/*
* Copyright (c) 2018, 2019, Red Hat, Inc. All rights reserved.
*
* 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 "gc/shenandoah/shenandoahFreeSet.hpp"
#include "gc/shenandoah/shenandoahHeap.inline.hpp"
#include "gc/shenandoah/shenandoahPacer.hpp"
#include "runtime/atomic.hpp"
/*
* In normal concurrent cycle, we have to pace the application to let GC finish.
*
* Here, we do not know how large would be the collection set, and what are the
* relative performances of the each stage in the concurrent cycle, and so we have to
* make some assumptions.
*
* For concurrent mark, there is no clear notion of progress. The moderately accurate
* and easy to get metric is the amount of live objects the mark had encountered. But,
* that does directly correlate with the used heap, because the heap might be fully
* dead or fully alive. We cannot assume either of the extremes: we would either allow
* application to run out of memory if we assume heap is fully dead but it is not, and,
* conversely, we would pacify application excessively if we assume heap is fully alive
* but it is not. So we need to guesstimate the particular expected value for heap liveness.
* The best way to do this is apparently recording the past history.
*
* For concurrent evac and update-refs, we are walking the heap per-region, and so the
* notion of progress is clear: we get reported the "used" size from the processed regions
* and use the global heap-used as the baseline.
*
* The allocatable space when GC is running is "free" at the start of cycle, but the
* accounted budget is based on "used". So, we need to adjust the tax knowing that.
* Also, since we effectively count the used space three times (mark, evac, update-refs),
* we need to multiply the tax by 3. Example: for 10 MB free and 90 MB used, GC would
* come back with 3*90 MB budget, and thus for each 1 MB of allocation, we have to pay
* 3*90 / 10 MBs. In the end, we would pay back the entire budget.
*/
void ShenandoahPacer::setup_for_mark() {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
size_t live = update_and_get_progress_history();
size_t free = _heap->free_set()->available();
size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
size_t taxable = free - non_taxable;
double tax = 1.0 * live / taxable; // base tax for available free space
tax *= 3; // mark is phase 1 of 3, claim 1/3 of free for it
tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
restart_with(non_taxable, tax);
log_info(gc, ergo)("Pacer for Mark. Expected Live: " SIZE_FORMAT "%s, Free: " SIZE_FORMAT "%s, "
"Non-Taxable: " SIZE_FORMAT "%s, Alloc Tax Rate: %.1fx",
byte_size_in_proper_unit(live), proper_unit_for_byte_size(live),
byte_size_in_proper_unit(free), proper_unit_for_byte_size(free),
byte_size_in_proper_unit(non_taxable), proper_unit_for_byte_size(non_taxable),
tax);
}
void ShenandoahPacer::setup_for_evac() {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
size_t used = _heap->collection_set()->used();
size_t free = _heap->free_set()->available();
size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
size_t taxable = free - non_taxable;
double tax = 1.0 * used / taxable; // base tax for available free space
tax *= 2; // evac is phase 2 of 3, claim 1/2 of remaining free
tax = MAX2<double>(1, tax); // never allocate more than GC processes during the phase
tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
restart_with(non_taxable, tax);
log_info(gc, ergo)("Pacer for Evacuation. Used CSet: " SIZE_FORMAT "%s, Free: " SIZE_FORMAT "%s, "
"Non-Taxable: " SIZE_FORMAT "%s, Alloc Tax Rate: %.1fx",
byte_size_in_proper_unit(used), proper_unit_for_byte_size(used),
byte_size_in_proper_unit(free), proper_unit_for_byte_size(free),
byte_size_in_proper_unit(non_taxable), proper_unit_for_byte_size(non_taxable),
tax);
}
void ShenandoahPacer::setup_for_updaterefs() {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
size_t used = _heap->used();
size_t free = _heap->free_set()->available();
size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
size_t taxable = free - non_taxable;
double tax = 1.0 * used / taxable; // base tax for available free space
tax *= 1; // update-refs is phase 3 of 3, claim the remaining free
tax = MAX2<double>(1, tax); // never allocate more than GC processes during the phase
tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
restart_with(non_taxable, tax);
log_info(gc, ergo)("Pacer for Update Refs. Used: " SIZE_FORMAT "%s, Free: " SIZE_FORMAT "%s, "
"Non-Taxable: " SIZE_FORMAT "%s, Alloc Tax Rate: %.1fx",
byte_size_in_proper_unit(used), proper_unit_for_byte_size(used),
byte_size_in_proper_unit(free), proper_unit_for_byte_size(free),
byte_size_in_proper_unit(non_taxable), proper_unit_for_byte_size(non_taxable),
tax);
}
/*
* Traversal walks the entire heap once, and therefore we have to make assumptions about its
* liveness, like concurrent mark does.
*/
void ShenandoahPacer::setup_for_traversal() {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
size_t live = update_and_get_progress_history();
size_t free = _heap->free_set()->available();
size_t non_taxable = free * ShenandoahPacingCycleSlack / 100;
size_t taxable = free - non_taxable;
double tax = 1.0 * live / taxable; // base tax for available free space
tax *= ShenandoahPacingSurcharge; // additional surcharge to help unclutter heap
restart_with(non_taxable, tax);
log_info(gc, ergo)("Pacer for Traversal. Expected Live: " SIZE_FORMAT "%s, Free: " SIZE_FORMAT "%s, "
"Non-Taxable: " SIZE_FORMAT "%s, Alloc Tax Rate: %.1fx",
byte_size_in_proper_unit(live), proper_unit_for_byte_size(live),
byte_size_in_proper_unit(free), proper_unit_for_byte_size(free),
byte_size_in_proper_unit(non_taxable), proper_unit_for_byte_size(non_taxable),
tax);
}
/*
* In idle phase, we have to pace the application to let control thread react with GC start.
*
* Here, we have rendezvous with concurrent thread that adds up the budget as it acknowledges
* it had seen recent allocations. It will naturally pace the allocations if control thread is
* not catching up. To bootstrap this feedback cycle, we need to start with some initial budget
* for applications to allocate at.
*/
void ShenandoahPacer::setup_for_idle() {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
size_t initial = _heap->max_capacity() / 100 * ShenandoahPacingIdleSlack;
double tax = 1;
restart_with(initial, tax);
log_info(gc, ergo)("Pacer for Idle. Initial: " SIZE_FORMAT "%s, Alloc Tax Rate: %.1fx",
byte_size_in_proper_unit(initial), proper_unit_for_byte_size(initial),
tax);
}
size_t ShenandoahPacer::update_and_get_progress_history() {
if (_progress == -1) {
// First initialization, report some prior
Atomic::store(&_progress, (intptr_t)PACING_PROGRESS_ZERO);
return (size_t) (_heap->max_capacity() * 0.1);
} else {
// Record history, and reply historical data
_progress_history->add(_progress);
Atomic::store(&_progress, (intptr_t)PACING_PROGRESS_ZERO);
return (size_t) (_progress_history->avg() * HeapWordSize);
}
}
void ShenandoahPacer::restart_with(size_t non_taxable_bytes, double tax_rate) {
size_t initial = (size_t)(non_taxable_bytes * tax_rate) >> LogHeapWordSize;
STATIC_ASSERT(sizeof(size_t) <= sizeof(intptr_t));
Atomic::xchg(&_budget, (intptr_t)initial);
Atomic::store(&_tax_rate, tax_rate);
Atomic::inc(&_epoch);
}
bool ShenandoahPacer::claim_for_alloc(size_t words, bool force) {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
intptr_t tax = MAX2<intptr_t>(1, words * Atomic::load(&_tax_rate));
intptr_t cur = 0;
intptr_t new_val = 0;
do {
cur = Atomic::load(&_budget);
if (cur < tax && !force) {
// Progress depleted, alas.
return false;
}
new_val = cur - tax;
} while (Atomic::cmpxchg(&_budget, cur, new_val) != cur);
return true;
}
void ShenandoahPacer::unpace_for_alloc(intptr_t epoch, size_t words) {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
if (_epoch != epoch) {
// Stale ticket, no need to unpace.
return;
}
intptr_t tax = MAX2<intptr_t>(1, words * Atomic::load(&_tax_rate));
Atomic::add(&_budget, tax);
}
intptr_t ShenandoahPacer::epoch() {
return Atomic::load(&_epoch);
}
void ShenandoahPacer::pace_for_alloc(size_t words) {
assert(ShenandoahPacing, "Only be here when pacing is enabled");
// Fast path: try to allocate right away
if (claim_for_alloc(words, false)) {
return;
}
// Threads that are attaching should not block at all: they are not
// fully initialized yet. Calling sleep() on them would be awkward.
// This is probably the path that allocates the thread oop itself.
// Forcefully claim without waiting.
if (JavaThread::current()->is_attaching_via_jni()) {
claim_for_alloc(words, true);
return;
}
size_t max = ShenandoahPacingMaxDelay;
double start = os::elapsedTime();
size_t total = 0;
size_t cur = 0;
while (true) {
// We could instead assist GC, but this would suffice for now.
// This code should also participate in safepointing.
// Perform the exponential backoff, limited by max.
cur = cur * 2;
if (total + cur > max) {
cur = (max > total) ? (max - total) : 0;
}
cur = MAX2<size_t>(1, cur);
JavaThread::current()->sleep(cur);
double end = os::elapsedTime();
total = (size_t)((end - start) * 1000);
if (total > max) {
// Spent local time budget to wait for enough GC progress.
// Breaking out and allocating anyway, which may mean we outpace GC,
// and start Degenerated GC cycle.
_delays.add(total);
// Forcefully claim the budget: it may go negative at this point, and
// GC should replenish for this and subsequent allocations
claim_for_alloc(words, true);
break;
}
if (claim_for_alloc(words, false)) {
// Acquired enough permit, nice. Can allocate now.
_delays.add(total);
break;
}
}
}
void ShenandoahPacer::print_on(outputStream* out) const {
out->print_cr("ALLOCATION PACING:");
out->cr();
out->print_cr("Max pacing delay is set for " UINTX_FORMAT " ms.", ShenandoahPacingMaxDelay);
out->cr();
out->print_cr("Higher delay would prevent application outpacing the GC, but it will hide the GC latencies");
out->print_cr("from the STW pause times. Pacing affects the individual threads, and so it would also be");
out->print_cr("invisible to the usual profiling tools, but would add up to end-to-end application latency.");
out->print_cr("Raise max pacing delay with care.");
out->cr();
out->print_cr("Actual pacing delays histogram:");
out->cr();
out->print_cr("%10s - %10s %12s%12s", "From", "To", "Count", "Sum");
size_t total_count = 0;
size_t total_sum = 0;
for (int c = _delays.min_level(); c <= _delays.max_level(); c++) {
int l = (c == 0) ? 0 : 1 << (c - 1);
int r = 1 << c;
size_t count = _delays.level(c);
size_t sum = count * (r - l) / 2;
total_count += count;
total_sum += sum;
out->print_cr("%7d ms - %7d ms: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", l, r, count, sum);
}
out->print_cr("%23s: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", "Total", total_count, total_sum);
out->cr();
out->print_cr("Pacing delays are measured from entering the pacing code till exiting it. Therefore,");
out->print_cr("observed pacing delays may be higher than the threshold when paced thread spent more");
out->print_cr("time in the pacing code. It usually happens when thread is de-scheduled while paced,");
out->print_cr("OS takes longer to unblock the thread, or JVM experiences an STW pause.");
out->cr();
}