view src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp @ 7629:354d56cd6eb2

8048179: Early reclaim of large objects that are referenced by a few objects Summary: Push the remembered sets of large objects with few referenced into the dirty card queue at the beginning of the evacuation so that they may end up with zero remembered set entries at the end of the collection, and are potentially reclaimed. Also improve timing measurements of the early reclaim mechanism, and shorten flag names. Reviewed-by: brutisso, jmasa, dfazunen
author tschatzl
date Wed, 07 Jan 2015 15:15:37 +0100
parents a13726250f7f
children 0034df25722b
line wrap: on
line source
 * Copyright (c) 2001, 2014, Oracle and/or its affiliates. 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 if you need additional information or have any
 * questions.


#include "gc_implementation/g1/g1AllocationContext.hpp"
#include "gc_implementation/g1/g1Allocator.hpp"
#include "gc_implementation/g1/concurrentMark.hpp"
#include "gc_implementation/g1/evacuationInfo.hpp"
#include "gc_implementation/g1/g1AllocRegion.hpp"
#include "gc_implementation/g1/g1BiasedArray.hpp"
#include "gc_implementation/g1/g1HRPrinter.hpp"
#include "gc_implementation/g1/g1InCSetState.hpp"
#include "gc_implementation/g1/g1MonitoringSupport.hpp"
#include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
#include "gc_implementation/g1/g1YCTypes.hpp"
#include "gc_implementation/g1/heapRegionManager.hpp"
#include "gc_implementation/g1/heapRegionSet.hpp"
#include "gc_implementation/shared/hSpaceCounters.hpp"
#include "gc_implementation/shared/parGCAllocBuffer.hpp"
#include "memory/barrierSet.hpp"
#include "memory/memRegion.hpp"
#include "memory/sharedHeap.hpp"
#include "utilities/stack.hpp"

// A "G1CollectedHeap" is an implementation of a java heap for HotSpot.
// It uses the "Garbage First" heap organization and algorithm, which
// may combine concurrent marking with parallel, incremental compaction of
// heap subsets that will yield large amounts of garbage.

// Forward declarations
class HeapRegion;
class HRRSCleanupTask;
class GenerationSpec;
class OopsInHeapRegionClosure;
class G1KlassScanClosure;
class G1ScanHeapEvacClosure;
class ObjectClosure;
class SpaceClosure;
class CompactibleSpaceClosure;
class Space;
class G1CollectorPolicy;
class GenRemSet;
class G1RemSet;
class HeapRegionRemSetIterator;
class ConcurrentMark;
class ConcurrentMarkThread;
class ConcurrentG1Refine;
class ConcurrentGCTimer;
class GenerationCounters;
class STWGCTimer;
class G1NewTracer;
class G1OldTracer;
class EvacuationFailedInfo;
class nmethod;
class Ticks;

typedef OverflowTaskQueue<StarTask, mtGC>         RefToScanQueue;
typedef GenericTaskQueueSet<RefToScanQueue, mtGC> RefToScanQueueSet;

typedef int RegionIdx_t;   // needs to hold [ 0..max_regions() )
typedef int CardIdx_t;     // needs to hold [ 0..CardsPerRegion )

class YoungList : public CHeapObj<mtGC> {
  G1CollectedHeap* _g1h;

  HeapRegion* _head;

  HeapRegion* _survivor_head;
  HeapRegion* _survivor_tail;

  HeapRegion* _curr;

  uint        _length;
  uint        _survivor_length;

  size_t      _last_sampled_rs_lengths;
  size_t      _sampled_rs_lengths;

  void         empty_list(HeapRegion* list);

  YoungList(G1CollectedHeap* g1h);

  void         push_region(HeapRegion* hr);
  void         add_survivor_region(HeapRegion* hr);

  void         empty_list();
  bool         is_empty() { return _length == 0; }
  uint         length() { return _length; }
  uint         survivor_length() { return _survivor_length; }

  // Currently we do not keep track of the used byte sum for the
  // young list and the survivors and it'd be quite a lot of work to
  // do so. When we'll eventually replace the young list with
  // instances of HeapRegionLinkedList we'll get that for free. So,
  // we'll report the more accurate information then.
  size_t       eden_used_bytes() {
    assert(length() >= survivor_length(), "invariant");
    return (size_t) (length() - survivor_length()) * HeapRegion::GrainBytes;
  size_t       survivor_used_bytes() {
    return (size_t) survivor_length() * HeapRegion::GrainBytes;

  void rs_length_sampling_init();
  bool rs_length_sampling_more();
  void rs_length_sampling_next();

  void reset_sampled_info() {
    _last_sampled_rs_lengths =   0;
  size_t sampled_rs_lengths() { return _last_sampled_rs_lengths; }

  // for development purposes
  void reset_auxilary_lists();
  void clear() { _head = NULL; _length = 0; }

  void clear_survivors() {
    _survivor_head    = NULL;
    _survivor_tail    = NULL;
    _survivor_length  = 0;

  HeapRegion* first_region() { return _head; }
  HeapRegion* first_survivor_region() { return _survivor_head; }
  HeapRegion* last_survivor_region() { return _survivor_tail; }

  // debugging
  bool          check_list_well_formed();
  bool          check_list_empty(bool check_sample = true);
  void          print();

// The G1 STW is alive closure.
// An instance is embedded into the G1CH and used as the
// (optional) _is_alive_non_header closure in the STW
// reference processor. It is also extensively used during
// reference processing during STW evacuation pauses.
class G1STWIsAliveClosure: public BoolObjectClosure {
  G1CollectedHeap* _g1;
  G1STWIsAliveClosure(G1CollectedHeap* g1) : _g1(g1) {}
  bool do_object_b(oop p);

class RefineCardTableEntryClosure;

class G1RegionMappingChangedListener : public G1MappingChangedListener {
  void reset_from_card_cache(uint start_idx, size_t num_regions);
  virtual void on_commit(uint start_idx, size_t num_regions, bool zero_filled);

class G1CollectedHeap : public SharedHeap {
  friend class VM_CollectForMetadataAllocation;
  friend class VM_G1CollectForAllocation;
  friend class VM_G1CollectFull;
  friend class VM_G1IncCollectionPause;
  friend class VMStructs;
  friend class MutatorAllocRegion;
  friend class SurvivorGCAllocRegion;
  friend class OldGCAllocRegion;
  friend class G1Allocator;

  // Closures used in implementation.
  friend class G1ParScanThreadState;
  friend class G1ParTask;
  friend class G1ParGCAllocator;
  friend class G1PrepareCompactClosure;

  // Other related classes.
  friend class HeapRegionClaimer;

  // Testing classes.
  friend class G1CheckCSetFastTableClosure;

  // The one and only G1CollectedHeap, so static functions can find it.
  static G1CollectedHeap* _g1h;

  static size_t _humongous_object_threshold_in_words;

  // The secondary free list which contains regions that have been
  // freed up during the cleanup process. This will be appended to
  // the master free list when appropriate.
  FreeRegionList _secondary_free_list;

  // It keeps track of the old regions.
  HeapRegionSet _old_set;

  // It keeps track of the humongous regions.
  HeapRegionSet _humongous_set;

  void clear_humongous_is_live_table();
  void eagerly_reclaim_humongous_regions();

  // The number of regions we could create by expansion.
  uint _expansion_regions;

  // The block offset table for the G1 heap.
  G1BlockOffsetSharedArray* _bot_shared;

  // Tears down the region sets / lists so that they are empty and the
  // regions on the heap do not belong to a region set / list. The
  // only exception is the humongous set which we leave unaltered. If
  // free_list_only is true, it will only tear down the master free
  // list. It is called before a Full GC (free_list_only == false) or
  // before heap shrinking (free_list_only == true).
  void tear_down_region_sets(bool free_list_only);

  // Rebuilds the region sets / lists so that they are repopulated to
  // reflect the contents of the heap. The only exception is the
  // humongous set which was not torn down in the first place. If
  // free_list_only is true, it will only rebuild the master free
  // list. It is called after a Full GC (free_list_only == false) or
  // after heap shrinking (free_list_only == true).
  void rebuild_region_sets(bool free_list_only);

  // Callback for region mapping changed events.
  G1RegionMappingChangedListener _listener;

  // The sequence of all heap regions in the heap.
  HeapRegionManager _hrm;

  // Class that handles the different kinds of allocations.
  G1Allocator* _allocator;

  // Statistics for each allocation context
  AllocationContextStats _allocation_context_stats;

  // PLAB sizing policy for survivors.
  PLABStats _survivor_plab_stats;

  // PLAB sizing policy for tenured objects.
  PLABStats _old_plab_stats;

  // It specifies whether we should attempt to expand the heap after a
  // region allocation failure. If heap expansion fails we set this to
  // false so that we don't re-attempt the heap expansion (it's likely
  // that subsequent expansion attempts will also fail if one fails).
  // Currently, it is only consulted during GC and it's reset at the
  // start of each GC.
  bool _expand_heap_after_alloc_failure;

  // It resets the mutator alloc region before new allocations can take place.
  void init_mutator_alloc_region();

  // It releases the mutator alloc region.
  void release_mutator_alloc_region();

  // It initializes the GC alloc regions at the start of a GC.
  void init_gc_alloc_regions(EvacuationInfo& evacuation_info);

  // It releases the GC alloc regions at the end of a GC.
  void release_gc_alloc_regions(uint no_of_gc_workers, EvacuationInfo& evacuation_info);

  // It does any cleanup that needs to be done on the GC alloc regions
  // before a Full GC.
  void abandon_gc_alloc_regions();

  // Helper for monitoring and management support.
  G1MonitoringSupport* _g1mm;

  // Records whether the region at the given index is kept live by roots or
  // references from the young generation.
  class HumongousIsLiveBiasedMappedArray : public G1BiasedMappedArray<bool> {
    bool default_value() const { return false; }
    void clear() { G1BiasedMappedArray<bool>::clear(); }
    void set_live(uint region) {
      set_by_index(region, true);
    bool is_live(uint region) {
      return get_by_index(region);

  HumongousIsLiveBiasedMappedArray _humongous_is_live;
  // Stores whether during humongous object registration we found candidate regions.
  // If not, we can skip a few steps.
  bool _has_humongous_reclaim_candidates;

  volatile unsigned _gc_time_stamp;

  size_t* _surviving_young_words;

  G1HRPrinter _hr_printer;

  void setup_surviving_young_words();
  void update_surviving_young_words(size_t* surv_young_words);
  void cleanup_surviving_young_words();

  // It decides whether an explicit GC should start a concurrent cycle
  // instead of doing a STW GC. Currently, a concurrent cycle is
  // explicitly started if:
  // (a) cause == _gc_locker and +GCLockerInvokesConcurrent, or
  // (b) cause == _java_lang_system_gc and +ExplicitGCInvokesConcurrent.
  // (c) cause == _g1_humongous_allocation
  bool should_do_concurrent_full_gc(GCCause::Cause cause);

  // Keeps track of how many "old marking cycles" (i.e., Full GCs or
  // concurrent cycles) we have started.
  volatile unsigned int _old_marking_cycles_started;

  // Keeps track of how many "old marking cycles" (i.e., Full GCs or
  // concurrent cycles) we have completed.
  volatile unsigned int _old_marking_cycles_completed;

  bool _concurrent_cycle_started;
  bool _heap_summary_sent;

  // This is a non-product method that is helpful for testing. It is
  // called at the end of a GC and artificially expands the heap by
  // allocating a number of dead regions. This way we can induce very
  // frequent marking cycles and stress the cleanup / concurrent
  // cleanup code more (as all the regions that will be allocated by
  // this method will be found dead by the marking cycle).
  void allocate_dummy_regions() PRODUCT_RETURN;

  // Clear RSets after a compaction. It also resets the GC time stamps.
  void clear_rsets_post_compaction();

  // If the HR printer is active, dump the state of the regions in the
  // heap after a compaction.
  void print_hrm_post_compaction();

  double verify(bool guard, const char* msg);
  void verify_before_gc();
  void verify_after_gc();

  void log_gc_header();
  void log_gc_footer(double pause_time_sec);

  // These are macros so that, if the assert fires, we get the correct
  // line number, file, etc.

#define heap_locking_asserts_err_msg(_extra_message_)                         \
  err_msg("%s : Heap_lock locked: %s, at safepoint: %s, is VM thread: %s",    \
          (_extra_message_),                                                  \
          BOOL_TO_STR(Heap_lock->owned_by_self()),                            \
          BOOL_TO_STR(SafepointSynchronize::is_at_safepoint()),               \

#define assert_heap_locked()                                                  \
  do {                                                                        \
    assert(Heap_lock->owned_by_self(),                                        \
           heap_locking_asserts_err_msg("should be holding the Heap_lock"));  \
  } while (0)

#define assert_heap_locked_or_at_safepoint(_should_be_vm_thread_)             \
  do {                                                                        \
    assert(Heap_lock->owned_by_self() ||                                      \
           (SafepointSynchronize::is_at_safepoint() &&                        \
             ((_should_be_vm_thread_) == Thread::current()->is_VM_thread())), \
           heap_locking_asserts_err_msg("should be holding the Heap_lock or " \
                                        "should be at a safepoint"));         \
  } while (0)

#define assert_heap_locked_and_not_at_safepoint()                             \
  do {                                                                        \
    assert(Heap_lock->owned_by_self() &&                                      \
                                    !SafepointSynchronize::is_at_safepoint(), \
          heap_locking_asserts_err_msg("should be holding the Heap_lock and " \
                                       "should not be at a safepoint"));      \
  } while (0)

#define assert_heap_not_locked()                                              \
  do {                                                                        \
    assert(!Heap_lock->owned_by_self(),                                       \
        heap_locking_asserts_err_msg("should not be holding the Heap_lock")); \
  } while (0)

#define assert_heap_not_locked_and_not_at_safepoint()                         \
  do {                                                                        \
    assert(!Heap_lock->owned_by_self() &&                                     \
                                    !SafepointSynchronize::is_at_safepoint(), \
      heap_locking_asserts_err_msg("should not be holding the Heap_lock and " \
                                   "should not be at a safepoint"));          \
  } while (0)

#define assert_at_safepoint(_should_be_vm_thread_)                            \
  do {                                                                        \
    assert(SafepointSynchronize::is_at_safepoint() &&                         \
              ((_should_be_vm_thread_) == Thread::current()->is_VM_thread()), \
           heap_locking_asserts_err_msg("should be at a safepoint"));         \
  } while (0)

#define assert_not_at_safepoint()                                             \
  do {                                                                        \
    assert(!SafepointSynchronize::is_at_safepoint(),                          \
           heap_locking_asserts_err_msg("should not be at a safepoint"));     \
  } while (0)


  // The young region list.
  YoungList*  _young_list;

  // The current policy object for the collector.
  G1CollectorPolicy* _g1_policy;

  // This is the second level of trying to allocate a new region. If
  // new_region() didn't find a region on the free_list, this call will
  // check whether there's anything available on the
  // secondary_free_list and/or wait for more regions to appear on
  // that list, if _free_regions_coming is set.
  HeapRegion* new_region_try_secondary_free_list(bool is_old);

  // Try to allocate a single non-humongous HeapRegion sufficient for
  // an allocation of the given word_size. If do_expand is true,
  // attempt to expand the heap if necessary to satisfy the allocation
  // request. If the region is to be used as an old region or for a
  // humongous object, set is_old to true. If not, to false.
  HeapRegion* new_region(size_t word_size, bool is_old, bool do_expand);

  // Initialize a contiguous set of free regions of length num_regions
  // and starting at index first so that they appear as a single
  // humongous region.
  HeapWord* humongous_obj_allocate_initialize_regions(uint first,
                                                      uint num_regions,
                                                      size_t word_size,
                                                      AllocationContext_t context);

  // Attempt to allocate a humongous object of the given size. Return
  // NULL if unsuccessful.
  HeapWord* humongous_obj_allocate(size_t word_size, AllocationContext_t context);

  // The following two methods, allocate_new_tlab() and
  // mem_allocate(), are the two main entry points from the runtime
  // into the G1's allocation routines. They have the following
  // assumptions:
  // * They should both be called outside safepoints.
  // * They should both be called without holding the Heap_lock.
  // * All allocation requests for new TLABs should go to
  //   allocate_new_tlab().
  // * All non-TLAB allocation requests should go to mem_allocate().
  // * If either call cannot satisfy the allocation request using the
  //   current allocating region, they will try to get a new one. If
  //   this fails, they will attempt to do an evacuation pause and
  //   retry the allocation.
  // * If all allocation attempts fail, even after trying to schedule
  //   an evacuation pause, allocate_new_tlab() will return NULL,
  //   whereas mem_allocate() will attempt a heap expansion and/or
  //   schedule a Full GC.
  // * We do not allow humongous-sized TLABs. So, allocate_new_tlab
  //   should never be called with word_size being humongous. All
  //   humongous allocation requests should go to mem_allocate() which
  //   will satisfy them with a special path.

  virtual HeapWord* allocate_new_tlab(size_t word_size);

  virtual HeapWord* mem_allocate(size_t word_size,
                                 bool*  gc_overhead_limit_was_exceeded);

  // The following three methods take a gc_count_before_ret
  // parameter which is used to return the GC count if the method
  // returns NULL. Given that we are required to read the GC count
  // while holding the Heap_lock, and these paths will take the
  // Heap_lock at some point, it's easier to get them to read the GC
  // count while holding the Heap_lock before they return NULL instead
  // of the caller (namely: mem_allocate()) having to also take the
  // Heap_lock just to read the GC count.

  // First-level mutator allocation attempt: try to allocate out of
  // the mutator alloc region without taking the Heap_lock. This
  // should only be used for non-humongous allocations.
  inline HeapWord* attempt_allocation(size_t word_size,
                                      unsigned int* gc_count_before_ret,
                                      int* gclocker_retry_count_ret);

  // Second-level mutator allocation attempt: take the Heap_lock and
  // retry the allocation attempt, potentially scheduling a GC
  // pause. This should only be used for non-humongous allocations.
  HeapWord* attempt_allocation_slow(size_t word_size,
                                    AllocationContext_t context,
                                    unsigned int* gc_count_before_ret,
                                    int* gclocker_retry_count_ret);

  // Takes the Heap_lock and attempts a humongous allocation. It can
  // potentially schedule a GC pause.
  HeapWord* attempt_allocation_humongous(size_t word_size,
                                         unsigned int* gc_count_before_ret,
                                         int* gclocker_retry_count_ret);

  // Allocation attempt that should be called during safepoints (e.g.,
  // at the end of a successful GC). expect_null_mutator_alloc_region
  // specifies whether the mutator alloc region is expected to be NULL
  // or not.
  HeapWord* attempt_allocation_at_safepoint(size_t word_size,
                                            AllocationContext_t context,
                                            bool expect_null_mutator_alloc_region);

  // It dirties the cards that cover the block so that so that the post
  // write barrier never queues anything when updating objects on this
  // block. It is assumed (and in fact we assert) that the block
  // belongs to a young region.
  inline void dirty_young_block(HeapWord* start, size_t word_size);

  // Allocate blocks during garbage collection. Will ensure an
  // allocation region, either by picking one or expanding the
  // heap, and then allocate a block of the given size. The block
  // may not be a humongous - it must fit into a single heap region.
  inline HeapWord* par_allocate_during_gc(InCSetState dest,
                                          size_t word_size,
                                          AllocationContext_t context);
  // Ensure that no further allocations can happen in "r", bearing in mind
  // that parallel threads might be attempting allocations.
  void par_allocate_remaining_space(HeapRegion* r);

  // Allocation attempt during GC for a survivor object / PLAB.
  inline HeapWord* survivor_attempt_allocation(size_t word_size,
                                               AllocationContext_t context);

  // Allocation attempt during GC for an old object / PLAB.
  inline HeapWord* old_attempt_allocation(size_t word_size,
                                          AllocationContext_t context);

  // These methods are the "callbacks" from the G1AllocRegion class.

  // For mutator alloc regions.
  HeapRegion* new_mutator_alloc_region(size_t word_size, bool force);
  void retire_mutator_alloc_region(HeapRegion* alloc_region,
                                   size_t allocated_bytes);

  // For GC alloc regions.
  HeapRegion* new_gc_alloc_region(size_t word_size, uint count,
                                  InCSetState dest);
  void retire_gc_alloc_region(HeapRegion* alloc_region,
                              size_t allocated_bytes, InCSetState dest);

  // - if explicit_gc is true, the GC is for a System.gc() or a heap
  //   inspection request and should collect the entire heap
  // - if clear_all_soft_refs is true, all soft references should be
  //   cleared during the GC
  // - if explicit_gc is false, word_size describes the allocation that
  //   the GC should attempt (at least) to satisfy
  // - it returns false if it is unable to do the collection due to the
  //   GC locker being active, true otherwise
  bool do_collection(bool explicit_gc,
                     bool clear_all_soft_refs,
                     size_t word_size);

  // Callback from VM_G1CollectFull operation.
  // Perform a full collection.
  virtual void do_full_collection(bool clear_all_soft_refs);

  // Resize the heap if necessary after a full collection.  If this is
  // after a collect-for allocation, "word_size" is the allocation size,
  // and will be considered part of the used portion of the heap.
  void resize_if_necessary_after_full_collection(size_t word_size);

  // Callback from VM_G1CollectForAllocation operation.
  // This function does everything necessary/possible to satisfy a
  // failed allocation request (including collection, expansion, etc.)
  HeapWord* satisfy_failed_allocation(size_t word_size,
                                      AllocationContext_t context,
                                      bool* succeeded);

  // Attempting to expand the heap sufficiently
  // to support an allocation of the given "word_size".  If
  // successful, perform the allocation and return the address of the
  // allocated block, or else "NULL".
  HeapWord* expand_and_allocate(size_t word_size, AllocationContext_t context);

  // Process any reference objects discovered during
  // an incremental evacuation pause.
  void process_discovered_references(uint no_of_gc_workers);

  // Enqueue any remaining discovered references
  // after processing.
  void enqueue_discovered_references(uint no_of_gc_workers);


  G1Allocator* allocator() {
    return _allocator;

  G1MonitoringSupport* g1mm() {
    assert(_g1mm != NULL, "should have been initialized");
    return _g1mm;

  // Expand the garbage-first heap by at least the given size (in bytes!).
  // Returns true if the heap was expanded by the requested amount;
  // false otherwise.
  // (Rounds up to a HeapRegion boundary.)
  bool expand(size_t expand_bytes);

  // Returns the PLAB statistics for a given destination.
  inline PLABStats* alloc_buffer_stats(InCSetState dest);

  // Determines PLAB size for a given destination.
  inline size_t desired_plab_sz(InCSetState dest);

  inline AllocationContextStats& allocation_context_stats();

  // Do anything common to GC's.
  virtual void gc_prologue(bool full);
  virtual void gc_epilogue(bool full);

  inline void set_humongous_is_live(oop obj);

  bool humongous_is_live(uint region) {
    return _humongous_is_live.is_live(region);

  // Returns whether the given region (which must be a humongous (start) region)
  // is to be considered conservatively live regardless of any other conditions.
  bool humongous_region_is_always_live(uint index);
  // Returns whether the given region (which must be a humongous (start) region)
  // is considered a candidate for eager reclamation.
  bool humongous_region_is_candidate(uint index);
  // Register the given region to be part of the collection set.
  inline void register_humongous_region_with_in_cset_fast_test(uint index);
  // Register regions with humongous objects (actually on the start region) in
  // the in_cset_fast_test table.
  void register_humongous_regions_with_in_cset_fast_test();
  // We register a region with the fast "in collection set" test. We
  // simply set to true the array slot corresponding to this region.
  void register_young_region_with_in_cset_fast_test(HeapRegion* r) {
  void register_old_region_with_in_cset_fast_test(HeapRegion* r) {

  // This is a fast test on whether a reference points into the
  // collection set or not. Assume that the reference
  // points into the heap.
  inline bool in_cset_fast_test(oop obj);

  void clear_cset_fast_test() {

  // This is called at the start of either a concurrent cycle or a Full
  // GC to update the number of old marking cycles started.
  void increment_old_marking_cycles_started();

  // This is called at the end of either a concurrent cycle or a Full
  // GC to update the number of old marking cycles completed. Those two
  // can happen in a nested fashion, i.e., we start a concurrent
  // cycle, a Full GC happens half-way through it which ends first,
  // and then the cycle notices that a Full GC happened and ends
  // too. The concurrent parameter is a boolean to help us do a bit
  // tighter consistency checking in the method. If concurrent is
  // false, the caller is the inner caller in the nesting (i.e., the
  // Full GC). If concurrent is true, the caller is the outer caller
  // in this nesting (i.e., the concurrent cycle). Further nesting is
  // not currently supported. The end of this call also notifies
  // the FullGCCount_lock in case a Java thread is waiting for a full
  // GC to happen (e.g., it called System.gc() with
  // +ExplicitGCInvokesConcurrent).
  void increment_old_marking_cycles_completed(bool concurrent);

  unsigned int old_marking_cycles_completed() {
    return _old_marking_cycles_completed;

  void register_concurrent_cycle_start(const Ticks& start_time);
  void register_concurrent_cycle_end();
  void trace_heap_after_concurrent_cycle();

  G1YCType yc_type();

  G1HRPrinter* hr_printer() { return &_hr_printer; }

  // Frees a non-humongous region by initializing its contents and
  // adding it to the free list that's passed as a parameter (this is
  // usually a local list which will be appended to the master free
  // list later). The used bytes of freed regions are accumulated in
  // pre_used. If par is true, the region's RSet will not be freed
  // up. The assumption is that this will be done later.
  // The locked parameter indicates if the caller has already taken
  // care of proper synchronization. This may allow some optimizations.
  void free_region(HeapRegion* hr,
                   FreeRegionList* free_list,
                   bool par,
                   bool locked = false);

  // Frees a humongous region by collapsing it into individual regions
  // and calling free_region() for each of them. The freed regions
  // will be added to the free list that's passed as a parameter (this
  // is usually a local list which will be appended to the master free
  // list later). The used bytes of freed regions are accumulated in
  // pre_used. If par is true, the region's RSet will not be freed
  // up. The assumption is that this will be done later.
  void free_humongous_region(HeapRegion* hr,
                             FreeRegionList* free_list,
                             bool par);

  // Shrink the garbage-first heap by at most the given size (in bytes!).
  // (Rounds down to a HeapRegion boundary.)
  virtual void shrink(size_t expand_bytes);
  void shrink_helper(size_t expand_bytes);

  static void print_taskqueue_stats_hdr(outputStream* const st = gclog_or_tty);
  void print_taskqueue_stats(outputStream* const st = gclog_or_tty) const;
  void reset_taskqueue_stats();

  // Schedule the VM operation that will do an evacuation pause to
  // satisfy an allocation request of word_size. *succeeded will
  // return whether the VM operation was successful (it did do an
  // evacuation pause) or not (another thread beat us to it or the GC
  // locker was active). Given that we should not be holding the
  // Heap_lock when we enter this method, we will pass the
  // gc_count_before (i.e., total_collections()) as a parameter since
  // it has to be read while holding the Heap_lock. Currently, both
  // methods that call do_collection_pause() release the Heap_lock
  // before the call, so it's easy to read gc_count_before just before.
  HeapWord* do_collection_pause(size_t         word_size,
                                unsigned int   gc_count_before,
                                bool*          succeeded,
                                GCCause::Cause gc_cause);

  // The guts of the incremental collection pause, executed by the vm
  // thread. It returns false if it is unable to do the collection due
  // to the GC locker being active, true otherwise
  bool do_collection_pause_at_safepoint(double target_pause_time_ms);

  // Actually do the work of evacuating the collection set.
  void evacuate_collection_set(EvacuationInfo& evacuation_info);

  // The g1 remembered set of the heap.
  G1RemSet* _g1_rem_set;

  // A set of cards that cover the objects for which the Rsets should be updated
  // concurrently after the collection.
  DirtyCardQueueSet _dirty_card_queue_set;

  // The closure used to refine a single card.
  RefineCardTableEntryClosure* _refine_cte_cl;

  // A DirtyCardQueueSet that is used to hold cards that contain
  // references into the current collection set. This is used to
  // update the remembered sets of the regions in the collection
  // set in the event of an evacuation failure.
  DirtyCardQueueSet _into_cset_dirty_card_queue_set;

  // After a collection pause, make the regions in the CS into free
  // regions.
  void free_collection_set(HeapRegion* cs_head, EvacuationInfo& evacuation_info);

  // Abandon the current collection set without recording policy
  // statistics or updating free lists.
  void abandon_collection_set(HeapRegion* cs_head);

  // Applies "scan_non_heap_roots" to roots outside the heap,
  // "scan_rs" to roots inside the heap (having done "set_region" to
  // indicate the region in which the root resides),
  // and does "scan_metadata" If "scan_rs" is
  // NULL, then this step is skipped.  The "worker_i"
  // param is for use with parallel roots processing, and should be
  // the "i" of the calling parallel worker thread's work(i) function.
  // In the sequential case this param will be ignored.
  void g1_process_roots(OopClosure* scan_non_heap_roots,
                        OopClosure* scan_non_heap_weak_roots,
                        G1ParPushHeapRSClosure* scan_rs,
                        CLDClosure* scan_strong_clds,
                        CLDClosure* scan_weak_clds,
                        CodeBlobClosure* scan_strong_code,
                        uint worker_i);

  // The concurrent marker (and the thread it runs in.)
  ConcurrentMark* _cm;
  ConcurrentMarkThread* _cmThread;
  bool _mark_in_progress;

  // The concurrent refiner.
  ConcurrentG1Refine* _cg1r;

  // The parallel task queues
  RefToScanQueueSet *_task_queues;

  // True iff a evacuation has failed in the current collection.
  bool _evacuation_failed;

  EvacuationFailedInfo* _evacuation_failed_info_array;

  // Failed evacuations cause some logical from-space objects to have
  // forwarding pointers to themselves.  Reset them.
  void remove_self_forwarding_pointers();

  // Together, these store an object with a preserved mark, and its mark value.
  Stack<oop, mtGC>     _objs_with_preserved_marks;
  Stack<markOop, mtGC> _preserved_marks_of_objs;

  // Preserve the mark of "obj", if necessary, in preparation for its mark
  // word being overwritten with a self-forwarding-pointer.
  void preserve_mark_if_necessary(oop obj, markOop m);

  // The stack of evac-failure objects left to be scanned.
  GrowableArray<oop>*    _evac_failure_scan_stack;
  // The closure to apply to evac-failure objects.

  OopsInHeapRegionClosure* _evac_failure_closure;
  // Set the field above.
  set_evac_failure_closure(OopsInHeapRegionClosure* evac_failure_closure) {
    _evac_failure_closure = evac_failure_closure;

  // Push "obj" on the scan stack.
  void push_on_evac_failure_scan_stack(oop obj);
  // Process scan stack entries until the stack is empty.
  void drain_evac_failure_scan_stack();
  // True iff an invocation of "drain_scan_stack" is in progress; to
  // prevent unnecessary recursion.
  bool _drain_in_progress;

  // Do any necessary initialization for evacuation-failure handling.
  // "cl" is the closure that will be used to process evac-failure
  // objects.
  void init_for_evac_failure(OopsInHeapRegionClosure* cl);
  // Do any necessary cleanup for evacuation-failure handling data
  // structures.
  void finalize_for_evac_failure();

  // An attempt to evacuate "obj" has failed; take necessary steps.
  oop handle_evacuation_failure_par(G1ParScanThreadState* _par_scan_state, oop obj);
  void handle_evacuation_failure_common(oop obj, markOop m);

#ifndef PRODUCT
  // Support for forcing evacuation failures. Analogous to
  // PromotionFailureALot for the other collectors.

  // Records whether G1EvacuationFailureALot should be in effect
  // for the current GC
  bool _evacuation_failure_alot_for_current_gc;

  // Used to record the GC number for interval checking when
  // determining whether G1EvaucationFailureALot is in effect
  // for the current GC.
  size_t _evacuation_failure_alot_gc_number;

  // Count of the number of evacuations between failures.
  volatile size_t _evacuation_failure_alot_count;

  // Set whether G1EvacuationFailureALot should be in effect
  // for the current GC (based upon the type of GC and which
  // command line flags are set);
  inline bool evacuation_failure_alot_for_gc_type(bool gcs_are_young,
                                                  bool during_initial_mark,
                                                  bool during_marking);

  inline void set_evacuation_failure_alot_for_current_gc();

  // Return true if it's time to cause an evacuation failure.
  inline bool evacuation_should_fail();

  // Reset the G1EvacuationFailureALot counters.  Should be called at
  // the end of an evacuation pause in which an evacuation failure occurred.
  inline void reset_evacuation_should_fail();
#endif // !PRODUCT

  // ("Weak") Reference processing support.
  // G1 has 2 instances of the reference processor class. One
  // (_ref_processor_cm) handles reference object discovery
  // and subsequent processing during concurrent marking cycles.
  // The other (_ref_processor_stw) handles reference object
  // discovery and processing during full GCs and incremental
  // evacuation pauses.
  // During an incremental pause, reference discovery will be
  // temporarily disabled for _ref_processor_cm and will be
  // enabled for _ref_processor_stw. At the end of the evacuation
  // pause references discovered by _ref_processor_stw will be
  // processed and discovery will be disabled. The previous
  // setting for reference object discovery for _ref_processor_cm
  // will be re-instated.
  // At the start of marking:
  //  * Discovery by the CM ref processor is verified to be inactive
  //    and it's discovered lists are empty.
  //  * Discovery by the CM ref processor is then enabled.
  // At the end of marking:
  //  * Any references on the CM ref processor's discovered
  //    lists are processed (possibly MT).
  // At the start of full GC we:
  //  * Disable discovery by the CM ref processor and
  //    empty CM ref processor's discovered lists
  //    (without processing any entries).
  //  * Verify that the STW ref processor is inactive and it's
  //    discovered lists are empty.
  //  * Temporarily set STW ref processor discovery as single threaded.
  //  * Temporarily clear the STW ref processor's _is_alive_non_header
  //    field.
  //  * Finally enable discovery by the STW ref processor.
  // The STW ref processor is used to record any discovered
  // references during the full GC.
  // At the end of a full GC we:
  //  * Enqueue any reference objects discovered by the STW ref processor
  //    that have non-live referents. This has the side-effect of
  //    making the STW ref processor inactive by disabling discovery.
  //  * Verify that the CM ref processor is still inactive
  //    and no references have been placed on it's discovered
  //    lists (also checked as a precondition during initial marking).

  // The (stw) reference processor...
  ReferenceProcessor* _ref_processor_stw;

  STWGCTimer* _gc_timer_stw;
  ConcurrentGCTimer* _gc_timer_cm;

  G1OldTracer* _gc_tracer_cm;
  G1NewTracer* _gc_tracer_stw;

  // During reference object discovery, the _is_alive_non_header
  // closure (if non-null) is applied to the referent object to
  // determine whether the referent is live. If so then the
  // reference object does not need to be 'discovered' and can
  // be treated as a regular oop. This has the benefit of reducing
  // the number of 'discovered' reference objects that need to
  // be processed.
  // Instance of the is_alive closure for embedding into the
  // STW reference processor as the _is_alive_non_header field.
  // Supplying a value for the _is_alive_non_header field is
  // optional but doing so prevents unnecessary additions to
  // the discovered lists during reference discovery.
  G1STWIsAliveClosure _is_alive_closure_stw;

  // The (concurrent marking) reference processor...
  ReferenceProcessor* _ref_processor_cm;

  // Instance of the concurrent mark is_alive closure for embedding
  // into the Concurrent Marking reference processor as the
  // _is_alive_non_header field. Supplying a value for the
  // _is_alive_non_header field is optional but doing so prevents
  // unnecessary additions to the discovered lists during reference
  // discovery.
  G1CMIsAliveClosure _is_alive_closure_cm;

  // Cache used by G1CollectedHeap::start_cset_region_for_worker().
  HeapRegion** _worker_cset_start_region;

  // Time stamp to validate the regions recorded in the cache
  // used by G1CollectedHeap::start_cset_region_for_worker().
  // The heap region entry for a given worker is valid iff
  // the associated time stamp value matches the current value
  // of G1CollectedHeap::_gc_time_stamp.
  unsigned int* _worker_cset_start_region_time_stamp;

  enum G1H_process_roots_tasks {
    // Leave this one last.

  SubTasksDone* _process_strong_tasks;

  volatile bool _free_regions_coming;


  SubTasksDone* process_strong_tasks() { return _process_strong_tasks; }

  void set_refine_cte_cl_concurrency(bool concurrent);

  RefToScanQueue *task_queue(int i) const;

  // A set of cards where updates happened during the GC
  DirtyCardQueueSet& dirty_card_queue_set() { return _dirty_card_queue_set; }

  // A DirtyCardQueueSet that is used to hold cards that contain
  // references into the current collection set. This is used to
  // update the remembered sets of the regions in the collection
  // set in the event of an evacuation failure.
  DirtyCardQueueSet& into_cset_dirty_card_queue_set()
        { return _into_cset_dirty_card_queue_set; }

  // Create a G1CollectedHeap with the specified policy.
  // Must call the initialize method afterwards.
  // May not return if something goes wrong.
  G1CollectedHeap(G1CollectorPolicy* policy);

  // Initialize the G1CollectedHeap to have the initial and
  // maximum sizes and remembered and barrier sets
  // specified by the policy object.
  jint initialize();

  virtual void stop();

  // Return the (conservative) maximum heap alignment for any G1 heap
  static size_t conservative_max_heap_alignment();

  // Initialize weak reference processing.
  virtual void ref_processing_init();

  void set_par_threads(uint t) {
    // Done in SharedHeap but oddly there are
    // two _process_strong_tasks's in a G1CollectedHeap
    // so do it here too.

  // Set _n_par_threads according to a policy TBD.
  void set_par_threads();

  void set_n_termination(int t) {

  virtual CollectedHeap::Name kind() const {
    return CollectedHeap::G1CollectedHeap;

  // The current policy object for the collector.
  G1CollectorPolicy* g1_policy() const { return _g1_policy; }

  virtual CollectorPolicy* collector_policy() const { return (CollectorPolicy*) g1_policy(); }

  // Adaptive size policy.  No such thing for g1.
  virtual AdaptiveSizePolicy* size_policy() { return NULL; }

  // The rem set and barrier set.
  G1RemSet* g1_rem_set() const { return _g1_rem_set; }

  unsigned get_gc_time_stamp() {
    return _gc_time_stamp;

  inline void reset_gc_time_stamp();

  void check_gc_time_stamps() PRODUCT_RETURN;

  inline void increment_gc_time_stamp();

  // Reset the given region's GC timestamp. If it's starts humongous,
  // also reset the GC timestamp of its corresponding
  // continues humongous regions too.
  void reset_gc_time_stamps(HeapRegion* hr);

  void iterate_dirty_card_closure(CardTableEntryClosure* cl,
                                  DirtyCardQueue* into_cset_dcq,
                                  bool concurrent, uint worker_i);

  // The shared block offset table array.
  G1BlockOffsetSharedArray* bot_shared() const { return _bot_shared; }

  // Reference Processing accessors

  // The STW reference processor....
  ReferenceProcessor* ref_processor_stw() const { return _ref_processor_stw; }

  // The Concurrent Marking reference processor...
  ReferenceProcessor* ref_processor_cm() const { return _ref_processor_cm; }

  ConcurrentGCTimer* gc_timer_cm() const { return _gc_timer_cm; }
  G1OldTracer* gc_tracer_cm() const { return _gc_tracer_cm; }

  virtual size_t capacity() const;
  virtual size_t used() const;
  // This should be called when we're not holding the heap lock. The
  // result might be a bit inaccurate.
  size_t used_unlocked() const;
  size_t recalculate_used() const;

  // These virtual functions do the actual allocation.
  // Some heaps may offer a contiguous region for shared non-blocking
  // allocation, via inlined code (by exporting the address of the top and
  // end fields defining the extent of the contiguous allocation region.)
  // But G1CollectedHeap doesn't yet support this.

  virtual bool is_maximal_no_gc() const {
    return _hrm.available() == 0;

  // The current number of regions in the heap.
  uint num_regions() const { return _hrm.length(); }

  // The max number of regions in the heap.
  uint max_regions() const { return _hrm.max_length(); }

  // The number of regions that are completely free.
  uint num_free_regions() const { return _hrm.num_free_regions(); }

  // The number of regions that are not completely free.
  uint num_used_regions() const { return num_regions() - num_free_regions(); }

  void verify_not_dirty_region(HeapRegion* hr) PRODUCT_RETURN;
  void verify_dirty_region(HeapRegion* hr) PRODUCT_RETURN;
  void verify_dirty_young_list(HeapRegion* head) PRODUCT_RETURN;
  void verify_dirty_young_regions() PRODUCT_RETURN;

#ifndef PRODUCT
  // Make sure that the given bitmap has no marked objects in the
  // range [from,limit). If it does, print an error message and return
  // false. Otherwise, just return true. bitmap_name should be "prev"
  // or "next".
  bool verify_no_bits_over_tams(const char* bitmap_name, CMBitMapRO* bitmap,
                                HeapWord* from, HeapWord* limit);

  // Verify that the prev / next bitmap range [tams,end) for the given
  // region has no marks. Return true if all is well, false if errors
  // are detected.
  bool verify_bitmaps(const char* caller, HeapRegion* hr);
#endif // PRODUCT

  // If G1VerifyBitmaps is set, verify that the marking bitmaps for
  // the given region do not have any spurious marks. If errors are
  // detected, print appropriate error messages and crash.
  void check_bitmaps(const char* caller, HeapRegion* hr) PRODUCT_RETURN;

  // If G1VerifyBitmaps is set, verify that the marking bitmaps do not
  // have any spurious marks. If errors are detected, print
  // appropriate error messages and crash.
  void check_bitmaps(const char* caller) PRODUCT_RETURN;

  // Do sanity check on the contents of the in-cset fast test table.
  bool check_cset_fast_test() PRODUCT_RETURN_( return true; );

  // verify_region_sets() performs verification over the region
  // lists. It will be compiled in the product code to be used when
  // necessary (i.e., during heap verification).
  void verify_region_sets();

  // verify_region_sets_optional() is planted in the code for
  // list verification in non-product builds (and it can be enabled in
  // product builds by defining HEAP_REGION_SET_FORCE_VERIFY to be 1).
  void verify_region_sets_optional() {
  void verify_region_sets_optional() { }

#ifdef ASSERT
  bool is_on_master_free_list(HeapRegion* hr) {
    return _hrm.is_free(hr);
#endif // ASSERT

  // Wrapper for the region list operations that can be called from
  // methods outside this class.

  void secondary_free_list_add(FreeRegionList* list) {

  void append_secondary_free_list() {

  void append_secondary_free_list_if_not_empty_with_lock() {
    // If the secondary free list looks empty there's no reason to
    // take the lock and then try to append it.
    if (!_secondary_free_list.is_empty()) {
      MutexLockerEx x(SecondaryFreeList_lock, Mutex::_no_safepoint_check_flag);

  inline void old_set_remove(HeapRegion* hr);

  size_t non_young_capacity_bytes() {
    return _old_set.total_capacity_bytes() + _humongous_set.total_capacity_bytes();

  void set_free_regions_coming();
  void reset_free_regions_coming();
  bool free_regions_coming() { return _free_regions_coming; }
  void wait_while_free_regions_coming();

  // Determine whether the given region is one that we are using as an
  // old GC alloc region.
  bool is_old_gc_alloc_region(HeapRegion* hr) {
    return _allocator->is_retained_old_region(hr);

  // Perform a collection of the heap; intended for use in implementing
  // "System.gc".  This probably implies as full a collection as the
  // "CollectedHeap" supports.
  virtual void collect(GCCause::Cause cause);

  // The same as above but assume that the caller holds the Heap_lock.
  void collect_locked(GCCause::Cause cause);

  virtual bool copy_allocation_context_stats(const jint* contexts,
                                             jlong* totals,
                                             jbyte* accuracy,
                                             jint len);

  // True iff an evacuation has failed in the most-recent collection.
  bool evacuation_failed() { return _evacuation_failed; }

  void remove_from_old_sets(const HeapRegionSetCount& old_regions_removed, const HeapRegionSetCount& humongous_regions_removed);
  void prepend_to_freelist(FreeRegionList* list);
  void decrement_summary_bytes(size_t bytes);

  // Returns "TRUE" iff "p" points into the committed areas of the heap.
  virtual bool is_in(const void* p) const;
#ifdef ASSERT
  // Returns whether p is in one of the available areas of the heap. Slow but
  // extensive version.
  bool is_in_exact(const void* p) const;

  // Return "TRUE" iff the given object address is within the collection
  // set. Slow implementation.
  inline bool obj_in_cs(oop obj);

  inline bool is_in_cset(oop obj);

  inline bool is_in_cset_or_humongous(const oop obj);

  // This array is used for a quick test on whether a reference points into
  // the collection set or not. Each of the array's elements denotes whether the
  // corresponding region is in the collection set or not.
  G1InCSetStateFastTestBiasedMappedArray _in_cset_fast_test;


  inline InCSetState in_cset_state(const oop obj);

  // Return "TRUE" iff the given object address is in the reserved
  // region of g1.
  bool is_in_g1_reserved(const void* p) const {
    return _hrm.reserved().contains(p);

  // Returns a MemRegion that corresponds to the space that has been
  // reserved for the heap
  MemRegion g1_reserved() const {
    return _hrm.reserved();

  virtual bool is_in_closed_subset(const void* p) const;

  G1SATBCardTableLoggingModRefBS* g1_barrier_set() {
    return (G1SATBCardTableLoggingModRefBS*) barrier_set();

  // This resets the card table to all zeros.  It is used after
  // a collection pause which used the card table to claim cards.
  void cleanUpCardTable();

  // Iteration functions.

  // Iterate over all the ref-containing fields of all objects, calling
  // "cl.do_oop" on each.
  virtual void oop_iterate(ExtendedOopClosure* cl);

  // Iterate over all objects, calling "cl.do_object" on each.
  virtual void object_iterate(ObjectClosure* cl);

  virtual void safe_object_iterate(ObjectClosure* cl) {

  // Iterate over all spaces in use in the heap, in ascending address order.
  virtual void space_iterate(SpaceClosure* cl);

  // Iterate over heap regions, in address order, terminating the
  // iteration early if the "doHeapRegion" method returns "true".
  void heap_region_iterate(HeapRegionClosure* blk) const;

  // Return the region with the given index. It assumes the index is valid.
  inline HeapRegion* region_at(uint index) const;

  // Calculate the region index of the given address. Given address must be
  // within the heap.
  inline uint addr_to_region(HeapWord* addr) const;

  inline HeapWord* bottom_addr_for_region(uint index) const;

  // Iterate over the heap regions in parallel. Assumes that this will be called
  // in parallel by ParallelGCThreads worker threads with distinct worker ids
  // in the range [0..max(ParallelGCThreads-1, 1)]. Applies "blk->doHeapRegion"
  // to each of the regions, by attempting to claim the region using the
  // HeapRegionClaimer and, if successful, applying the closure to the claimed
  // region. The concurrent argument should be set to true if iteration is
  // performed concurrently, during which no assumptions are made for consistent
  // attributes of the heap regions (as they might be modified while iterating).
  void heap_region_par_iterate(HeapRegionClosure* cl,
                               uint worker_id,
                               HeapRegionClaimer* hrclaimer,
                               bool concurrent = false) const;

  // Clear the cached cset start regions and (more importantly)
  // the time stamps. Called when we reset the GC time stamp.
  void clear_cset_start_regions();

  // Given the id of a worker, obtain or calculate a suitable
  // starting region for iterating over the current collection set.
  HeapRegion* start_cset_region_for_worker(uint worker_i);

  // Iterate over the regions (if any) in the current collection set.
  void collection_set_iterate(HeapRegionClosure* blk);

  // As above but starting from region r
  void collection_set_iterate_from(HeapRegion* r, HeapRegionClosure *blk);

  HeapRegion* next_compaction_region(const HeapRegion* from) const;

  // A CollectedHeap will contain some number of spaces.  This finds the
  // space containing a given address, or else returns NULL.
  virtual Space* space_containing(const void* addr) const;

  // Returns the HeapRegion that contains addr. addr must not be NULL.
  template <class T>
  inline HeapRegion* heap_region_containing_raw(const T addr) const;

  // Returns the HeapRegion that contains addr. addr must not be NULL.
  // If addr is within a humongous continues region, it returns its humongous start region.
  template <class T>
  inline HeapRegion* heap_region_containing(const T addr) const;

  // A CollectedHeap is divided into a dense sequence of "blocks"; that is,
  // each address in the (reserved) heap is a member of exactly
  // one block.  The defining characteristic of a block is that it is
  // possible to find its size, and thus to progress forward to the next
  // block.  (Blocks may be of different sizes.)  Thus, blocks may
  // represent Java objects, or they might be free blocks in a
  // free-list-based heap (or subheap), as long as the two kinds are
  // distinguishable and the size of each is determinable.

  // Returns the address of the start of the "block" that contains the
  // address "addr".  We say "blocks" instead of "object" since some heaps
  // may not pack objects densely; a chunk may either be an object or a
  // non-object.
  virtual HeapWord* block_start(const void* addr) const;

  // Requires "addr" to be the start of a chunk, and returns its size.
  // "addr + size" is required to be the start of a new chunk, or the end
  // of the active area of the heap.
  virtual size_t block_size(const HeapWord* addr) const;

  // Requires "addr" to be the start of a block, and returns "TRUE" iff
  // the block is an object.
  virtual bool block_is_obj(const HeapWord* addr) const;

  // Does this heap support heap inspection? (+PrintClassHistogram)
  virtual bool supports_heap_inspection() const { return true; }

  // Section on thread-local allocation buffers (TLABs)
  // See CollectedHeap for semantics.

  bool supports_tlab_allocation() const;
  size_t tlab_capacity(Thread* ignored) const;
  size_t tlab_used(Thread* ignored) const;
  size_t max_tlab_size() const;
  size_t unsafe_max_tlab_alloc(Thread* ignored) const;

  // Can a compiler initialize a new object without store barriers?
  // This permission only extends from the creation of a new object
  // via a TLAB up to the first subsequent safepoint. If such permission
  // is granted for this heap type, the compiler promises to call
  // defer_store_barrier() below on any slow path allocation of
  // a new object for which such initializing store barriers will
  // have been elided. G1, like CMS, allows this, but should be
  // ready to provide a compensating write barrier as necessary
  // if that storage came out of a non-young region. The efficiency
  // of this implementation depends crucially on being able to
  // answer very efficiently in constant time whether a piece of
  // storage in the heap comes from a young region or not.
  // See ReduceInitialCardMarks.
  virtual bool can_elide_tlab_store_barriers() const {
    return true;

  virtual bool card_mark_must_follow_store() const {
    return true;

  inline bool is_in_young(const oop obj);

#ifdef ASSERT
  virtual bool is_in_partial_collection(const void* p);

  virtual bool is_scavengable(const void* addr);

  // We don't need barriers for initializing stores to objects
  // in the young gen: for the SATB pre-barrier, there is no
  // pre-value that needs to be remembered; for the remembered-set
  // update logging post-barrier, we don't maintain remembered set
  // information for young gen objects.
  virtual inline bool can_elide_initializing_store_barrier(oop new_obj);

  // Returns "true" iff the given word_size is "very large".
  static bool is_humongous(size_t word_size) {
    // Note this has to be strictly greater-than as the TLABs
    // are capped at the humongous threshold and we want to
    // ensure that we don't try to allocate a TLAB as
    // humongous and that we don't allocate a humongous
    // object in a TLAB.
    return word_size > _humongous_object_threshold_in_words;

  // Update mod union table with the set of dirty cards.
  void updateModUnion();

  // Set the mod union bits corresponding to the given memRegion.  Note
  // that this is always a safe operation, since it doesn't clear any
  // bits.
  void markModUnionRange(MemRegion mr);

  // Records the fact that a marking phase is no longer in progress.
  void set_marking_complete() {
    _mark_in_progress = false;
  void set_marking_started() {
    _mark_in_progress = true;
  bool mark_in_progress() {
    return _mark_in_progress;

  // Print the maximum heap capacity.
  virtual size_t max_capacity() const;

  virtual jlong millis_since_last_gc();

  // Convenience function to be used in situations where the heap type can be
  // asserted to be this type.
  static G1CollectedHeap* heap();

  void set_region_short_lived_locked(HeapRegion* hr);
  // add appropriate methods for any other surv rate groups

  YoungList* young_list() const { return _young_list; }

  // debugging
  bool check_young_list_well_formed() {
    return _young_list->check_list_well_formed();

  bool check_young_list_empty(bool check_heap,
                              bool check_sample = true);

  // *** Stuff related to concurrent marking.  It's not clear to me that so
  // many of these need to be public.

  // The functions below are helper functions that a subclass of
  // "CollectedHeap" can use in the implementation of its virtual
  // functions.
  // This performs a concurrent marking of the live objects in a
  // bitmap off to the side.
  void doConcurrentMark();

  bool isMarkedPrev(oop obj) const;
  bool isMarkedNext(oop obj) const;

  // Determine if an object is dead, given the object and also
  // the region to which the object belongs. An object is dead
  // iff a) it was not allocated since the last mark and b) it
  // is not marked.
  bool is_obj_dead(const oop obj, const HeapRegion* hr) const {
      !hr->obj_allocated_since_prev_marking(obj) &&

  // This function returns true when an object has been
  // around since the previous marking and hasn't yet
  // been marked during this marking.
  bool is_obj_ill(const oop obj, const HeapRegion* hr) const {
      !hr->obj_allocated_since_next_marking(obj) &&

  // Determine if an object is dead, given only the object itself.
  // This will find the region to which the object belongs and
  // then call the region version of the same function.

  // Added if it is NULL it isn't dead.

  inline bool is_obj_dead(const oop obj) const;

  inline bool is_obj_ill(const oop obj) const;

  bool allocated_since_marking(oop obj, HeapRegion* hr, VerifyOption vo);
  HeapWord* top_at_mark_start(HeapRegion* hr, VerifyOption vo);
  bool is_marked(oop obj, VerifyOption vo);
  const char* top_at_mark_start_str(VerifyOption vo);

  ConcurrentMark* concurrent_mark() const { return _cm; }

  // Refinement

  ConcurrentG1Refine* concurrent_g1_refine() const { return _cg1r; }

  // The dirty cards region list is used to record a subset of regions
  // whose cards need clearing. The list if populated during the
  // remembered set scanning and drained during the card table
  // cleanup. Although the methods are reentrant, population/draining
  // phases must not overlap. For synchronization purposes the last
  // element on the list points to itself.
  HeapRegion* _dirty_cards_region_list;
  void push_dirty_cards_region(HeapRegion* hr);
  HeapRegion* pop_dirty_cards_region();

  // Optimized nmethod scanning support routines

  // Register the given nmethod with the G1 heap.
  virtual void register_nmethod(nmethod* nm);

  // Unregister the given nmethod from the G1 heap.
  virtual void unregister_nmethod(nmethod* nm);

  // Free up superfluous code root memory.
  void purge_code_root_memory();

  // Rebuild the strong code root lists for each region
  // after a full GC.
  void rebuild_strong_code_roots();

  // Delete entries for dead interned string and clean up unreferenced symbols
  // in symbol table, possibly in parallel.
  void unlink_string_and_symbol_table(BoolObjectClosure* is_alive, bool unlink_strings = true, bool unlink_symbols = true);

  // Parallel phase of unloading/cleaning after G1 concurrent mark.
  void parallel_cleaning(BoolObjectClosure* is_alive, bool process_strings, bool process_symbols, bool class_unloading_occurred);

  // Redirty logged cards in the refinement queue.
  void redirty_logged_cards();
  // Verification

  // The following is just to alert the verification code
  // that a full collection has occurred and that the
  // remembered sets are no longer up to date.
  bool _full_collection;
  void set_full_collection() { _full_collection = true;}
  void clear_full_collection() {_full_collection = false;}
  bool full_collection() {return _full_collection;}

  // Perform any cleanup actions necessary before allowing a verification.
  virtual void prepare_for_verify();

  // Perform verification.

  // vo == UsePrevMarking  -> use "prev" marking information,
  // vo == UseNextMarking -> use "next" marking information
  // vo == UseMarkWord    -> use the mark word in the object header
  // NOTE: Only the "prev" marking information is guaranteed to be
  // consistent most of the time, so most calls to this should use
  // vo == UsePrevMarking.
  // Currently, there is only one case where this is called with
  // vo == UseNextMarking, which is to verify the "next" marking
  // information at the end of remark.
  // Currently there is only one place where this is called with
  // vo == UseMarkWord, which is to verify the marking during a
  // full GC.
  void verify(bool silent, VerifyOption vo);

  // Override; it uses the "prev" marking information
  virtual void verify(bool silent);

  // The methods below are here for convenience and dispatch the
  // appropriate method depending on value of the given VerifyOption
  // parameter. The values for that parameter, and their meanings,
  // are the same as those above.

  bool is_obj_dead_cond(const oop obj,
                        const HeapRegion* hr,
                        const VerifyOption vo) const;

  bool is_obj_dead_cond(const oop obj,
                        const VerifyOption vo) const;

  // Printing

  virtual void print_on(outputStream* st) const;
  virtual void print_extended_on(outputStream* st) const;
  virtual void print_on_error(outputStream* st) const;

  virtual void print_gc_threads_on(outputStream* st) const;
  virtual void gc_threads_do(ThreadClosure* tc) const;

  // Override
  void print_tracing_info() const;

  // The following two methods are helpful for debugging RSet issues.
  void print_cset_rsets() PRODUCT_RETURN;
  void print_all_rsets() PRODUCT_RETURN;

  size_t pending_card_num();
  size_t cards_scanned();

  size_t _max_heap_capacity;