view src/share/vm/gc_implementation/g1/concurrentMark.hpp @ 2740:4dfb2df418f2

6484982: G1: process references during evacuation pauses Summary: G1 now uses two reference processors - one is used by concurrent marking and the other is used by STW GCs (both full and incremental evacuation pauses). In an evacuation pause, the reference processor is embedded into the closures used to scan objects. Doing so causes causes reference objects to be 'discovered' by the reference processor. At the end of the evacuation pause, these discovered reference objects are processed - preserving (and copying) referent objects (and their reachable graphs) as appropriate. Reviewed-by: ysr, jwilhelm, brutisso, stefank, tonyp
author johnc
date Thu, 22 Sep 2011 10:57:37 -0700
parents ff53346271fe
children bca17e38de00
line wrap: on
line source
 * Copyright (c) 2001, 2011, 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/heapRegionSets.hpp"
#include "utilities/taskqueue.hpp"

class G1CollectedHeap;
class CMTask;
typedef GenericTaskQueue<oop>            CMTaskQueue;
typedef GenericTaskQueueSet<CMTaskQueue> CMTaskQueueSet;

// Closure used by CM during concurrent reference discovery
// and reference processing (during remarking) to determine
// if a particular object is alive. It is primarily used
// to determine if referents of discovered reference objects
// are alive. An instance is also embedded into the
// reference processor as the _is_alive_non_header field
class G1CMIsAliveClosure: public BoolObjectClosure {
  G1CollectedHeap* _g1;
  G1CMIsAliveClosure(G1CollectedHeap* g1) :

  void do_object(oop obj) {
  bool do_object_b(oop obj);

// A generic CM bit map.  This is essentially a wrapper around the BitMap
// class, with one bit per (1<<_shifter) HeapWords.

  HeapWord* _bmStartWord;      // base address of range covered by map
  size_t    _bmWordSize;       // map size (in #HeapWords covered)
  const int _shifter;          // map to char or bit
  VirtualSpace _virtual_space; // underlying the bit map
  BitMap    _bm;               // the bit map itself

  // constructor
  CMBitMapRO(ReservedSpace rs, int shifter);

  enum { do_yield = true };

  // inquiries
  HeapWord* startWord()   const { return _bmStartWord; }
  size_t    sizeInWords() const { return _bmWordSize;  }
  // the following is one past the last word in space
  HeapWord* endWord()     const { return _bmStartWord + _bmWordSize; }

  // read marks

  bool isMarked(HeapWord* addr) const {
    assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize),
           "outside underlying space?");

  // iteration
  bool iterate(BitMapClosure* cl) { return _bm.iterate(cl); }
  bool iterate(BitMapClosure* cl, MemRegion mr);

  // Return the address corresponding to the next marked bit at or after
  // "addr", and before "limit", if "limit" is non-NULL.  If there is no
  // such bit, returns "limit" if that is non-NULL, or else "endWord()".
  HeapWord* getNextMarkedWordAddress(HeapWord* addr,
                                     HeapWord* limit = NULL) const;
  // Return the address corresponding to the next unmarked bit at or after
  // "addr", and before "limit", if "limit" is non-NULL.  If there is no
  // such bit, returns "limit" if that is non-NULL, or else "endWord()".
  HeapWord* getNextUnmarkedWordAddress(HeapWord* addr,
                                       HeapWord* limit = NULL) const;

  // conversion utilities
  // XXX Fix these so that offsets are size_t's...
  HeapWord* offsetToHeapWord(size_t offset) const {
    return _bmStartWord + (offset << _shifter);
  size_t heapWordToOffset(HeapWord* addr) const {
    return pointer_delta(addr, _bmStartWord) >> _shifter;
  int heapWordDiffToOffsetDiff(size_t diff) const;
  HeapWord* nextWord(HeapWord* addr) {
    return offsetToHeapWord(heapWordToOffset(addr) + 1);

  void mostly_disjoint_range_union(BitMap*   from_bitmap,
                                   size_t    from_start_index,
                                   HeapWord* to_start_word,
                                   size_t    word_num);

  // debugging
  NOT_PRODUCT(bool covers(ReservedSpace rs) const;)

class CMBitMap : public CMBitMapRO {

  // constructor
  CMBitMap(ReservedSpace rs, int shifter) :
    CMBitMapRO(rs, shifter) {}

  // write marks
  void mark(HeapWord* addr) {
    assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize),
           "outside underlying space?");
  void clear(HeapWord* addr) {
    assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize),
           "outside underlying space?");
  bool parMark(HeapWord* addr) {
    assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize),
           "outside underlying space?");
    return _bm.par_set_bit(heapWordToOffset(addr));
  bool parClear(HeapWord* addr) {
    assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize),
           "outside underlying space?");
    return _bm.par_clear_bit(heapWordToOffset(addr));
  void markRange(MemRegion mr);
  void clearAll();
  void clearRange(MemRegion mr);

  // Starting at the bit corresponding to "addr" (inclusive), find the next
  // "1" bit, if any.  This bit starts some run of consecutive "1"'s; find
  // the end of this run (stopping at "end_addr").  Return the MemRegion
  // covering from the start of the region corresponding to the first bit
  // of the run to the end of the region corresponding to the last bit of
  // the run.  If there is no "1" bit at or after "addr", return an empty
  // MemRegion.
  MemRegion getAndClearMarkedRegion(HeapWord* addr, HeapWord* end_addr);

// Represents a marking stack used by the CM collector.
// Ideally this should be GrowableArray<> just like MSC's marking stack(s).
  ConcurrentMark* _cm;
  oop*   _base;      // bottom of stack
  jint   _index;     // one more than last occupied index
  jint   _capacity;  // max #elements
  jint   _oops_do_bound;  // Number of elements to include in next iteration.
  NOT_PRODUCT(jint _max_depth;)  // max depth plumbed during run

  bool   _overflow;
  DEBUG_ONLY(bool _drain_in_progress;)
  DEBUG_ONLY(bool _drain_in_progress_yields;)

  CMMarkStack(ConcurrentMark* cm);

  void allocate(size_t size);

  oop pop() {
    if (!isEmpty()) {
      return _base[--_index] ;
    return NULL;

  // If overflow happens, don't do the push, and record the overflow.
  // *Requires* that "ptr" is already marked.
  void push(oop ptr) {
    if (isFull()) {
      // Record overflow.
      _overflow = true;
    } else {
      _base[_index++] = ptr;
      NOT_PRODUCT(_max_depth = MAX2(_max_depth, _index));
  // Non-block impl.  Note: concurrency is allowed only with other
  // "par_push" operations, not with "pop" or "drain".  We would need
  // parallel versions of them if such concurrency was desired.
  void par_push(oop ptr);

  // Pushes the first "n" elements of "ptr_arr" on the stack.
  // Non-block impl.  Note: concurrency is allowed only with other
  // "par_adjoin_arr" or "push" operations, not with "pop" or "drain".
  void par_adjoin_arr(oop* ptr_arr, int n);

  // Pushes the first "n" elements of "ptr_arr" on the stack.
  // Locking impl: concurrency is allowed only with
  // "par_push_arr" and/or "par_pop_arr" operations, which use the same
  // locking strategy.
  void par_push_arr(oop* ptr_arr, int n);

  // If returns false, the array was empty.  Otherwise, removes up to "max"
  // elements from the stack, and transfers them to "ptr_arr" in an
  // unspecified order.  The actual number transferred is given in "n" ("n
  // == 0" is deliberately redundant with the return value.)  Locking impl:
  // concurrency is allowed only with "par_push_arr" and/or "par_pop_arr"
  // operations, which use the same locking strategy.
  bool par_pop_arr(oop* ptr_arr, int max, int* n);

  // Drain the mark stack, applying the given closure to all fields of
  // objects on the stack.  (That is, continue until the stack is empty,
  // even if closure applications add entries to the stack.)  The "bm"
  // argument, if non-null, may be used to verify that only marked objects
  // are on the mark stack.  If "yield_after" is "true", then the
  // concurrent marker performing the drain offers to yield after
  // processing each object.  If a yield occurs, stops the drain operation
  // and returns false.  Otherwise, returns true.
  template<class OopClosureClass>
  bool drain(OopClosureClass* cl, CMBitMap* bm, bool yield_after = false);

  bool isEmpty()    { return _index == 0; }
  bool isFull()     { return _index == _capacity; }
  int maxElems()    { return _capacity; }

  bool overflow() { return _overflow; }
  void clear_overflow() { _overflow = false; }

  int  size() { return _index; }

  void setEmpty()   { _index = 0; clear_overflow(); }

  // Record the current size; a subsequent "oops_do" will iterate only over
  // indices valid at the time of this call.
  void set_oops_do_bound(jint bound = -1) {
    if (bound == -1) {
      _oops_do_bound = _index;
    } else {
      _oops_do_bound = bound;
  jint oops_do_bound() { return _oops_do_bound; }
  // iterate over the oops in the mark stack, up to the bound recorded via
  // the call above.
  void oops_do(OopClosure* f);

class CMRegionStack VALUE_OBJ_CLASS_SPEC {
  MemRegion* _base;
  jint _capacity;
  jint _index;
  jint _oops_do_bound;
  bool _overflow;
  void allocate(size_t size);

  // This is lock-free; assumes that it will only be called in parallel
  // with other "push" operations (no pops).
  void push_lock_free(MemRegion mr);

  // Lock-free; assumes that it will only be called in parallel
  // with other "pop" operations (no pushes).
  MemRegion pop_lock_free();

#if 0
  // The routines that manipulate the region stack with a lock are
  // not currently used. They should be retained, however, as a
  // diagnostic aid.

  // These two are the implementations that use a lock. They can be
  // called concurrently with each other but they should not be called
  // concurrently with the lock-free versions (push() / pop()).
  void push_with_lock(MemRegion mr);
  MemRegion pop_with_lock();

  bool isEmpty()    { return _index == 0; }
  bool isFull()     { return _index == _capacity; }

  bool overflow() { return _overflow; }
  void clear_overflow() { _overflow = false; }

  int  size() { return _index; }

  // It iterates over the entries in the region stack and it
  // invalidates (i.e. assigns MemRegion()) the ones that point to
  // regions in the collection set.
  bool invalidate_entries_into_cset();

  // This gives an upper bound up to which the iteration in
  // invalidate_entries_into_cset() will reach. This prevents
  // newly-added entries to be unnecessarily scanned.
  void set_oops_do_bound() {
    _oops_do_bound = _index;

  void setEmpty()   { _index = 0; clear_overflow(); }

class ForceOverflowSettings VALUE_OBJ_CLASS_SPEC {
#ifndef PRODUCT
  uintx _num_remaining;
  bool _force;
#endif // !defined(PRODUCT)

  void init() PRODUCT_RETURN;
  void update() PRODUCT_RETURN;
  bool should_force() PRODUCT_RETURN_( return false; );

// this will enable a variety of different statistics per GC task
#define _MARKING_STATS_       0
// this will enable the higher verbose levels
#define _MARKING_VERBOSE_     0

#define statsOnly(statement)  \
do {                          \
  statement ;                 \
} while (0)
#else // _MARKING_STATS_
#define statsOnly(statement)  \
do {                          \
} while (0)
#endif // _MARKING_STATS_

typedef enum {
  no_verbose  = 0,   // verbose turned off
  stats_verbose,     // only prints stats at the end of marking
  low_verbose,       // low verbose, mostly per region and per major event
  medium_verbose,    // a bit more detailed than low
  high_verbose       // per object verbose
} CMVerboseLevel;

class ConcurrentMarkThread;

class ConcurrentMark: public CHeapObj {
  friend class ConcurrentMarkThread;
  friend class CMTask;
  friend class CMBitMapClosure;
  friend class CSMarkOopClosure;
  friend class CMGlobalObjectClosure;
  friend class CMRemarkTask;
  friend class CMConcurrentMarkingTask;
  friend class G1ParNoteEndTask;
  friend class CalcLiveObjectsClosure;
  friend class G1CMRefProcTaskProxy;
  friend class G1CMRefProcTaskExecutor;
  friend class G1CMParKeepAliveAndDrainClosure;
  friend class G1CMParDrainMarkingStackClosure;

  ConcurrentMarkThread* _cmThread;   // the thread doing the work
  G1CollectedHeap*      _g1h;        // the heap.
  size_t                _parallel_marking_threads; // the number of marking
                                                   // threads we'll use
  double                _sleep_factor; // how much we have to sleep, with
                                       // respect to the work we just did, to
                                       // meet the marking overhead goal
  double                _marking_task_overhead; // marking target overhead for
                                                // a single task

  // same as the two above, but for the cleanup task
  double                _cleanup_sleep_factor;
  double                _cleanup_task_overhead;

  FreeRegionList        _cleanup_list;

  // CMS marking support structures
  CMBitMap                _markBitMap1;
  CMBitMap                _markBitMap2;
  CMBitMapRO*             _prevMarkBitMap; // completed mark bitmap
  CMBitMap*               _nextMarkBitMap; // under-construction mark bitmap
  bool                    _at_least_one_mark_complete;

  BitMap                  _region_bm;
  BitMap                  _card_bm;

  // Heap bounds
  HeapWord*               _heap_start;
  HeapWord*               _heap_end;

  // For gray objects
  CMMarkStack             _markStack; // Grey objects behind global finger.
  CMRegionStack           _regionStack; // Grey regions behind global finger.
  HeapWord* volatile      _finger;  // the global finger, region aligned,
                                    // always points to the end of the
                                    // last claimed region

  // marking tasks
  size_t                  _max_task_num; // maximum task number
  size_t                  _active_tasks; // task num currently active
  CMTask**                _tasks;        // task queue array (max_task_num len)
  CMTaskQueueSet*         _task_queues;  // task queue set
  ParallelTaskTerminator  _terminator;   // for termination

  // Two sync barriers that are used to synchronise tasks when an
  // overflow occurs. The algorithm is the following. All tasks enter
  // the first one to ensure that they have all stopped manipulating
  // the global data structures. After they exit it, they re-initialise
  // their data structures and task 0 re-initialises the global data
  // structures. Then, they enter the second sync barrier. This
  // ensure, that no task starts doing work before all data
  // structures (local and global) have been re-initialised. When they
  // exit it, they are free to start working again.
  WorkGangBarrierSync     _first_overflow_barrier_sync;
  WorkGangBarrierSync     _second_overflow_barrier_sync;

  // this is set by any task, when an overflow on the global data
  // structures is detected.
  volatile bool           _has_overflown;
  // true: marking is concurrent, false: we're in remark
  volatile bool           _concurrent;
  // set at the end of a Full GC so that marking aborts
  volatile bool           _has_aborted;

  // used when remark aborts due to an overflow to indicate that
  // another concurrent marking phase should start
  volatile bool           _restart_for_overflow;

  // This is true from the very start of concurrent marking until the
  // point when all the tasks complete their work. It is really used
  // to determine the points between the end of concurrent marking and
  // time of remark.
  volatile bool           _concurrent_marking_in_progress;

  // verbose level
  CMVerboseLevel          _verbose_level;

  // These two fields are used to implement the optimisation that
  // avoids pushing objects on the global/region stack if there are
  // no collection set regions above the lowest finger.

  // This is the lowest finger (among the global and local fingers),
  // which is calculated before a new collection set is chosen.
  HeapWord* _min_finger;
  // If this flag is true, objects/regions that are marked below the
  // finger should be pushed on the stack(s). If this is flag is
  // false, it is safe not to push them on the stack(s).
  bool      _should_gray_objects;

  // All of these times are in ms.
  NumberSeq _init_times;
  NumberSeq _remark_times;
  NumberSeq   _remark_mark_times;
  NumberSeq   _remark_weak_ref_times;
  NumberSeq _cleanup_times;
  double    _total_counting_time;
  double    _total_rs_scrub_time;

  double*   _accum_task_vtime;   // accumulated task vtime

  WorkGang* _parallel_workers;

  ForceOverflowSettings _force_overflow_conc;
  ForceOverflowSettings _force_overflow_stw;

  void weakRefsWork(bool clear_all_soft_refs);

  void swapMarkBitMaps();

  // It resets the global marking data structures, as well as the
  // task local ones; should be called during initial mark.
  void reset();
  // It resets all the marking data structures.
  void clear_marking_state(bool clear_overflow = true);

  // It should be called to indicate which phase we're in (concurrent
  // mark or remark) and how many threads are currently active.
  void set_phase(size_t active_tasks, bool concurrent);
  // We do this after we're done with marking so that the marking data
  // structures are initialised to a sensible and predictable state.
  void set_non_marking_state();

  // prints all gathered CM-related statistics
  void print_stats();

  bool cleanup_list_is_empty() {
    return _cleanup_list.is_empty();

  // accessor methods
  size_t parallel_marking_threads() { return _parallel_marking_threads; }
  double sleep_factor()             { return _sleep_factor; }
  double marking_task_overhead()    { return _marking_task_overhead;}
  double cleanup_sleep_factor()     { return _cleanup_sleep_factor; }
  double cleanup_task_overhead()    { return _cleanup_task_overhead;}

  HeapWord*               finger()        { return _finger;   }
  bool                    concurrent()    { return _concurrent; }
  size_t                  active_tasks()  { return _active_tasks; }
  ParallelTaskTerminator* terminator()    { return &_terminator; }

  // It claims the next available region to be scanned by a marking
  // task. It might return NULL if the next region is empty or we have
  // run out of regions. In the latter case, out_of_regions()
  // determines whether we've really run out of regions or the task
  // should call claim_region() again.  This might seem a bit
  // awkward. Originally, the code was written so that claim_region()
  // either successfully returned with a non-empty region or there
  // were no more regions to be claimed. The problem with this was
  // that, in certain circumstances, it iterated over large chunks of
  // the heap finding only empty regions and, while it was working, it
  // was preventing the calling task to call its regular clock
  // method. So, this way, each task will spend very little time in
  // claim_region() and is allowed to call the regular clock method
  // frequently.
  HeapRegion* claim_region(int task);

  // It determines whether we've run out of regions to scan.
  bool        out_of_regions() { return _finger == _heap_end; }

  // Returns the task with the given id
  CMTask* task(int id) {
    assert(0 <= id && id < (int) _active_tasks,
           "task id not within active bounds");
    return _tasks[id];

  // Returns the task queue with the given id
  CMTaskQueue* task_queue(int id) {
    assert(0 <= id && id < (int) _active_tasks,
           "task queue id not within active bounds");
    return (CMTaskQueue*) _task_queues->queue(id);

  // Returns the task queue set
  CMTaskQueueSet* task_queues()  { return _task_queues; }

  // Access / manipulation of the overflow flag which is set to
  // indicate that the global stack or region stack has overflown
  bool has_overflown()           { return _has_overflown; }
  void set_has_overflown()       { _has_overflown = true; }
  void clear_has_overflown()     { _has_overflown = false; }

  bool has_aborted()             { return _has_aborted; }
  bool restart_for_overflow()    { return _restart_for_overflow; }

  // Methods to enter the two overflow sync barriers
  void enter_first_sync_barrier(int task_num);
  void enter_second_sync_barrier(int task_num);

  ForceOverflowSettings* force_overflow_conc() {
    return &_force_overflow_conc;

  ForceOverflowSettings* force_overflow_stw() {
    return &_force_overflow_stw;

  ForceOverflowSettings* force_overflow() {
    if (concurrent()) {
      return force_overflow_conc();
    } else {
      return force_overflow_stw();

  // Manipulation of the global mark stack.
  // Notice that the first mark_stack_push is CAS-based, whereas the
  // two below are Mutex-based. This is OK since the first one is only
  // called during evacuation pauses and doesn't compete with the
  // other two (which are called by the marking tasks during
  // concurrent marking or remark).
  bool mark_stack_push(oop p) {
    if (_markStack.overflow()) {
      return false;
    return true;
  bool mark_stack_push(oop* arr, int n) {
    _markStack.par_push_arr(arr, n);
    if (_markStack.overflow()) {
      return false;
    return true;
  void mark_stack_pop(oop* arr, int max, int* n) {
    _markStack.par_pop_arr(arr, max, n);
  size_t mark_stack_size()                { return _markStack.size(); }
  size_t partial_mark_stack_size_target() { return _markStack.maxElems()/3; }
  bool mark_stack_overflow()              { return _markStack.overflow(); }
  bool mark_stack_empty()                 { return _markStack.isEmpty(); }

  // (Lock-free) Manipulation of the region stack
  bool region_stack_push_lock_free(MemRegion mr) {
    // Currently we only call the lock-free version during evacuation
    // pauses.
    assert(SafepointSynchronize::is_at_safepoint(), "world should be stopped");

    if (_regionStack.overflow()) {
      return false;
    return true;

  // Lock-free version of region-stack pop. Should only be
  // called in tandem with other lock-free pops.
  MemRegion region_stack_pop_lock_free() {
    return _regionStack.pop_lock_free();

#if 0
  // The routines that manipulate the region stack with a lock are
  // not currently used. They should be retained, however, as a
  // diagnostic aid.

  bool region_stack_push_with_lock(MemRegion mr) {
    // Currently we only call the lock-based version during either
    // concurrent marking or remark.
    assert(!SafepointSynchronize::is_at_safepoint() || !concurrent(),
           "if we are at a safepoint it should be the remark safepoint");

    if (_regionStack.overflow()) {
      return false;
    return true;

  MemRegion region_stack_pop_with_lock() {
    // Currently we only call the lock-based version during either
    // concurrent marking or remark.
    assert(!SafepointSynchronize::is_at_safepoint() || !concurrent(),
           "if we are at a safepoint it should be the remark safepoint");

    return _regionStack.pop_with_lock();

  int region_stack_size()               { return _regionStack.size(); }
  bool region_stack_overflow()          { return _regionStack.overflow(); }
  bool region_stack_empty()             { return _regionStack.isEmpty(); }

  // Iterate over any regions that were aborted while draining the
  // region stack (any such regions are saved in the corresponding
  // CMTask) and invalidate (i.e. assign to the empty MemRegion())
  // any regions that point into the collection set.
  bool invalidate_aborted_regions_in_cset();

  // Returns true if there are any aborted memory regions.
  bool has_aborted_regions();

  bool concurrent_marking_in_progress() {
    return _concurrent_marking_in_progress;
  void set_concurrent_marking_in_progress() {
    _concurrent_marking_in_progress = true;
  void clear_concurrent_marking_in_progress() {
    _concurrent_marking_in_progress = false;

  void update_accum_task_vtime(int i, double vtime) {
    _accum_task_vtime[i] += vtime;

  double all_task_accum_vtime() {
    double ret = 0.0;
    for (int i = 0; i < (int)_max_task_num; ++i)
      ret += _accum_task_vtime[i];
    return ret;

  // Attempts to steal an object from the task queues of other tasks
  bool try_stealing(int task_num, int* hash_seed, oop& obj) {
    return _task_queues->steal(task_num, hash_seed, obj);

  // It grays an object by first marking it. Then, if it's behind the
  // global finger, it also pushes it on the global stack.
  void deal_with_reference(oop obj);

  ConcurrentMark(ReservedSpace rs, int max_regions);
  ConcurrentMarkThread* cmThread() { return _cmThread; }

  CMBitMapRO* prevMarkBitMap() const { return _prevMarkBitMap; }
  CMBitMap*   nextMarkBitMap() const { return _nextMarkBitMap; }

  // The following three are interaction between CM and
  // G1CollectedHeap

  // This notifies CM that a root during initial-mark needs to be
  // grayed and it's MT-safe. Currently, we just mark it. But, in the
  // future, we can experiment with pushing it on the stack and we can
  // do this without changing G1CollectedHeap.
  void grayRoot(oop p);
  // It's used during evacuation pauses to gray a region, if
  // necessary, and it's MT-safe. It assumes that the caller has
  // marked any objects on that region. If _should_gray_objects is
  // true and we're still doing concurrent marking, the region is
  // pushed on the region stack, if it is located below the global
  // finger, otherwise we do nothing.
  void grayRegionIfNecessary(MemRegion mr);
  // It's used during evacuation pauses to mark and, if necessary,
  // gray a single object and it's MT-safe. It assumes the caller did
  // not mark the object. If _should_gray_objects is true and we're
  // still doing concurrent marking, the objects is pushed on the
  // global stack, if it is located below the global finger, otherwise
  // we do nothing.
  void markAndGrayObjectIfNecessary(oop p);

  // It iterates over the heap and for each object it comes across it
  // will dump the contents of its reference fields, as well as
  // liveness information for the object and its referents. The dump
  // will be written to a file with the following name:
  // G1PrintReachableBaseFile + "." + str.
  // vo decides whether the prev (vo == UsePrevMarking), the next
  // (vo == UseNextMarking) marking information, or the mark word
  // (vo == UseMarkWord) will be used to determine the liveness of
  // each object / referent.
  // If all is true, all objects in the heap will be dumped, otherwise
  // only the live ones. In the dump the following symbols / breviations
  // are used:
  //   M : an explicitly live object (its bitmap bit is set)
  //   > : an implicitly live object (over tams)
  //   O : an object outside the G1 heap (typically: in the perm gen)
  //   NOT : a reference field whose referent is not live
  //   AND MARKED : indicates that an object is both explicitly and
  //   implicitly live (it should be one or the other, not both)
  void print_reachable(const char* str,
                       VerifyOption vo, bool all) PRODUCT_RETURN;

  // Clear the next marking bitmap (will be called concurrently).
  void clearNextBitmap();

  // These two do the work that needs to be done before and after the
  // initial root checkpoint. Since this checkpoint can be done at two
  // different points (i.e. an explicit pause or piggy-backed on a
  // young collection), then it's nice to be able to easily share the
  // pre/post code. It might be the case that we can put everything in
  // the post method. TP
  void checkpointRootsInitialPre();
  void checkpointRootsInitialPost();

  // Do concurrent phase of marking, to a tentative transitive closure.
  void markFromRoots();

  // Process all unprocessed SATB buffers. It is called at the
  // beginning of an evacuation pause.
  void drainAllSATBBuffers();

  void checkpointRootsFinal(bool clear_all_soft_refs);
  void checkpointRootsFinalWork();
  void calcDesiredRegions();
  void cleanup();
  void completeCleanup();

  // Mark in the previous bitmap.  NB: this is usually read-only, so use
  // this carefully!
  void markPrev(oop p);
  void clear(oop p);
  // Clears marks for all objects in the given range, for both prev and
  // next bitmaps.  NB: the previous bitmap is usually read-only, so use
  // this carefully!
  void clearRangeBothMaps(MemRegion mr);

  // Record the current top of the mark and region stacks; a
  // subsequent oops_do() on the mark stack and
  // invalidate_entries_into_cset() on the region stack will iterate
  // only over indices valid at the time of this call.
  void set_oops_do_bound() {
  // Iterate over the oops in the mark stack and all local queues. It
  // also calls invalidate_entries_into_cset() on the region stack.
  void oops_do(OopClosure* f);
  // It is called at the end of an evacuation pause during marking so
  // that CM is notified of where the new end of the heap is. It
  // doesn't do anything if concurrent_marking_in_progress() is false,
  // unless the force parameter is true.
  void update_g1_committed(bool force = false);

  void complete_marking_in_collection_set();

  // It indicates that a new collection set is being chosen.
  void newCSet();

  // It registers a collection set heap region with CM. This is used
  // to determine whether any heap regions are located above the finger.
  void registerCSetRegion(HeapRegion* hr);

  // Resets the region fields of any active CMTask whose region fields
  // are in the collection set (i.e. the region currently claimed by
  // the CMTask will be evacuated and may be used, subsequently, as
  // an alloc region). When this happens the region fields in the CMTask
  // are stale and, hence, should be cleared causing the worker thread
  // to claim a new region.
  void reset_active_task_region_fields_in_cset();

  // Registers the maximum region-end associated with a set of
  // regions with CM. Again this is used to determine whether any
  // heap regions are located above the finger.
  void register_collection_set_finger(HeapWord* max_finger) {
    // max_finger is the highest heap region end of the regions currently
    // contained in the collection set. If this value is larger than
    // _min_finger then we need to gray objects.
    // This routine is like registerCSetRegion but for an entire
    // collection of regions.
    if (max_finger > _min_finger) {
      _should_gray_objects = true;

  // Returns "true" if at least one mark has been completed.
  bool at_least_one_mark_complete() { return _at_least_one_mark_complete; }

  bool isMarked(oop p) const {
    assert(p != NULL && p->is_oop(), "expected an oop");
    HeapWord* addr = (HeapWord*)p;
    assert(addr >= _nextMarkBitMap->startWord() ||
           addr < _nextMarkBitMap->endWord(), "in a region");

    return _nextMarkBitMap->isMarked(addr);

  inline bool not_yet_marked(oop p) const;

  // XXX Debug code
  bool containing_card_is_marked(void* p);
  bool containing_cards_are_marked(void* start, void* last);

  bool isPrevMarked(oop p) const {
    assert(p != NULL && p->is_oop(), "expected an oop");
    HeapWord* addr = (HeapWord*)p;
    assert(addr >= _prevMarkBitMap->startWord() ||
           addr < _prevMarkBitMap->endWord(), "in a region");

    return _prevMarkBitMap->isMarked(addr);

  inline bool do_yield_check(int worker_i = 0);
  inline bool should_yield();

  // Called to abort the marking cycle after a Full GC takes palce.
  void abort();

  // This prints the global/local fingers. It is used for debugging.
  NOT_PRODUCT(void print_finger();)

  void print_summary_info();

  void print_worker_threads_on(outputStream* st) const;

  // The following indicate whether a given verbose level has been
  // set. Notice that anything above stats is conditional to
  // _MARKING_VERBOSE_ having been set to 1
  bool verbose_stats() {
    return _verbose_level >= stats_verbose;
  bool verbose_low() {
    return _MARKING_VERBOSE_ && _verbose_level >= low_verbose;
  bool verbose_medium() {
    return _MARKING_VERBOSE_ && _verbose_level >= medium_verbose;
  bool verbose_high() {
    return _MARKING_VERBOSE_ && _verbose_level >= high_verbose;

// A class representing a marking task.
class CMTask : public TerminatorTerminator {
  enum PrivateConstants {
    // the regular clock call is called once the scanned words reaches
    // this limit
    words_scanned_period          = 12*1024,
    // the regular clock call is called once the number of visited
    // references reaches this limit
    refs_reached_period           = 384,
    // initial value for the hash seed, used in the work stealing code
    init_hash_seed                = 17,
    // how many entries will be transferred between global stack and
    // local queues
    global_stack_transfer_size    = 16

  int                         _task_id;
  G1CollectedHeap*            _g1h;
  ConcurrentMark*             _cm;
  CMBitMap*                   _nextMarkBitMap;
  // the task queue of this task
  CMTaskQueue*                _task_queue;
  // the task queue set---needed for stealing
  CMTaskQueueSet*             _task_queues;
  // indicates whether the task has been claimed---this is only  for
  // debugging purposes
  bool                        _claimed;

  // number of calls to this task
  int                         _calls;

  // when the virtual timer reaches this time, the marking step should
  // exit
  double                      _time_target_ms;
  // the start time of the current marking step
  double                      _start_time_ms;

  // the oop closure used for iterations over oops
  G1CMOopClosure*             _cm_oop_closure;

  // the region this task is scanning, NULL if we're not scanning any
  HeapRegion*                 _curr_region;
  // the local finger of this task, NULL if we're not scanning a region
  HeapWord*                   _finger;
  // limit of the region this task is scanning, NULL if we're not scanning one
  HeapWord*                   _region_limit;

  // This is used only when we scan regions popped from the region
  // stack. It records what the last object on such a region we
  // scanned was. It is used to ensure that, if we abort region
  // iteration, we do not rescan the first part of the region. This
  // should be NULL when we're not scanning a region from the region
  // stack.
  HeapWord*                   _region_finger;

  // If we abort while scanning a region we record the remaining
  // unscanned portion and check this field when marking restarts.
  // This avoids having to push on the region stack while other
  // marking threads may still be popping regions.
  // If we were to push the unscanned portion directly to the
  // region stack then we would need to using locking versions
  // of the push and pop operations.
  MemRegion                   _aborted_region;

  // the number of words this task has scanned
  size_t                      _words_scanned;
  // When _words_scanned reaches this limit, the regular clock is
  // called. Notice that this might be decreased under certain
  // circumstances (i.e. when we believe that we did an expensive
  // operation).
  size_t                      _words_scanned_limit;
  // the initial value of _words_scanned_limit (i.e. what it was
  // before it was decreased).
  size_t                      _real_words_scanned_limit;

  // the number of references this task has visited
  size_t                      _refs_reached;
  // When _refs_reached reaches this limit, the regular clock is
  // called. Notice this this might be decreased under certain
  // circumstances (i.e. when we believe that we did an expensive
  // operation).
  size_t                      _refs_reached_limit;
  // the initial value of _refs_reached_limit (i.e. what it was before
  // it was decreased).
  size_t                      _real_refs_reached_limit;

  // used by the work stealing stuff
  int                         _hash_seed;
  // if this is true, then the task has aborted for some reason
  bool                        _has_aborted;
  // set when the task aborts because it has met its time quota
  bool                        _has_timed_out;
  // true when we're draining SATB buffers; this avoids the task
  // aborting due to SATB buffers being available (as we're already
  // dealing with them)
  bool                        _draining_satb_buffers;

  // number sequence of past step times
  NumberSeq                   _step_times_ms;
  // elapsed time of this task
  double                      _elapsed_time_ms;
  // termination time of this task
  double                      _termination_time_ms;
  // when this task got into the termination protocol
  double                      _termination_start_time_ms;

  // true when the task is during a concurrent phase, false when it is
  // in the remark phase (so, in the latter case, we do not have to
  // check all the things that we have to check during the concurrent
  // phase, i.e. SATB buffer availability...)
  bool                        _concurrent;

  TruncatedSeq                _marking_step_diffs_ms;

  // LOTS of statistics related with this task
  NumberSeq                   _all_clock_intervals_ms;
  double                      _interval_start_time_ms;

  int                         _aborted;
  int                         _aborted_overflow;
  int                         _aborted_cm_aborted;
  int                         _aborted_yield;
  int                         _aborted_timed_out;
  int                         _aborted_satb;
  int                         _aborted_termination;

  int                         _steal_attempts;
  int                         _steals;

  int                         _clock_due_to_marking;
  int                         _clock_due_to_scanning;

  int                         _local_pushes;
  int                         _local_pops;
  int                         _local_max_size;
  int                         _objs_scanned;

  int                         _global_pushes;
  int                         _global_pops;
  int                         _global_max_size;

  int                         _global_transfers_to;
  int                         _global_transfers_from;

  int                         _region_stack_pops;

  int                         _regions_claimed;
  int                         _objs_found_on_bitmap;

  int                         _satb_buffers_processed;
#endif // _MARKING_STATS_

  // it updates the local fields after this task has claimed
  // a new region to scan
  void setup_for_region(HeapRegion* hr);
  // it brings up-to-date the limit of the region
  void update_region_limit();

  // called when either the words scanned or the refs visited limit
  // has been reached
  void reached_limit();
  // recalculates the words scanned and refs visited limits
  void recalculate_limits();
  // decreases the words scanned and refs visited limits when we reach
  // an expensive operation
  void decrease_limits();
  // it checks whether the words scanned or refs visited reached their
  // respective limit and calls reached_limit() if they have
  void check_limits() {
    if (_words_scanned >= _words_scanned_limit ||
        _refs_reached >= _refs_reached_limit) {
  // this is supposed to be called regularly during a marking step as
  // it checks a bunch of conditions that might cause the marking step
  // to abort
  void regular_clock_call();
  bool concurrent() { return _concurrent; }

  // It resets the task; it should be called right at the beginning of
  // a marking phase.
  void reset(CMBitMap* _nextMarkBitMap);
  // it clears all the fields that correspond to a claimed region.
  void clear_region_fields();

  void set_concurrent(bool concurrent) { _concurrent = concurrent; }

  // The main method of this class which performs a marking step
  // trying not to exceed the given duration. However, it might exit
  // prematurely, according to some conditions (i.e. SATB buffers are
  // available for processing).
  void do_marking_step(double target_ms, bool do_stealing, bool do_termination);

  // These two calls start and stop the timer
  void record_start_time() {
    _elapsed_time_ms = os::elapsedTime() * 1000.0;
  void record_end_time() {
    _elapsed_time_ms = os::elapsedTime() * 1000.0 - _elapsed_time_ms;

  // returns the task ID
  int task_id() { return _task_id; }

  // From TerminatorTerminator. It determines whether this task should
  // exit the termination protocol after it's entered it.
  virtual bool should_exit_termination();

  // Resets the local region fields after a task has finished scanning a
  // region; or when they have become stale as a result of the region
  // being evacuated.
  void giveup_current_region();

  HeapWord* finger()            { return _finger; }

  bool has_aborted()            { return _has_aborted; }
  void set_has_aborted()        { _has_aborted = true; }
  void clear_has_aborted()      { _has_aborted = false; }
  bool has_timed_out()          { return _has_timed_out; }
  bool claimed()                { return _claimed; }

  // Support routines for the partially scanned region that may be
  // recorded as a result of aborting while draining the CMRegionStack
  MemRegion aborted_region()    { return _aborted_region; }
  void set_aborted_region(MemRegion mr)
                                { _aborted_region = mr; }

  // Clears any recorded partially scanned region
  void clear_aborted_region()   { set_aborted_region(MemRegion()); }

  void set_cm_oop_closure(G1CMOopClosure* cm_oop_closure);

  // It grays the object by marking it and, if necessary, pushing it
  // on the local queue
  inline void deal_with_reference(oop obj);

  // It scans an object and visits its children.
  void scan_object(oop obj);

  // It pushes an object on the local queue.
  inline void push(oop obj);

  // These two move entries to/from the global stack.
  void move_entries_to_global_stack();
  void get_entries_from_global_stack();

  // It pops and scans objects from the local queue. If partially is
  // true, then it stops when the queue size is of a given limit. If
  // partially is false, then it stops when the queue is empty.
  void drain_local_queue(bool partially);
  // It moves entries from the global stack to the local queue and
  // drains the local queue. If partially is true, then it stops when
  // both the global stack and the local queue reach a given size. If
  // partially if false, it tries to empty them totally.
  void drain_global_stack(bool partially);
  // It keeps picking SATB buffers and processing them until no SATB
  // buffers are available.
  void drain_satb_buffers();
  // It keeps popping regions from the region stack and processing
  // them until the region stack is empty.
  void drain_region_stack(BitMapClosure* closure);

  // moves the local finger to a new location
  inline void move_finger_to(HeapWord* new_finger) {
    assert(new_finger >= _finger && new_finger < _region_limit, "invariant");
    _finger = new_finger;

  // moves the region finger to a new location
  inline void move_region_finger_to(HeapWord* new_finger) {
    assert(new_finger < _cm->finger(), "invariant");
    _region_finger = new_finger;

  CMTask(int task_num, ConcurrentMark *cm,
         CMTaskQueue* task_queue, CMTaskQueueSet* task_queues);

  // it prints statistics associated with this task
  void print_stats();

  void increase_objs_found_on_bitmap() { ++_objs_found_on_bitmap; }
#endif // _MARKING_STATS_

// Class that's used to to print out per-region liveness
// information. It's currently used at the end of marking and also
// after we sort the old regions at the end of the cleanup operation.
class G1PrintRegionLivenessInfoClosure: public HeapRegionClosure {
  outputStream* _out;

  // Accumulators for these values.
  size_t _total_used_bytes;
  size_t _total_capacity_bytes;
  size_t _total_prev_live_bytes;
  size_t _total_next_live_bytes;

  // These are set up when we come across a "stars humongous" region
  // (as this is where most of this information is stored, not in the
  // subsequent "continues humongous" regions). After that, for every
  // region in a given humongous region series we deduce the right
  // values for it by simply subtracting the appropriate amount from
  // these fields. All these values should reach 0 after we've visited
  // the last region in the series.
  size_t _hum_used_bytes;
  size_t _hum_capacity_bytes;
  size_t _hum_prev_live_bytes;
  size_t _hum_next_live_bytes;

  static double perc(size_t val, size_t total) {
    if (total == 0) {
      return 0.0;
    } else {
      return 100.0 * ((double) val / (double) total);

  static double bytes_to_mb(size_t val) {
    return (double) val / (double) M;

  // See the .cpp file.
  size_t get_hum_bytes(size_t* hum_bytes);
  void get_hum_bytes(size_t* used_bytes, size_t* capacity_bytes,
                     size_t* prev_live_bytes, size_t* next_live_bytes);

  // The header and footer are printed in the constructor and
  // destructor respectively.
  G1PrintRegionLivenessInfoClosure(outputStream* out, const char* phase_name);
  virtual bool doHeapRegion(HeapRegion* r);