view src/share/vm/gc_interface/collectedHeap.hpp @ 2534:6747fd0512e0

7004681: G1: Extend marking verification to Full GCs Summary: Perform a heap verification after the first phase of G1's full GC using objects' mark words to determine liveness. The third parameter of the heap verification routines, which was used in G1 to determine which marking bitmap to use in liveness calculations, has been changed from a boolean to an enum with values defined for using the mark word, and the 'prev' and 'next' bitmaps. Reviewed-by: tonyp, ysr
author johnc
date Tue, 14 Jun 2011 11:01:10 -0700
parents 2aa9ddbb9e60
children c9ca3f51cf41
line wrap: on
line source
 * Copyright (c) 2001, 2010, 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_interface/gcCause.hpp"
#include "memory/allocation.hpp"
#include "memory/barrierSet.hpp"
#include "runtime/handles.hpp"
#include "runtime/perfData.hpp"
#include "runtime/safepoint.hpp"

// A "CollectedHeap" is an implementation of a java heap for HotSpot.  This
// is an abstract class: there may be many different kinds of heaps.  This
// class defines the functions that a heap must implement, and contains
// infrastructure common to all heaps.

class BarrierSet;
class ThreadClosure;
class AdaptiveSizePolicy;
class Thread;
class CollectorPolicy;

// CollectedHeap
//   SharedHeap
//     GenCollectedHeap
//     G1CollectedHeap
//   ParallelScavengeHeap
class CollectedHeap : public CHeapObj {
  friend class VMStructs;
  friend class IsGCActiveMark; // Block structured external access to _is_gc_active
  friend class constantPoolCacheKlass; // allocate() method inserts is_conc_safe

#ifdef ASSERT
  static int       _fire_out_of_memory_count;

  // Used for filler objects (static, but initialized in ctor).
  static size_t _filler_array_max_size;

  // Used in support of ReduceInitialCardMarks; only consulted if COMPILER2 is being used
  bool _defer_initial_card_mark;

  MemRegion _reserved;
  BarrierSet* _barrier_set;
  bool _is_gc_active;
  int _n_par_threads;

  unsigned int _total_collections;          // ... started
  unsigned int _total_full_collections;     // ... started
  NOT_PRODUCT(volatile size_t _promotion_failure_alot_count;)
  NOT_PRODUCT(volatile size_t _promotion_failure_alot_gc_number;)

  // Reason for current garbage collection.  Should be set to
  // a value reflecting no collection between collections.
  GCCause::Cause _gc_cause;
  GCCause::Cause _gc_lastcause;
  PerfStringVariable* _perf_gc_cause;
  PerfStringVariable* _perf_gc_lastcause;

  // Constructor

  // Do common initializations that must follow instance construction,
  // for example, those needing virtual calls.
  // This code could perhaps be moved into initialize() but would
  // be slightly more awkward because we want the latter to be a
  // pure virtual.
  void pre_initialize();

  // Create a new tlab
  virtual HeapWord* allocate_new_tlab(size_t size);

  // Accumulate statistics on all tlabs.
  virtual void accumulate_statistics_all_tlabs();

  // Reinitialize tlabs before resuming mutators.
  virtual void resize_all_tlabs();

  // Allocate from the current thread's TLAB, with broken-out slow path.
  inline static HeapWord* allocate_from_tlab(Thread* thread, size_t size);
  static HeapWord* allocate_from_tlab_slow(Thread* thread, size_t size);

  // Allocate an uninitialized block of the given size, or returns NULL if
  // this is impossible.
  inline static HeapWord* common_mem_allocate_noinit(size_t size, bool is_noref, TRAPS);

  // Like allocate_init, but the block returned by a successful allocation
  // is guaranteed initialized to zeros.
  inline static HeapWord* common_mem_allocate_init(size_t size, bool is_noref, TRAPS);

  // Same as common_mem version, except memory is allocated in the permanent area
  // If there is no permanent area, revert to common_mem_allocate_noinit
  inline static HeapWord* common_permanent_mem_allocate_noinit(size_t size, TRAPS);

  // Same as common_mem version, except memory is allocated in the permanent area
  // If there is no permanent area, revert to common_mem_allocate_init
  inline static HeapWord* common_permanent_mem_allocate_init(size_t size, TRAPS);

  // Helper functions for (VM) allocation.
  inline static void post_allocation_setup_common(KlassHandle klass,
                                                  HeapWord* obj, size_t size);
  inline static void post_allocation_setup_no_klass_install(KlassHandle klass,
                                                            HeapWord* objPtr,
                                                            size_t size);

  inline static void post_allocation_setup_obj(KlassHandle klass,
                                               HeapWord* obj, size_t size);

  inline static void post_allocation_setup_array(KlassHandle klass,
                                                 HeapWord* obj, size_t size,
                                                 int length);

  // Clears an allocated object.
  inline static void init_obj(HeapWord* obj, size_t size);

  // Filler object utilities.
  static inline size_t filler_array_hdr_size();
  static inline size_t filler_array_min_size();
  static inline size_t filler_array_max_size();

  DEBUG_ONLY(static void fill_args_check(HeapWord* start, size_t words);)
  DEBUG_ONLY(static void zap_filler_array(HeapWord* start, size_t words, bool zap = true);)

  // Fill with a single array; caller must ensure filler_array_min_size() <=
  // words <= filler_array_max_size().
  static inline void fill_with_array(HeapWord* start, size_t words, bool zap = true);

  // Fill with a single object (either an int array or a java.lang.Object).
  static inline void fill_with_object_impl(HeapWord* start, size_t words, bool zap = true);

  // Verification functions
  virtual void check_for_bad_heap_word_value(HeapWord* addr, size_t size)
  virtual void check_for_non_bad_heap_word_value(HeapWord* addr, size_t size)
  debug_only(static void check_for_valid_allocation_state();)

  enum Name {

  virtual CollectedHeap::Name kind() const { return CollectedHeap::Abstract; }

   * Returns JNI error code JNI_ENOMEM if memory could not be allocated,
   * and JNI_OK on success.
  virtual jint initialize() = 0;

  // In many heaps, there will be a need to perform some initialization activities
  // after the Universe is fully formed, but before general heap allocation is allowed.
  // This is the correct place to place such initialization methods.
  virtual void post_initialize() = 0;

  MemRegion reserved_region() const { return _reserved; }
  address base() const { return (address)reserved_region().start(); }

  // Future cleanup here. The following functions should specify bytes or
  // heapwords as part of their signature.
  virtual size_t capacity() const = 0;
  virtual size_t used() const = 0;

  // Return "true" if the part of the heap that allocates Java
  // objects has reached the maximal committed limit that it can
  // reach, without a garbage collection.
  virtual bool is_maximal_no_gc() const = 0;

  virtual size_t permanent_capacity() const = 0;
  virtual size_t permanent_used() const = 0;

  // Support for java.lang.Runtime.maxMemory():  return the maximum amount of
  // memory that the vm could make available for storing 'normal' java objects.
  // This is based on the reserved address space, but should not include space
  // that the vm uses internally for bookkeeping or temporary storage (e.g.,
  // perm gen space or, in the case of the young gen, one of the survivor
  // spaces).
  virtual size_t max_capacity() const = 0;

  // Returns "TRUE" if "p" points into the reserved area of the heap.
  bool is_in_reserved(const void* p) const {
    return _reserved.contains(p);

  bool is_in_reserved_or_null(const void* p) const {
    return p == NULL || is_in_reserved(p);

  // Returns "TRUE" if "p" points to the head of an allocated object in the
  // heap. Since this method can be expensive in general, we restrict its
  // use to assertion checking only.
  virtual bool is_in(const void* p) const = 0;

  bool is_in_or_null(const void* p) const {
    return p == NULL || is_in(p);

  // Let's define some terms: a "closed" subset of a heap is one that
  // 1) contains all currently-allocated objects, and
  // 2) is closed under reference: no object in the closed subset
  //    references one outside the closed subset.
  // Membership in a heap's closed subset is useful for assertions.
  // Clearly, the entire heap is a closed subset, so the default
  // implementation is to use "is_in_reserved".  But this may not be too
  // liberal to perform useful checking.  Also, the "is_in" predicate
  // defines a closed subset, but may be too expensive, since "is_in"
  // verifies that its argument points to an object head.  The
  // "closed_subset" method allows a heap to define an intermediate
  // predicate, allowing more precise checking than "is_in_reserved" at
  // lower cost than "is_in."

  // One important case is a heap composed of disjoint contiguous spaces,
  // such as the Garbage-First collector.  Such heaps have a convenient
  // closed subset consisting of the allocated portions of those
  // contiguous spaces.

  // Return "TRUE" iff the given pointer points into the heap's defined
  // closed subset (which defaults to the entire heap).
  virtual bool is_in_closed_subset(const void* p) const {
    return is_in_reserved(p);

  bool is_in_closed_subset_or_null(const void* p) const {
    return p == NULL || is_in_closed_subset(p);

  // XXX is_permanent() and is_in_permanent() should be better named
  // to distinguish one from the other.

  // Returns "TRUE" if "p" is allocated as "permanent" data.
  // If the heap does not use "permanent" data, returns the same
  // value is_in_reserved() would return.
  // NOTE: this actually returns true if "p" is in reserved space
  // for the space not that it is actually allocated (i.e. in committed
  // space). If you need the more conservative answer use is_permanent().
  virtual bool is_in_permanent(const void *p) const = 0;

#ifdef ASSERT
  // Returns true if "p" is in the part of the
  // heap being collected.
  virtual bool is_in_partial_collection(const void *p) = 0;

  bool is_in_permanent_or_null(const void *p) const {
    return p == NULL || is_in_permanent(p);

  // Returns "TRUE" if "p" is in the committed area of  "permanent" data.
  // If the heap does not use "permanent" data, returns the same
  // value is_in() would return.
  virtual bool is_permanent(const void *p) const = 0;

  bool is_permanent_or_null(const void *p) const {
    return p == NULL || is_permanent(p);

  // An object is scavengable if its location may move during a scavenge.
  // (A scavenge is a GC which is not a full GC.)
  virtual bool is_scavengable(const void *p) = 0;

  // Returns "TRUE" if "p" is a method oop in the
  // current heap, with high probability. This predicate
  // is not stable, in general.
  bool is_valid_method(oop p) const;

  void set_gc_cause(GCCause::Cause v) {
     if (UsePerfData) {
       _gc_lastcause = _gc_cause;
    _gc_cause = v;
  GCCause::Cause gc_cause() { return _gc_cause; }

  // Number of threads currently working on GC tasks.
  int n_par_threads() { return _n_par_threads; }

  // May be overridden to set additional parallelism.
  virtual void set_par_threads(int t) { _n_par_threads = t; };

  // Preload classes into the shared portion of the heap, and then dump
  // that data to a file so that it can be loaded directly by another
  // VM (then terminate).
  virtual void preload_and_dump(TRAPS) { ShouldNotReachHere(); }

  // General obj/array allocation facilities.
  inline static oop obj_allocate(KlassHandle klass, int size, TRAPS);
  inline static oop array_allocate(KlassHandle klass, int size, int length, TRAPS);
  inline static oop large_typearray_allocate(KlassHandle klass, int size, int length, TRAPS);

  // Special obj/array allocation facilities.
  // Some heaps may want to manage "permanent" data uniquely. These default
  // to the general routines if the heap does not support such handling.
  inline static oop permanent_obj_allocate(KlassHandle klass, int size, TRAPS);
  // permanent_obj_allocate_no_klass_install() does not do the installation of
  // the klass pointer in the newly created object (as permanent_obj_allocate()
  // above does).  This allows for a delay in the installation of the klass
  // pointer that is needed during the create of klassKlass's.  The
  // method post_allocation_install_obj_klass() is used to install the
  // klass pointer.
  inline static oop permanent_obj_allocate_no_klass_install(KlassHandle klass,
                                                            int size,
  inline static void post_allocation_install_obj_klass(KlassHandle klass,
                                                       oop obj,
                                                       int size);
  inline static oop permanent_array_allocate(KlassHandle klass, int size, int length, TRAPS);

  // Raw memory allocation facilities
  // The obj and array allocate methods are covers for these methods.
  // The permanent allocation method should default to mem_allocate if
  // permanent memory isn't supported.
  virtual HeapWord* mem_allocate(size_t size,
                                 bool is_noref,
                                 bool is_tlab,
                                 bool* gc_overhead_limit_was_exceeded) = 0;
  virtual HeapWord* permanent_mem_allocate(size_t size) = 0;

  // The boundary between a "large" and "small" array of primitives, in words.
  virtual size_t large_typearray_limit() = 0;

  // Utilities for turning raw memory into filler objects.
  // min_fill_size() is the smallest region that can be filled.
  // fill_with_objects() can fill arbitrary-sized regions of the heap using
  // multiple objects.  fill_with_object() is for regions known to be smaller
  // than the largest array of integers; it uses a single object to fill the
  // region and has slightly less overhead.
  static size_t min_fill_size() {
    return size_t(align_object_size(oopDesc::header_size()));

  static void fill_with_objects(HeapWord* start, size_t words, bool zap = true);

  static void fill_with_object(HeapWord* start, size_t words, bool zap = true);
  static void fill_with_object(MemRegion region, bool zap = true) {
    fill_with_object(region.start(), region.word_size(), zap);
  static void fill_with_object(HeapWord* start, HeapWord* end, bool zap = true) {
    fill_with_object(start, pointer_delta(end, start), zap);

  // 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.)

  // This function returns "true" iff the heap supports this kind of
  // allocation.  (Default is "no".)
  virtual bool supports_inline_contig_alloc() const {
    return false;
  // These functions return the addresses of the fields that define the
  // boundaries of the contiguous allocation area.  (These fields should be
  // physically near to one another.)
  virtual HeapWord** top_addr() const {
    guarantee(false, "inline contiguous allocation not supported");
    return NULL;
  virtual HeapWord** end_addr() const {
    guarantee(false, "inline contiguous allocation not supported");
    return NULL;

  // Some heaps may be in an unparseable state at certain times between
  // collections. This may be necessary for efficient implementation of
  // certain allocation-related activities. Calling this function before
  // attempting to parse a heap ensures that the heap is in a parsable
  // state (provided other concurrent activity does not introduce
  // unparsability). It is normally expected, therefore, that this
  // method is invoked with the world stopped.
  // NOTE: if you override this method, make sure you call
  // super::ensure_parsability so that the non-generational
  // part of the work gets done. See implementation of
  // CollectedHeap::ensure_parsability and, for instance,
  // that of GenCollectedHeap::ensure_parsability().
  // The argument "retire_tlabs" controls whether existing TLABs
  // are merely filled or also retired, thus preventing further
  // allocation from them and necessitating allocation of new TLABs.
  virtual void ensure_parsability(bool retire_tlabs);

  // Return an estimate of the maximum allocation that could be performed
  // without triggering any collection or expansion activity.  In a
  // generational collector, for example, this is probably the largest
  // allocation that could be supported (without expansion) in the youngest
  // generation.  It is "unsafe" because no locks are taken; the result
  // should be treated as an approximation, not a guarantee, for use in
  // heuristic resizing decisions.
  virtual size_t unsafe_max_alloc() = 0;

  // Section on thread-local allocation buffers (TLABs)
  // If the heap supports thread-local allocation buffers, it should override
  // the following methods:
  // Returns "true" iff the heap supports thread-local allocation buffers.
  // The default is "no".
  virtual bool supports_tlab_allocation() const {
    return false;
  // The amount of space available for thread-local allocation buffers.
  virtual size_t tlab_capacity(Thread *thr) const {
    guarantee(false, "thread-local allocation buffers not supported");
    return 0;
  // An estimate of the maximum allocation that could be performed
  // for thread-local allocation buffers without triggering any
  // collection or expansion activity.
  virtual size_t unsafe_max_tlab_alloc(Thread *thr) const {
    guarantee(false, "thread-local allocation buffers not supported");
    return 0;

  // 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.
  virtual bool can_elide_tlab_store_barriers() const = 0;

  // If a compiler is eliding store barriers for TLAB-allocated objects,
  // there is probably a corresponding slow path which can produce
  // an object allocated anywhere.  The compiler's runtime support
  // promises to call this function on such a slow-path-allocated
  // object before performing initializations that have elided
  // store barriers. Returns new_obj, or maybe a safer copy thereof.
  virtual oop new_store_pre_barrier(JavaThread* thread, oop new_obj);

  // Answers whether an initializing store to a new object currently
  // allocated at the given address doesn't need a store
  // barrier. Returns "true" if it doesn't need an initializing
  // store barrier; answers "false" if it does.
  virtual bool can_elide_initializing_store_barrier(oop new_obj) = 0;

  // If a compiler is eliding store barriers for TLAB-allocated objects,
  // we will be informed of a slow-path allocation by a call
  // to new_store_pre_barrier() above. Such a call precedes the
  // initialization of the object itself, and no post-store-barriers will
  // be issued. Some heap types require that the barrier strictly follows
  // the initializing stores. (This is currently implemented by deferring the
  // barrier until the next slow-path allocation or gc-related safepoint.)
  // This interface answers whether a particular heap type needs the card
  // mark to be thus strictly sequenced after the stores.
  virtual bool card_mark_must_follow_store() const = 0;

  // If the CollectedHeap was asked to defer a store barrier above,
  // this informs it to flush such a deferred store barrier to the
  // remembered set.
  virtual void flush_deferred_store_barrier(JavaThread* thread);

  // Can a compiler elide a store barrier when it writes
  // a permanent oop into the heap?  Applies when the compiler
  // is storing x to the heap, where x->is_perm() is true.
  virtual bool can_elide_permanent_oop_store_barriers() const = 0;

  // Does this heap support heap inspection (+PrintClassHistogram?)
  virtual bool supports_heap_inspection() const = 0;

  // 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) = 0;

  // This interface assumes that it's being called by the
  // vm thread. It collects the heap assuming that the
  // heap lock is already held and that we are executing in
  // the context of the vm thread.
  virtual void collect_as_vm_thread(GCCause::Cause cause) = 0;

  // Returns the barrier set for this heap
  BarrierSet* barrier_set() { return _barrier_set; }

  // Returns "true" iff there is a stop-world GC in progress.  (I assume
  // that it should answer "false" for the concurrent part of a concurrent
  // collector -- dld).
  bool is_gc_active() const { return _is_gc_active; }

  // Total number of GC collections (started)
  unsigned int total_collections() const { return _total_collections; }
  unsigned int total_full_collections() const { return _total_full_collections;}

  // Increment total number of GC collections (started)
  // Should be protected but used by PSMarkSweep - cleanup for 1.4.2
  void increment_total_collections(bool full = false) {
    if (full) {

  void increment_total_full_collections() { _total_full_collections++; }

  // Return the AdaptiveSizePolicy for the heap.
  virtual AdaptiveSizePolicy* size_policy() = 0;

  // Return the CollectorPolicy for the heap
  virtual CollectorPolicy* collector_policy() const = 0;

  // Iterate over all the ref-containing fields of all objects, calling
  // "cl.do_oop" on each. This includes objects in permanent memory.
  virtual void oop_iterate(OopClosure* cl) = 0;

  // Iterate over all objects, calling "cl.do_object" on each.
  // This includes objects in permanent memory.
  virtual void object_iterate(ObjectClosure* cl) = 0;

  // Similar to object_iterate() except iterates only
  // over live objects.
  virtual void safe_object_iterate(ObjectClosure* cl) = 0;

  // Behaves the same as oop_iterate, except only traverses
  // interior pointers contained in permanent memory. If there
  // is no permanent memory, does nothing.
  virtual void permanent_oop_iterate(OopClosure* cl) = 0;

  // Behaves the same as object_iterate, except only traverses
  // object contained in permanent memory. If there is no
  // permanent memory, does nothing.
  virtual void permanent_object_iterate(ObjectClosure* cl) = 0;

  // NOTE! There is no requirement that a collector implement these
  // functions.
  // 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 = 0;

  // 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 = 0;

  // 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 = 0;

  // Returns the longest time (in ms) that has elapsed since the last
  // time that any part of the heap was examined by a garbage collection.
  virtual jlong millis_since_last_gc() = 0;

  // Perform any cleanup actions necessary before allowing a verification.
  virtual void prepare_for_verify() = 0;

  // Generate any dumps preceding or following a full gc
  void pre_full_gc_dump();
  void post_full_gc_dump();

  virtual void print() const = 0;
  virtual void print_on(outputStream* st) const = 0;

  // Print all GC threads (other than the VM thread)
  // used by this heap.
  virtual void print_gc_threads_on(outputStream* st) const = 0;
  void print_gc_threads() { print_gc_threads_on(tty); }
  // Iterator for all GC threads (other than VM thread)
  virtual void gc_threads_do(ThreadClosure* tc) const = 0;

  // Print any relevant tracing info that flags imply.
  // Default implementation does nothing.
  virtual void print_tracing_info() const = 0;

  // Heap verification
  virtual void verify(bool allow_dirty, bool silent, VerifyOption option) = 0;

  // Non product verification and debugging.
#ifndef PRODUCT
  // Support for PromotionFailureALot.  Return true if it's time to cause a
  // promotion failure.  The no-argument version uses
  // this->_promotion_failure_alot_count as the counter.
  inline bool promotion_should_fail(volatile size_t* count);
  inline bool promotion_should_fail();

  // Reset the PromotionFailureALot counters.  Should be called at the end of a
  // GC in which promotion failure ocurred.
  inline void reset_promotion_should_fail(volatile size_t* count);
  inline void reset_promotion_should_fail();
#endif  // #ifndef PRODUCT

#ifdef ASSERT
  static int fired_fake_oom() {
    return (CIFireOOMAt > 1 && _fire_out_of_memory_count >= CIFireOOMAt);

  // This is a convenience method that is used in cases where
  // the actual number of GC worker threads is not pertinent but
  // only whether there more than 0.  Use of this method helps
  // reduce the occurrence of ParallelGCThreads to uses where the
  // actual number may be germane.
  static bool use_parallel_gc_threads() { return ParallelGCThreads > 0; }

// Class to set and reset the GC cause for a CollectedHeap.

class GCCauseSetter : StackObj {
  CollectedHeap* _heap;
  GCCause::Cause _previous_cause;
  GCCauseSetter(CollectedHeap* heap, GCCause::Cause cause) {
           "This method manipulates heap state without locking");
    _heap = heap;
    _previous_cause = _heap->gc_cause();

  ~GCCauseSetter() {
          "This method manipulates heap state without locking");