src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp - platform/libcore - Git at Google

 /*
  * 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"

 /*
  * 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(new_val, &_budget, cur) != 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();
 }
	/*
	* 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"

	/*
	* 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(new_val, &_budget, cur) != 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();
	}