gc/parallel/psMarkSweep.cpp

/*
 * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
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 */

#include "precompiled.hpp"
#include "classfile/stringTable.hpp"
#include "classfile/symbolTable.hpp"
#include "classfile/systemDictionary.hpp"
#include "code/codeCache.hpp"
#include "gc/parallel/parallelScavengeHeap.hpp"
#include "gc/parallel/psAdaptiveSizePolicy.hpp"
#include "gc/parallel/psMarkSweep.hpp"
#include "gc/parallel/psMarkSweepDecorator.hpp"
#include "gc/parallel/psOldGen.hpp"
#include "gc/parallel/psScavenge.hpp"
#include "gc/parallel/psYoungGen.hpp"
#include "gc/serial/markSweep.hpp"
#include "gc/shared/gcCause.hpp"
#include "gc/shared/gcHeapSummary.hpp"
#include "gc/shared/gcId.hpp"
#include "gc/shared/gcLocker.inline.hpp"
#include "gc/shared/gcTimer.hpp"
#include "gc/shared/gcTrace.hpp"
#include "gc/shared/gcTraceTime.inline.hpp"
#include "gc/shared/isGCActiveMark.hpp"
#include "gc/shared/referencePolicy.hpp"
#include "gc/shared/referenceProcessor.hpp"
#include "gc/shared/spaceDecorator.hpp"
#include "logging/log.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/biasedLocking.hpp"
#include "runtime/fprofiler.hpp"
#include "runtime/safepoint.hpp"
#include "runtime/vmThread.hpp"
#include "services/management.hpp"
#include "services/memoryService.hpp"
#include "utilities/events.hpp"
#include "utilities/stack.inline.hpp"

elapsedTimer        PSMarkSweep::_accumulated_time;
jlong               PSMarkSweep::_time_of_last_gc   = 0;
CollectorCounters*  PSMarkSweep::_counters = NULL;

void PSMarkSweep::initialize() {
  MemRegion mr = ParallelScavengeHeap::heap()->reserved_region();
  set_ref_processor(new ReferenceProcessor(mr));     // a vanilla ref proc
  _counters = new CollectorCounters("PSMarkSweep", 1);
}

// This method contains all heap specific policy for invoking mark sweep.
// PSMarkSweep::invoke_no_policy() will only attempt to mark-sweep-compact
// the heap. It will do nothing further. If we need to bail out for policy
// reasons, scavenge before full gc, or any other specialized behavior, it
// needs to be added here.
//
// Note that this method should only be called from the vm_thread while
// at a safepoint!
//
// Note that the all_soft_refs_clear flag in the collector policy
// may be true because this method can be called without intervening
// activity.  For example when the heap space is tight and full measure
// are being taken to free space.

void PSMarkSweep::invoke(bool maximum_heap_compaction) {
  assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
  assert(Thread::current() == (Thread*)VMThread::vm_thread(), "should be in vm thread");
  assert(!ParallelScavengeHeap::heap()->is_gc_active(), "not reentrant");

  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  GCCause::Cause gc_cause = heap->gc_cause();
  PSAdaptiveSizePolicy* policy = heap->size_policy();
  IsGCActiveMark mark;

  if (ScavengeBeforeFullGC) {
    PSScavenge::invoke_no_policy();
  }

  const bool clear_all_soft_refs =
    heap->collector_policy()->should_clear_all_soft_refs();

  uint count = maximum_heap_compaction ? 1 : MarkSweepAlwaysCompactCount;
  UIntXFlagSetting flag_setting(MarkSweepAlwaysCompactCount, count);
  PSMarkSweep::invoke_no_policy(clear_all_soft_refs || maximum_heap_compaction);
}

// This method contains no policy. You should probably
// be calling invoke() instead.
bool PSMarkSweep::invoke_no_policy(bool clear_all_softrefs) {
  assert(SafepointSynchronize::is_at_safepoint(), "must be at a safepoint");
  assert(ref_processor() != NULL, "Sanity");

  if (GC_locker::check_active_before_gc()) {
    return false;
  }

  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  GCCause::Cause gc_cause = heap->gc_cause();

  GCIdMark gc_id_mark;
  _gc_timer->register_gc_start();
  _gc_tracer->report_gc_start(gc_cause, _gc_timer->gc_start());

  PSAdaptiveSizePolicy* size_policy = heap->size_policy();

  // The scope of casr should end after code that can change
  // CollectorPolicy::_should_clear_all_soft_refs.
  ClearedAllSoftRefs casr(clear_all_softrefs, heap->collector_policy());

  PSYoungGen* young_gen = heap->young_gen();
  PSOldGen* old_gen = heap->old_gen();

  // Increment the invocation count
  heap->increment_total_collections(true /* full */);

  // Save information needed to minimize mangling
  heap->record_gen_tops_before_GC();

  // We need to track unique mark sweep invocations as well.
  _total_invocations++;

  heap->print_heap_before_gc();
  heap->trace_heap_before_gc(_gc_tracer);

  // Fill in TLABs
  heap->accumulate_statistics_all_tlabs();
  heap->ensure_parsability(true);  // retire TLABs

  if (VerifyBeforeGC && heap->total_collections() >= VerifyGCStartAt) {
    HandleMark hm;  // Discard invalid handles created during verification
    Universe::verify("Before GC");
  }

  // Verify object start arrays
  if (VerifyObjectStartArray &&
      VerifyBeforeGC) {
    old_gen->verify_object_start_array();
  }

  heap->pre_full_gc_dump(_gc_timer);

  // Filled in below to track the state of the young gen after the collection.
  bool eden_empty;
  bool survivors_empty;
  bool young_gen_empty;

  {
    HandleMark hm;

    GCTraceCPUTime tcpu;
    GCTraceTime(Info, gc) t("Pause Full", NULL, gc_cause, true);
    TraceCollectorStats tcs(counters());
    TraceMemoryManagerStats tms(true /* Full GC */,gc_cause);

    if (TraceOldGenTime) accumulated_time()->start();

    // Let the size policy know we're starting
    size_policy->major_collection_begin();

    CodeCache::gc_prologue();
    BiasedLocking::preserve_marks();

    // Capture metadata size before collection for sizing.
    size_t metadata_prev_used = MetaspaceAux::used_bytes();

    size_t old_gen_prev_used = old_gen->used_in_bytes();
    size_t young_gen_prev_used = young_gen->used_in_bytes();

    allocate_stacks();

#if defined(COMPILER2) || INCLUDE_JVMCI
    DerivedPointerTable::clear();
#endif

    ref_processor()->enable_discovery();
    ref_processor()->setup_policy(clear_all_softrefs);

    mark_sweep_phase1(clear_all_softrefs);

    mark_sweep_phase2();

#if defined(COMPILER2) || INCLUDE_JVMCI
    // Don't add any more derived pointers during phase3
    assert(DerivedPointerTable::is_active(), "Sanity");
    DerivedPointerTable::set_active(false);
#endif

    mark_sweep_phase3();

    mark_sweep_phase4();

    restore_marks();

    deallocate_stacks();

    if (ZapUnusedHeapArea) {
      // Do a complete mangle (top to end) because the usage for
      // scratch does not maintain a top pointer.
      young_gen->to_space()->mangle_unused_area_complete();
    }

    eden_empty = young_gen->eden_space()->is_empty();
    if (!eden_empty) {
      eden_empty = absorb_live_data_from_eden(size_policy, young_gen, old_gen);
    }

    // Update heap occupancy information which is used as
    // input to soft ref clearing policy at the next gc.
    Universe::update_heap_info_at_gc();

    survivors_empty = young_gen->from_space()->is_empty() &&
                      young_gen->to_space()->is_empty();
    young_gen_empty = eden_empty && survivors_empty;

    ModRefBarrierSet* modBS = barrier_set_cast<ModRefBarrierSet>(heap->barrier_set());
    MemRegion old_mr = heap->old_gen()->reserved();
    if (young_gen_empty) {
      modBS->clear(MemRegion(old_mr.start(), old_mr.end()));
    } else {
      modBS->invalidate(MemRegion(old_mr.start(), old_mr.end()));
    }

    // Delete metaspaces for unloaded class loaders and clean up loader_data graph
    ClassLoaderDataGraph::purge();
    MetaspaceAux::verify_metrics();

    BiasedLocking::restore_marks();
    CodeCache::gc_epilogue();
    JvmtiExport::gc_epilogue();

#if defined(COMPILER2) || INCLUDE_JVMCI
    DerivedPointerTable::update_pointers();
#endif

    ref_processor()->enqueue_discovered_references(NULL);

    // Update time of last GC
    reset_millis_since_last_gc();

    // Let the size policy know we're done
    size_policy->major_collection_end(old_gen->used_in_bytes(), gc_cause);

    if (UseAdaptiveSizePolicy) {

     log_debug(gc, ergo)("AdaptiveSizeStart: collection: %d ", heap->total_collections());
     log_trace(gc, ergo)("old_gen_capacity: " SIZE_FORMAT " young_gen_capacity: " SIZE_FORMAT,
                         old_gen->capacity_in_bytes(), young_gen->capacity_in_bytes());

      // Don't check if the size_policy is ready here.  Let
      // the size_policy check that internally.
      if (UseAdaptiveGenerationSizePolicyAtMajorCollection &&
          AdaptiveSizePolicy::should_update_promo_stats(gc_cause)) {
        // Swap the survivor spaces if from_space is empty. The
        // resize_young_gen() called below is normally used after
        // a successful young GC and swapping of survivor spaces;
        // otherwise, it will fail to resize the young gen with
        // the current implementation.
        if (young_gen->from_space()->is_empty()) {
          young_gen->from_space()->clear(SpaceDecorator::Mangle);
          young_gen->swap_spaces();
        }

        // Calculate optimal free space amounts
        assert(young_gen->max_size() >
          young_gen->from_space()->capacity_in_bytes() +
          young_gen->to_space()->capacity_in_bytes(),
          "Sizes of space in young gen are out-of-bounds");

        size_t young_live = young_gen->used_in_bytes();
        size_t eden_live = young_gen->eden_space()->used_in_bytes();
        size_t old_live = old_gen->used_in_bytes();
        size_t cur_eden = young_gen->eden_space()->capacity_in_bytes();
        size_t max_old_gen_size = old_gen->max_gen_size();
        size_t max_eden_size = young_gen->max_size() -
          young_gen->from_space()->capacity_in_bytes() -
          young_gen->to_space()->capacity_in_bytes();

        // Used for diagnostics
        size_policy->clear_generation_free_space_flags();

        size_policy->compute_generations_free_space(young_live,
                                                    eden_live,
                                                    old_live,
                                                    cur_eden,
                                                    max_old_gen_size,
                                                    max_eden_size,
                                                    true /* full gc*/);

        size_policy->check_gc_overhead_limit(young_live,
                                             eden_live,
                                             max_old_gen_size,
                                             max_eden_size,
                                             true /* full gc*/,
                                             gc_cause,
                                             heap->collector_policy());

        size_policy->decay_supplemental_growth(true /* full gc*/);

        heap->resize_old_gen(size_policy->calculated_old_free_size_in_bytes());

        heap->resize_young_gen(size_policy->calculated_eden_size_in_bytes(),
                               size_policy->calculated_survivor_size_in_bytes());
      }
      log_debug(gc, ergo)("AdaptiveSizeStop: collection: %d ", heap->total_collections());
    }

    if (UsePerfData) {
      heap->gc_policy_counters()->update_counters();
      heap->gc_policy_counters()->update_old_capacity(
        old_gen->capacity_in_bytes());
      heap->gc_policy_counters()->update_young_capacity(
        young_gen->capacity_in_bytes());
    }

    heap->resize_all_tlabs();

    // We collected the heap, recalculate the metaspace capacity
    MetaspaceGC::compute_new_size();

    if (TraceOldGenTime) accumulated_time()->stop();

    young_gen->print_used_change(young_gen_prev_used);
    old_gen->print_used_change(old_gen_prev_used);
    MetaspaceAux::print_metaspace_change(metadata_prev_used);

    // Track memory usage and detect low memory
    MemoryService::track_memory_usage();
    heap->update_counters();
  }

  if (VerifyAfterGC && heap->total_collections() >= VerifyGCStartAt) {
    HandleMark hm;  // Discard invalid handles created during verification
    Universe::verify("After GC");
  }

  // Re-verify object start arrays
  if (VerifyObjectStartArray &&
      VerifyAfterGC) {
    old_gen->verify_object_start_array();
  }

  if (ZapUnusedHeapArea) {
    old_gen->object_space()->check_mangled_unused_area_complete();
  }

  NOT_PRODUCT(ref_processor()->verify_no_references_recorded());

  heap->print_heap_after_gc();
  heap->trace_heap_after_gc(_gc_tracer);

  heap->post_full_gc_dump(_gc_timer);

#ifdef TRACESPINNING
  ParallelTaskTerminator::print_termination_counts();
#endif

  AdaptiveSizePolicyOutput::print(size_policy, heap->total_collections());

  _gc_timer->register_gc_end();

  _gc_tracer->report_gc_end(_gc_timer->gc_end(), _gc_timer->time_partitions());

  return true;
}

bool PSMarkSweep::absorb_live_data_from_eden(PSAdaptiveSizePolicy* size_policy,
                                             PSYoungGen* young_gen,
                                             PSOldGen* old_gen) {
  MutableSpace* const eden_space = young_gen->eden_space();
  assert(!eden_space->is_empty(), "eden must be non-empty");
  assert(young_gen->virtual_space()->alignment() ==
         old_gen->virtual_space()->alignment(), "alignments do not match");

  if (!(UseAdaptiveSizePolicy && UseAdaptiveGCBoundary)) {
    return false;
  }

  // Both generations must be completely committed.
  if (young_gen->virtual_space()->uncommitted_size() != 0) {
    return false;
  }
  if (old_gen->virtual_space()->uncommitted_size() != 0) {
    return false;
  }

  // Figure out how much to take from eden.  Include the average amount promoted
  // in the total; otherwise the next young gen GC will simply bail out to a
  // full GC.
  const size_t alignment = old_gen->virtual_space()->alignment();
  const size_t eden_used = eden_space->used_in_bytes();
  const size_t promoted = (size_t)size_policy->avg_promoted()->padded_average();
  const size_t absorb_size = align_size_up(eden_used + promoted, alignment);
  const size_t eden_capacity = eden_space->capacity_in_bytes();

  if (absorb_size >= eden_capacity) {
    return false; // Must leave some space in eden.
  }

  const size_t new_young_size = young_gen->capacity_in_bytes() - absorb_size;
  if (new_young_size < young_gen->min_gen_size()) {
    return false; // Respect young gen minimum size.
  }

  log_trace(heap, ergo)(" absorbing " SIZE_FORMAT "K:  "
                        "eden " SIZE_FORMAT "K->" SIZE_FORMAT "K "
                        "from " SIZE_FORMAT "K, to " SIZE_FORMAT "K "
                        "young_gen " SIZE_FORMAT "K->" SIZE_FORMAT "K ",
                        absorb_size / K,
                        eden_capacity / K, (eden_capacity - absorb_size) / K,
                        young_gen->from_space()->used_in_bytes() / K,
                        young_gen->to_space()->used_in_bytes() / K,
                        young_gen->capacity_in_bytes() / K, new_young_size / K);

  // Fill the unused part of the old gen.
  MutableSpace* const old_space = old_gen->object_space();
  HeapWord* const unused_start = old_space->top();
  size_t const unused_words = pointer_delta(old_space->end(), unused_start);

  if (unused_words > 0) {
    if (unused_words < CollectedHeap::min_fill_size()) {
      return false;  // If the old gen cannot be filled, must give up.
    }
    CollectedHeap::fill_with_objects(unused_start, unused_words);
  }

  // Take the live data from eden and set both top and end in the old gen to
  // eden top.  (Need to set end because reset_after_change() mangles the region
  // from end to virtual_space->high() in debug builds).
  HeapWord* const new_top = eden_space->top();
  old_gen->virtual_space()->expand_into(young_gen->virtual_space(),
                                        absorb_size);
  young_gen->reset_after_change();
  old_space->set_top(new_top);
  old_space->set_end(new_top);
  old_gen->reset_after_change();

  // Update the object start array for the filler object and the data from eden.
  ObjectStartArray* const start_array = old_gen->start_array();
  for (HeapWord* p = unused_start; p < new_top; p += oop(p)->size()) {
    start_array->allocate_block(p);
  }

  // Could update the promoted average here, but it is not typically updated at
  // full GCs and the value to use is unclear.  Something like
  //
  // cur_promoted_avg + absorb_size / number_of_scavenges_since_last_full_gc.

  size_policy->set_bytes_absorbed_from_eden(absorb_size);
  return true;
}

void PSMarkSweep::allocate_stacks() {
  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  PSYoungGen* young_gen = heap->young_gen();

  MutableSpace* to_space = young_gen->to_space();
  _preserved_marks = (PreservedMark*)to_space->top();
  _preserved_count = 0;

  // We want to calculate the size in bytes first.
  _preserved_count_max  = pointer_delta(to_space->end(), to_space->top(), sizeof(jbyte));
  // Now divide by the size of a PreservedMark
  _preserved_count_max /= sizeof(PreservedMark);
}


void PSMarkSweep::deallocate_stacks() {
  _preserved_mark_stack.clear(true);
  _preserved_oop_stack.clear(true);
  _marking_stack.clear();
  _objarray_stack.clear(true);
}

void PSMarkSweep::mark_sweep_phase1(bool clear_all_softrefs) {
  // Recursively traverse all live objects and mark them
  GCTraceTime(Trace, gc) tm("Phase 1: Mark live objects", _gc_timer);

  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();

  // Need to clear claim bits before the tracing starts.
  ClassLoaderDataGraph::clear_claimed_marks();

  // General strong roots.
  {
    ParallelScavengeHeap::ParStrongRootsScope psrs;
    Universe::oops_do(mark_and_push_closure());
    JNIHandles::oops_do(mark_and_push_closure());   // Global (strong) JNI handles
    CLDToOopClosure mark_and_push_from_cld(mark_and_push_closure());
    MarkingCodeBlobClosure each_active_code_blob(mark_and_push_closure(), !CodeBlobToOopClosure::FixRelocations);
    Threads::oops_do(mark_and_push_closure(), &mark_and_push_from_cld, &each_active_code_blob);
    ObjectSynchronizer::oops_do(mark_and_push_closure());
    FlatProfiler::oops_do(mark_and_push_closure());
    Management::oops_do(mark_and_push_closure());
    JvmtiExport::oops_do(mark_and_push_closure());
    SystemDictionary::always_strong_oops_do(mark_and_push_closure());
    ClassLoaderDataGraph::always_strong_cld_do(follow_cld_closure());
    // Do not treat nmethods as strong roots for mark/sweep, since we can unload them.
    //CodeCache::scavenge_root_nmethods_do(CodeBlobToOopClosure(mark_and_push_closure()));
  }

  // Flush marking stack.
  follow_stack();

  // Process reference objects found during marking
  {
    ref_processor()->setup_policy(clear_all_softrefs);
    const ReferenceProcessorStats& stats =
      ref_processor()->process_discovered_references(
        is_alive_closure(), mark_and_push_closure(), follow_stack_closure(), NULL, _gc_timer);
    gc_tracer()->report_gc_reference_stats(stats);
  }

  // This is the point where the entire marking should have completed.
  assert(_marking_stack.is_empty(), "Marking should have completed");

  // Unload classes and purge the SystemDictionary.
  bool purged_class = SystemDictionary::do_unloading(is_alive_closure());

  // Unload nmethods.
  CodeCache::do_unloading(is_alive_closure(), purged_class);

  // Prune dead klasses from subklass/sibling/implementor lists.
  Klass::clean_weak_klass_links(is_alive_closure());

  // Delete entries for dead interned strings.
  StringTable::unlink(is_alive_closure());

  // Clean up unreferenced symbols in symbol table.
  SymbolTable::unlink();
  _gc_tracer->report_object_count_after_gc(is_alive_closure());
}


void PSMarkSweep::mark_sweep_phase2() {
  GCTraceTime(Trace, gc) tm("Phase 2: Compute new object addresses", _gc_timer);

  // Now all live objects are marked, compute the new object addresses.

  // It is not required that we traverse spaces in the same order in
  // phase2, phase3 and phase4, but the ValidateMarkSweep live oops
  // tracking expects us to do so. See comment under phase4.

  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  PSOldGen* old_gen = heap->old_gen();

  // Begin compacting into the old gen
  PSMarkSweepDecorator::set_destination_decorator_tenured();

  // This will also compact the young gen spaces.
  old_gen->precompact();
}

// This should be moved to the shared markSweep code!
class PSAlwaysTrueClosure: public BoolObjectClosure {
public:
  bool do_object_b(oop p) { return true; }
};
static PSAlwaysTrueClosure always_true;

void PSMarkSweep::mark_sweep_phase3() {
  // Adjust the pointers to reflect the new locations
  GCTraceTime(Trace, gc) tm("Phase 3: Adjust pointers", _gc_timer);

  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  PSYoungGen* young_gen = heap->young_gen();
  PSOldGen* old_gen = heap->old_gen();

  // Need to clear claim bits before the tracing starts.
  ClassLoaderDataGraph::clear_claimed_marks();

  // General strong roots.
  Universe::oops_do(adjust_pointer_closure());
  JNIHandles::oops_do(adjust_pointer_closure());   // Global (strong) JNI handles
  CLDToOopClosure adjust_from_cld(adjust_pointer_closure());
  Threads::oops_do(adjust_pointer_closure(), &adjust_from_cld, NULL);
  ObjectSynchronizer::oops_do(adjust_pointer_closure());
  FlatProfiler::oops_do(adjust_pointer_closure());
  Management::oops_do(adjust_pointer_closure());
  JvmtiExport::oops_do(adjust_pointer_closure());
  SystemDictionary::oops_do(adjust_pointer_closure());
  ClassLoaderDataGraph::cld_do(adjust_cld_closure());

  // Now adjust pointers in remaining weak roots.  (All of which should
  // have been cleared if they pointed to non-surviving objects.)
  // Global (weak) JNI handles
  JNIHandles::weak_oops_do(&always_true, adjust_pointer_closure());

  CodeBlobToOopClosure adjust_from_blobs(adjust_pointer_closure(), CodeBlobToOopClosure::FixRelocations);
  CodeCache::blobs_do(&adjust_from_blobs);
  StringTable::oops_do(adjust_pointer_closure());
  ref_processor()->weak_oops_do(adjust_pointer_closure());
  PSScavenge::reference_processor()->weak_oops_do(adjust_pointer_closure());

  adjust_marks();

  young_gen->adjust_pointers();
  old_gen->adjust_pointers();
}

void PSMarkSweep::mark_sweep_phase4() {
  EventMark m("4 compact heap");
  GCTraceTime(Trace, gc) tm("Phase 4: Move objects", _gc_timer);

  // All pointers are now adjusted, move objects accordingly

  ParallelScavengeHeap* heap = ParallelScavengeHeap::heap();
  PSYoungGen* young_gen = heap->young_gen();
  PSOldGen* old_gen = heap->old_gen();

  old_gen->compact();
  young_gen->compact();
}

jlong PSMarkSweep::millis_since_last_gc() {
  // We need a monotonically non-decreasing time in ms but
  // os::javaTimeMillis() does not guarantee monotonicity.
  jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  jlong ret_val = now - _time_of_last_gc;
  // XXX See note in genCollectedHeap::millis_since_last_gc().
  if (ret_val < 0) {
    NOT_PRODUCT(warning("time warp: " JLONG_FORMAT, ret_val);)
    return 0;
  }
  return ret_val;
}

void PSMarkSweep::reset_millis_since_last_gc() {
  // We need a monotonically non-decreasing time in ms but
  // os::javaTimeMillis() does not guarantee monotonicity.
  _time_of_last_gc = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
}