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"
 #include "gc/shenandoah/shenandoahPhaseTimings.hpp"
 #include "runtime/mutexLocker.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 phase, but the
  * accounted budget is based on "used". So, we need to adjust the tax knowing that.
  */

 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 *= 1;                          // mark can succeed with immediate garbage, claim all available space
   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 followed by update-refs, 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 the last phase, 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);
 }

 /*
  * 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);
 }

 /*
  * There is no useful notion of progress for these operations. To avoid stalling
  * the allocators unnecessarily, allow them to run unimpeded.
  */

 void ShenandoahPacer::setup_for_preclean() {
   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   size_t initial = _heap->max_capacity();
   restart_with(initial, 1.0);

   log_info(gc, ergo)("Pacer for Precleaning. Non-Taxable: " SIZE_FORMAT "%s",
                      byte_size_in_proper_unit(initial), proper_unit_for_byte_size(initial));
 }

 void ShenandoahPacer::setup_for_reset() {
   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   size_t initial = _heap->max_capacity();
   restart_with(initial, 1.0);

   log_info(gc, ergo)("Pacer for Reset. Non-Taxable: " SIZE_FORMAT "%s",
                      byte_size_in_proper_unit(initial), proper_unit_for_byte_size(initial));
 }

 size_t ShenandoahPacer::update_and_get_progress_history() {
   if (_progress == -1) {
     // First initialization, report some prior
     Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);
     return (size_t) (_heap->max_capacity() * 0.1);
   } else {
     // Record history, and reply historical data
     _progress_history->add(_progress);
     Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);
     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((intptr_t)initial, &_budget, memory_order_relaxed);
   Atomic::store(tax_rate, &_tax_rate);
   Atomic::inc(&_epoch);

   // Shake up stalled waiters after budget update.
   _need_notify_waiters.try_set();
 }

 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, memory_order_relaxed) != cur);
   return true;
 }

 void ShenandoahPacer::unpace_for_alloc(intptr_t epoch, size_t words) {
   assert(ShenandoahPacing, "Only be here when pacing is enabled");

   if (Atomic::load(&_epoch) != epoch) {
     // Stale ticket, no need to unpace.
     return;
   }

   size_t tax = MAX2<size_t>(1, words * Atomic::load(&_tax_rate));
   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
   bool claimed = claim_for_alloc(words, false);
   if (claimed) {
     return;
   }

   // Forcefully claim the budget: it may go negative at this point, and
   // GC should replenish for this and subsequent allocations. After this claim,
   // we would wait a bit until our claim is matched by additional progress,
   // or the time budget depletes.
   claimed = claim_for_alloc(words, true);
   assert(claimed, "Should always succeed");

   // Threads that are attaching should not block at all: they are not
   // fully initialized yet. Blocking them would be awkward.
   // This is probably the path that allocates the thread oop itself.
   if (JavaThread::current()->is_attaching_via_jni()) {
     return;
   }

   double start = os::elapsedTime();

   size_t max_ms = ShenandoahPacingMaxDelay;
   size_t total_ms = 0;

   while (true) {
     // We could instead assist GC, but this would suffice for now.
     size_t cur_ms = (max_ms > total_ms) ? (max_ms - total_ms) : 1;
     wait(cur_ms);

     double end = os::elapsedTime();
     total_ms = (size_t)((end - start) * 1000);

     if (total_ms > max_ms || Atomic::load(&_budget) >= 0) {
       // Exiting if either:
       //  a) 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.
       //  b) The budget had been replenished, which means our claim is satisfied.
       ShenandoahThreadLocalData::add_paced_time(JavaThread::current(), end - start);
       break;
     }
   }
 }

 void ShenandoahPacer::wait(size_t time_ms) {
   // Perform timed wait. It works like like sleep(), except without modifying
   // the thread interruptible status. MonitorLocker also checks for safepoints.
   assert(time_ms > 0, "Should not call this with zero argument, as it would stall until notify");
   assert(time_ms <= LONG_MAX, "Sanity");
   MonitorLockerEx locker(_wait_monitor);
   _wait_monitor->wait(!Mutex::_no_safepoint_check_flag, (long)time_ms);
 }

 void ShenandoahPacer::notify_waiters() {
   if (_need_notify_waiters.try_unset()) {
     MonitorLockerEx locker(_wait_monitor);
     _wait_monitor->notify_all();
   }
 }

 void ShenandoahPacer::flush_stats_to_cycle() {
   double sum = 0;
   for (JavaThreadIteratorWithHandle jtiwh; JavaThread *t = jtiwh.next(); ) {
     sum += ShenandoahThreadLocalData::paced_time(t);
   }
   ShenandoahHeap::heap()->phase_timings()->record_phase_time(ShenandoahPhaseTimings::pacing, sum);
 }

 void ShenandoahPacer::print_cycle_on(outputStream* out) {
   MutexLocker lock(Threads_lock);

   double now = os::elapsedTime();
   double total = now - _last_time;
   _last_time = now;

   out->cr();
   out->print_cr("Allocation pacing accrued:");

   size_t threads_total = 0;
   size_t threads_nz = 0;
   double sum = 0;
   for (JavaThreadIteratorWithHandle jtiwh; JavaThread *t = jtiwh.next(); ) {
     double d = ShenandoahThreadLocalData::paced_time(t);
     if (d > 0) {
       threads_nz++;
       sum += d;
       out->print_cr("  %5.0f of %5.0f ms (%5.1f%%): %s",
               d * 1000, total * 1000, d/total*100, t->name());
     }
     threads_total++;
     ShenandoahThreadLocalData::reset_paced_time(t);
   }
   out->print_cr("  %5.0f of %5.0f ms (%5.1f%%): <total>",
           sum * 1000, total * 1000, sum/total*100);

   if (threads_total > 0) {
     out->print_cr("  %5.0f of %5.0f ms (%5.1f%%): <average total>",
             sum / threads_total * 1000, total * 1000, sum / threads_total / total * 100);
   }
   if (threads_nz > 0) {
     out->print_cr("  %5.0f of %5.0f ms (%5.1f%%): <average non-zero>",
             sum / threads_nz * 1000, total * 1000, sum / threads_nz / total * 100);
   }
   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"
	#include "gc/shenandoah/shenandoahPhaseTimings.hpp"
	#include "runtime/mutexLocker.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 phase, but the
	* accounted budget is based on "used". So, we need to adjust the tax knowing that.
	*/

	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 *= 1; // mark can succeed with immediate garbage, claim all available space
	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 followed by update-refs, 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 the last phase, 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);
	}

	/*
	* 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);
	}

	/*
	* There is no useful notion of progress for these operations. To avoid stalling
	* the allocators unnecessarily, allow them to run unimpeded.
	*/

	void ShenandoahPacer::setup_for_preclean() {
	assert(ShenandoahPacing, "Only be here when pacing is enabled");

	size_t initial = _heap->max_capacity();
	restart_with(initial, 1.0);

	log_info(gc, ergo)("Pacer for Precleaning. Non-Taxable: " SIZE_FORMAT "%s",
	byte_size_in_proper_unit(initial), proper_unit_for_byte_size(initial));
	}

	void ShenandoahPacer::setup_for_reset() {
	assert(ShenandoahPacing, "Only be here when pacing is enabled");

	size_t initial = _heap->max_capacity();
	restart_with(initial, 1.0);

	log_info(gc, ergo)("Pacer for Reset. Non-Taxable: " SIZE_FORMAT "%s",
	byte_size_in_proper_unit(initial), proper_unit_for_byte_size(initial));
	}

	size_t ShenandoahPacer::update_and_get_progress_history() {
	if (_progress == -1) {
	// First initialization, report some prior
	Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);
	return (size_t) (_heap->max_capacity() * 0.1);
	} else {
	// Record history, and reply historical data
	_progress_history->add(_progress);
	Atomic::store((intptr_t)PACING_PROGRESS_ZERO, &_progress);
	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((intptr_t)initial, &_budget, memory_order_relaxed);
	Atomic::store(tax_rate, &_tax_rate);
	Atomic::inc(&_epoch);

	// Shake up stalled waiters after budget update.
	_need_notify_waiters.try_set();
	}

	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, memory_order_relaxed) != cur);
	return true;
	}

	void ShenandoahPacer::unpace_for_alloc(intptr_t epoch, size_t words) {
	assert(ShenandoahPacing, "Only be here when pacing is enabled");

	if (Atomic::load(&_epoch) != epoch) {
	// Stale ticket, no need to unpace.
	return;
	}

	size_t tax = MAX2<size_t>(1, words * Atomic::load(&_tax_rate));
	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
	bool claimed = claim_for_alloc(words, false);
	if (claimed) {
	return;
	}

	// Forcefully claim the budget: it may go negative at this point, and
	// GC should replenish for this and subsequent allocations. After this claim,
	// we would wait a bit until our claim is matched by additional progress,
	// or the time budget depletes.
	claimed = claim_for_alloc(words, true);
	assert(claimed, "Should always succeed");

	// Threads that are attaching should not block at all: they are not
	// fully initialized yet. Blocking them would be awkward.
	// This is probably the path that allocates the thread oop itself.
	if (JavaThread::current()->is_attaching_via_jni()) {
	return;
	}

	double start = os::elapsedTime();

	size_t max_ms = ShenandoahPacingMaxDelay;
	size_t total_ms = 0;

	while (true) {
	// We could instead assist GC, but this would suffice for now.
	size_t cur_ms = (max_ms > total_ms) ? (max_ms - total_ms) : 1;
	wait(cur_ms);

	double end = os::elapsedTime();
	total_ms = (size_t)((end - start) * 1000);

	if (total_ms > max_ms \|\| Atomic::load(&_budget) >= 0) {
	// Exiting if either:
	// a) 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.
	// b) The budget had been replenished, which means our claim is satisfied.
	ShenandoahThreadLocalData::add_paced_time(JavaThread::current(), end - start);
	break;
	}
	}
	}

	void ShenandoahPacer::wait(size_t time_ms) {
	// Perform timed wait. It works like like sleep(), except without modifying
	// the thread interruptible status. MonitorLocker also checks for safepoints.
	assert(time_ms > 0, "Should not call this with zero argument, as it would stall until notify");
	assert(time_ms <= LONG_MAX, "Sanity");
	MonitorLockerEx locker(_wait_monitor);
	_wait_monitor->wait(!Mutex::_no_safepoint_check_flag, (long)time_ms);
	}

	void ShenandoahPacer::notify_waiters() {
	if (_need_notify_waiters.try_unset()) {
	MonitorLockerEx locker(_wait_monitor);
	_wait_monitor->notify_all();
	}
	}

	void ShenandoahPacer::flush_stats_to_cycle() {
	double sum = 0;
	for (JavaThreadIteratorWithHandle jtiwh; JavaThread *t = jtiwh.next(); ) {
	sum += ShenandoahThreadLocalData::paced_time(t);
	}
	ShenandoahHeap::heap()->phase_timings()->record_phase_time(ShenandoahPhaseTimings::pacing, sum);
	}

	void ShenandoahPacer::print_cycle_on(outputStream* out) {
	MutexLocker lock(Threads_lock);

	double now = os::elapsedTime();
	double total = now - _last_time;
	_last_time = now;

	out->cr();
	out->print_cr("Allocation pacing accrued:");

	size_t threads_total = 0;
	size_t threads_nz = 0;
	double sum = 0;
	for (JavaThreadIteratorWithHandle jtiwh; JavaThread *t = jtiwh.next(); ) {
	double d = ShenandoahThreadLocalData::paced_time(t);
	if (d > 0) {
	threads_nz++;
	sum += d;
	out->print_cr(" %5.0f of %5.0f ms (%5.1f%%): %s",
	d * 1000, total * 1000, d/total*100, t->name());
	}
	threads_total++;
	ShenandoahThreadLocalData::reset_paced_time(t);
	}
	out->print_cr(" %5.0f of %5.0f ms (%5.1f%%): <total>",
	sum * 1000, total * 1000, sum/total*100);

	if (threads_total > 0) {
	out->print_cr(" %5.0f of %5.0f ms (%5.1f%%): <average total>",
	sum / threads_total * 1000, total * 1000, sum / threads_total / total * 100);
	}
	if (threads_nz > 0) {
	out->print_cr(" %5.0f of %5.0f ms (%5.1f%%): <average non-zero>",
	sum / threads_nz * 1000, total * 1000, sum / threads_nz / total * 100);
	}
	out->cr();
	}