REF_PHANTOM:
list = &_discoveredPhantomRefs[id];
break;
case REF_NONE:
// we should not reach here if we are an InstanceRefKlass
default:
ShouldNotReachHere();
}
log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
return list;
}
inline void
ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
oop obj,
HeapWord* discovered_addr) {
assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
// First we must make sure this object is only enqueued once. CAS in a non null
// discovered_addr.
oop current_head = refs_list.head();
// The last ref must have its discovered field pointing to itself.
oop next_discovered = (current_head != NULL) ? current_head : obj;
oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
if (retest == NULL) {
// This thread just won the right to enqueue the object.
// We have separate lists for enqueueing, so no synchronization
// is necessary.
refs_list.set_head(obj);
refs_list.inc_length(1);
log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
p2i(obj), obj->klass()->internal_name());
} else {
// If retest was non NULL, another thread beat us to it:
// The reference has already been discovered...
log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
p2i(obj), obj->klass()->internal_name());
}
}
#ifndef PRODUCT
// Non-atomic (i.e. concurrent) discovery might allow us
// to observe j.l.References with NULL referents, being those
// cleared concurrently by mutators during (or after) discovery.
void ReferenceProcessor::verify_referent(oop obj) {
bool da = discovery_is_atomic();
oop referent = java_lang_ref_Reference::referent(obj);
assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
"Bad referent " INTPTR_FORMAT " found in Reference "
INTPTR_FORMAT " during %satomic discovery ",
p2i(referent), p2i(obj), da ? "" : "non-");
}
#endif
bool ReferenceProcessor::is_subject_to_discovery(oop const obj) const {
return _is_subject_to_discovery->do_object_b(obj);
}
// We mention two of several possible choices here:
// #0: if the reference object is not in the "originating generation"
// (or part of the heap being collected, indicated by our "span"
// we don't treat it specially (i.e. we scan it as we would
// a normal oop, treating its references as strong references).
// This means that references can't be discovered unless their
// referent is also in the same span. This is the simplest,
// most "local" and most conservative approach, albeit one
// that may cause weak references to be enqueued least promptly.
// We call this choice the "ReferenceBasedDiscovery" policy.
// #1: the reference object may be in any generation (span), but if
// the referent is in the generation (span) being currently collected
// then we can discover the reference object, provided
// the object has not already been discovered by
// a different concurrently running collector (as may be the
// case, for instance, if the reference object is in CMS and
// the referent in DefNewGeneration), and provided the processing
// of this reference object by the current collector will
// appear atomic to every other collector in the system.
// (Thus, for instance, a concurrent collector may not
// discover references in other generations even if the
// referent is in its own generation). This policy may,
// in certain cases, enqueue references somewhat sooner than
// might Policy #0 above, but at marginally increased cost
// and complexity in processing these references.
// We call this choice the "RefeferentBasedDiscovery" policy.
bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
// Make sure we are discovering refs (rather than processing discovered refs).
if (!_discovering_refs || !RegisterReferences) {
return false;
}
// We only discover active references.
oop next = java_lang_ref_Reference::next(obj);
if (next != NULL) { // Ref is no longer active
return false;
}
if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
!is_subject_to_discovery(obj)) {
// Reference is not in the originating generation;
// don't treat it specially (i.e. we want to scan it as a normal
// object with strong references).
return false;
}
// We only discover references whose referents are not (yet)
// known to be strongly reachable.
if (is_alive_non_header() != NULL) {
verify_referent(obj);
if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
return false; // referent is reachable
}
}
if (rt == REF_SOFT) {
// For soft refs we can decide now if these are not
// current candidates for clearing, in which case we
// can mark through them now, rather than delaying that
// to the reference-processing phase. Since all current
// time-stamp policies advance the soft-ref clock only
// at a full collection cycle, this is always currently
// accurate.
if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
return false;
}
}
ResourceMark rm; // Needed for tracing.
HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
const oop discovered = java_lang_ref_Reference::discovered(obj);
assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
if (discovered != NULL) {
// The reference has already been discovered...
log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
p2i(obj), obj->klass()->internal_name());
if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
// assumes that an object is not processed twice;
// if it's been already discovered it must be on another
// generation's discovered list; so we won't discover it.
return false;
} else {
assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
"Unrecognized policy");
// Check assumption that an object is not potentially
// discovered twice except by concurrent collectors that potentially
// trace the same Reference object twice.
assert(UseConcMarkSweepGC || UseG1GC,
"Only possible with a concurrent marking collector");
return true;
}
}
if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
verify_referent(obj);
// Discover if and only if EITHER:
// .. reference is in our span, OR
// .. we are an atomic collector and referent is in our span
if (is_subject_to_discovery(obj) ||
(discovery_is_atomic() &&
is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
} else {
return false;
}
} else {
assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
is_subject_to_discovery(obj), "code inconsistency");
}
// Get the right type of discovered queue head.
DiscoveredList* list = get_discovered_list(rt);
if (list == NULL) {
return false; // nothing special needs to be done
}
if (_discovery_is_mt) {
add_to_discovered_list_mt(*list, obj, discovered_addr);
} else {
// We do a raw store here: the field will be visited later when processing
// the discovered references.
oop current_head = list->head();
// The last ref must have its discovered field pointing to itself.
oop next_discovered = (current_head != NULL) ? current_head : obj;
assert(discovered == NULL, "control point invariant");
RawAccess<>::oop_store(discovered_addr, next_discovered);
list->set_head(obj);
list->inc_length(1);
log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
}
assert(oopDesc::is_oop(obj), "Discovered a bad reference");
verify_referent(obj);
return true;
}
bool ReferenceProcessor::has_discovered_references() {
for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
if (!_discovered_refs[i].is_empty()) {
return true;
}
}
return false;
}
// Preclean the discovered references by removing those
// whose referents are alive, and by marking from those that
// are not active. These lists can be handled here
// in any order and, indeed, concurrently.
void ReferenceProcessor::preclean_discovered_references(
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc,
YieldClosure* yield,
GCTimer* gc_timer) {
// Soft references
{
GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
keep_alive, complete_gc, yield);
}
}
// Weak references
{
GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
keep_alive, complete_gc, yield);
}
}
// Final references
{
GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
keep_alive, complete_gc, yield);
}
}
// Phantom references
{
GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
for (uint i = 0; i < _max_num_queues; i++) {
if (yield->should_return()) {
return;
}
preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
keep_alive, complete_gc, yield);
}
}
}
// Walk the given discovered ref list, and remove all reference objects
// whose referents are still alive, whose referents are NULL or which
// are not active (have a non-NULL next field). NOTE: When we are
// thus precleaning the ref lists (which happens single-threaded today),
// we do not disable refs discovery to honor the correct semantics of
// java.lang.Reference. As a result, we need to be careful below
// that ref removal steps interleave safely with ref discovery steps
// (in this thread).
void
ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
BoolObjectClosure* is_alive,
OopClosure* keep_alive,
VoidClosure* complete_gc,
YieldClosure* yield) {
DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
while (iter.has_next()) {
iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
oop obj = iter.obj();
oop next = java_lang_ref_Reference::next(obj);
if (iter.referent() == NULL || iter.is_referent_alive() ||
next != NULL) {
// The referent has been cleared, or is alive, or the Reference is not
// active; we need to trace and mark its cohort.
log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
p2i(iter.obj()), iter.obj()->klass()->internal_name());
// Remove Reference object from list
iter.remove();
// Keep alive its cohort.
iter.make_referent_alive();
if (UseCompressedOops) {
narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr_raw(obj);
keep_alive->do_oop(next_addr);
} else {
oop* next_addr = (oop*)java_lang_ref_Reference::next_addr_raw(obj);
keep_alive->do_oop(next_addr);
}
iter.move_to_next();
} else {
iter.next();
}
}
// Close the reachable set
complete_gc->do_void();
NOT_PRODUCT(
if (iter.processed() > 0) {
log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
iter.removed(), iter.processed(), p2i(&refs_list));
}
)
}
const char* ReferenceProcessor::list_name(uint i) {
assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
"Out of bounds index");
int j = i / _max_num_queues;
switch (j) {
case 0: return "SoftRef";
case 1: return "WeakRef";
case 2: return "FinalRef";
case 3: return "PhantomRef";
}
ShouldNotReachHere();
return NULL;
}