view hotspot/src/share/vm/gc_implementation/g1/g1RemSet.cpp @ 26701:f8ff74a6c058

8052172: Evacuation failure handling in G1 does not evacuate all objects if -XX:-G1DeferredRSUpdate is set Summary: Remove -XX:-G1DeferredRSUpdate functionality as it is racy. During evacuation failure handling, threads where evacuation failure handling occurred may try to add remembered sets to regions which remembered sets are currently being scanned. The iterator to handle the remembered set scan does not support addition of entries during scan and so may skip valid references. Reviewed-by: iveresov, brutisso, mgerdin
author tschatzl
date Tue, 16 Sep 2014 10:28:15 +0200
parents 4ee5901e205e
children 7d4376f8560e
line wrap: on
line source
/*
 * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#include "precompiled.hpp"
#include "gc_implementation/g1/concurrentG1Refine.hpp"
#include "gc_implementation/g1/concurrentG1RefineThread.hpp"
#include "gc_implementation/g1/g1BlockOffsetTable.inline.hpp"
#include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
#include "gc_implementation/g1/g1CollectorPolicy.hpp"
#include "gc_implementation/g1/g1HotCardCache.hpp"
#include "gc_implementation/g1/g1GCPhaseTimes.hpp"
#include "gc_implementation/g1/g1OopClosures.inline.hpp"
#include "gc_implementation/g1/g1RemSet.inline.hpp"
#include "gc_implementation/g1/heapRegionManager.inline.hpp"
#include "gc_implementation/g1/heapRegionRemSet.hpp"
#include "memory/iterator.hpp"
#include "oops/oop.inline.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/intHisto.hpp"

PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC

#define CARD_REPEAT_HISTO 0

#if CARD_REPEAT_HISTO
static size_t ct_freq_sz;
static jbyte* ct_freq = NULL;

void init_ct_freq_table(size_t heap_sz_bytes) {
  if (ct_freq == NULL) {
    ct_freq_sz = heap_sz_bytes/CardTableModRefBS::card_size;
    ct_freq = new jbyte[ct_freq_sz];
    for (size_t j = 0; j < ct_freq_sz; j++) ct_freq[j] = 0;
  }
}

void ct_freq_note_card(size_t index) {
  assert(0 <= index && index < ct_freq_sz, "Bounds error.");
  if (ct_freq[index] < 100) { ct_freq[index]++; }
}

static IntHistogram card_repeat_count(10, 10);

void ct_freq_update_histo_and_reset() {
  for (size_t j = 0; j < ct_freq_sz; j++) {
    card_repeat_count.add_entry(ct_freq[j]);
    ct_freq[j] = 0;
  }

}
#endif

G1RemSet::G1RemSet(G1CollectedHeap* g1, CardTableModRefBS* ct_bs)
  : _g1(g1), _conc_refine_cards(0),
    _ct_bs(ct_bs), _g1p(_g1->g1_policy()),
    _cg1r(g1->concurrent_g1_refine()),
    _cset_rs_update_cl(NULL),
    _cards_scanned(NULL), _total_cards_scanned(0),
    _prev_period_summary()
{
  _seq_task = new SubTasksDone(NumSeqTasks);
  guarantee(n_workers() > 0, "There should be some workers");
  _cset_rs_update_cl = NEW_C_HEAP_ARRAY(OopsInHeapRegionClosure*, n_workers(), mtGC);
  for (uint i = 0; i < n_workers(); i++) {
    _cset_rs_update_cl[i] = NULL;
  }
  if (G1SummarizeRSetStats) {
    _prev_period_summary.initialize(this);
  }
}

G1RemSet::~G1RemSet() {
  delete _seq_task;
  for (uint i = 0; i < n_workers(); i++) {
    assert(_cset_rs_update_cl[i] == NULL, "it should be");
  }
  FREE_C_HEAP_ARRAY(OopsInHeapRegionClosure*, _cset_rs_update_cl, mtGC);
}

void CountNonCleanMemRegionClosure::do_MemRegion(MemRegion mr) {
  if (_g1->is_in_g1_reserved(mr.start())) {
    _n += (int) ((mr.byte_size() / CardTableModRefBS::card_size));
    if (_start_first == NULL) _start_first = mr.start();
  }
}

class ScanRSClosure : public HeapRegionClosure {
  size_t _cards_done, _cards;
  G1CollectedHeap* _g1h;

  OopsInHeapRegionClosure* _oc;
  CodeBlobClosure* _code_root_cl;

  G1BlockOffsetSharedArray* _bot_shared;
  G1SATBCardTableModRefBS *_ct_bs;

  double _strong_code_root_scan_time_sec;
  uint   _worker_i;
  int    _block_size;
  bool   _try_claimed;

public:
  ScanRSClosure(OopsInHeapRegionClosure* oc,
                CodeBlobClosure* code_root_cl,
                uint worker_i) :
    _oc(oc),
    _code_root_cl(code_root_cl),
    _strong_code_root_scan_time_sec(0.0),
    _cards(0),
    _cards_done(0),
    _worker_i(worker_i),
    _try_claimed(false)
  {
    _g1h = G1CollectedHeap::heap();
    _bot_shared = _g1h->bot_shared();
    _ct_bs = _g1h->g1_barrier_set();
    _block_size = MAX2<int>(G1RSetScanBlockSize, 1);
  }

  void set_try_claimed() { _try_claimed = true; }

  void scanCard(size_t index, HeapRegion *r) {
    // Stack allocate the DirtyCardToOopClosure instance
    HeapRegionDCTOC cl(_g1h, r, _oc,
                       CardTableModRefBS::Precise,
                       HeapRegionDCTOC::IntoCSFilterKind);

    // Set the "from" region in the closure.
    _oc->set_region(r);
    HeapWord* card_start = _bot_shared->address_for_index(index);
    HeapWord* card_end = card_start + G1BlockOffsetSharedArray::N_words;
    Space *sp = SharedHeap::heap()->space_containing(card_start);
    MemRegion sm_region = sp->used_region_at_save_marks();
    MemRegion mr = sm_region.intersection(MemRegion(card_start,card_end));
    if (!mr.is_empty() && !_ct_bs->is_card_claimed(index)) {
      // We make the card as "claimed" lazily (so races are possible
      // but they're benign), which reduces the number of duplicate
      // scans (the rsets of the regions in the cset can intersect).
      _ct_bs->set_card_claimed(index);
      _cards_done++;
      cl.do_MemRegion(mr);
    }
  }

  void printCard(HeapRegion* card_region, size_t card_index,
                 HeapWord* card_start) {
    gclog_or_tty->print_cr("T %u Region [" PTR_FORMAT ", " PTR_FORMAT ") "
                           "RS names card " SIZE_FORMAT_HEX ": "
                           "[" PTR_FORMAT ", " PTR_FORMAT ")",
                           _worker_i,
                           card_region->bottom(), card_region->end(),
                           card_index,
                           card_start, card_start + G1BlockOffsetSharedArray::N_words);
  }

  void scan_strong_code_roots(HeapRegion* r) {
    double scan_start = os::elapsedTime();
    r->strong_code_roots_do(_code_root_cl);
    _strong_code_root_scan_time_sec += (os::elapsedTime() - scan_start);
  }

  bool doHeapRegion(HeapRegion* r) {
    assert(r->in_collection_set(), "should only be called on elements of CS.");
    HeapRegionRemSet* hrrs = r->rem_set();
    if (hrrs->iter_is_complete()) return false; // All done.
    if (!_try_claimed && !hrrs->claim_iter()) return false;
    // If we ever free the collection set concurrently, we should also
    // clear the card table concurrently therefore we won't need to
    // add regions of the collection set to the dirty cards region.
    _g1h->push_dirty_cards_region(r);
    // If we didn't return above, then
    //   _try_claimed || r->claim_iter()
    // is true: either we're supposed to work on claimed-but-not-complete
    // regions, or we successfully claimed the region.

    HeapRegionRemSetIterator iter(hrrs);
    size_t card_index;

    // We claim cards in block so as to reduce the contention. The block size is determined by
    // the G1RSetScanBlockSize parameter.
    size_t jump_to_card = hrrs->iter_claimed_next(_block_size);
    for (size_t current_card = 0; iter.has_next(card_index); current_card++) {
      if (current_card >= jump_to_card + _block_size) {
        jump_to_card = hrrs->iter_claimed_next(_block_size);
      }
      if (current_card < jump_to_card) continue;
      HeapWord* card_start = _g1h->bot_shared()->address_for_index(card_index);
#if 0
      gclog_or_tty->print("Rem set iteration yielded card [" PTR_FORMAT ", " PTR_FORMAT ").\n",
                          card_start, card_start + CardTableModRefBS::card_size_in_words);
#endif

      HeapRegion* card_region = _g1h->heap_region_containing(card_start);
      _cards++;

      if (!card_region->is_on_dirty_cards_region_list()) {
        _g1h->push_dirty_cards_region(card_region);
      }

      // If the card is dirty, then we will scan it during updateRS.
      if (!card_region->in_collection_set() &&
          !_ct_bs->is_card_dirty(card_index)) {
        scanCard(card_index, card_region);
      }
    }
    if (!_try_claimed) {
      // Scan the strong code root list attached to the current region
      scan_strong_code_roots(r);

      hrrs->set_iter_complete();
    }
    return false;
  }

  double strong_code_root_scan_time_sec() {
    return _strong_code_root_scan_time_sec;
  }

  size_t cards_done() { return _cards_done;}
  size_t cards_looked_up() { return _cards;}
};

void G1RemSet::scanRS(OopsInHeapRegionClosure* oc,
                      CodeBlobClosure* code_root_cl,
                      uint worker_i) {
  double rs_time_start = os::elapsedTime();
  HeapRegion *startRegion = _g1->start_cset_region_for_worker(worker_i);

  ScanRSClosure scanRScl(oc, code_root_cl, worker_i);

  _g1->collection_set_iterate_from(startRegion, &scanRScl);
  scanRScl.set_try_claimed();
  _g1->collection_set_iterate_from(startRegion, &scanRScl);

  double scan_rs_time_sec = (os::elapsedTime() - rs_time_start)
                            - scanRScl.strong_code_root_scan_time_sec();

  assert(_cards_scanned != NULL, "invariant");
  _cards_scanned[worker_i] = scanRScl.cards_done();

  _g1p->phase_times()->record_scan_rs_time(worker_i, scan_rs_time_sec * 1000.0);
  _g1p->phase_times()->record_strong_code_root_scan_time(worker_i,
                                                         scanRScl.strong_code_root_scan_time_sec() * 1000.0);
}

// Closure used for updating RSets and recording references that
// point into the collection set. Only called during an
// evacuation pause.

class RefineRecordRefsIntoCSCardTableEntryClosure: public CardTableEntryClosure {
  G1RemSet* _g1rs;
  DirtyCardQueue* _into_cset_dcq;
public:
  RefineRecordRefsIntoCSCardTableEntryClosure(G1CollectedHeap* g1h,
                                              DirtyCardQueue* into_cset_dcq) :
    _g1rs(g1h->g1_rem_set()), _into_cset_dcq(into_cset_dcq)
  {}
  bool do_card_ptr(jbyte* card_ptr, uint worker_i) {
    // The only time we care about recording cards that
    // contain references that point into the collection set
    // is during RSet updating within an evacuation pause.
    // In this case worker_i should be the id of a GC worker thread.
    assert(SafepointSynchronize::is_at_safepoint(), "not during an evacuation pause");
    assert(worker_i < (ParallelGCThreads == 0 ? 1 : ParallelGCThreads), "should be a GC worker");

    if (_g1rs->refine_card(card_ptr, worker_i, true)) {
      // 'card_ptr' contains references that point into the collection
      // set. We need to record the card in the DCQS
      // (G1CollectedHeap::into_cset_dirty_card_queue_set())
      // that's used for that purpose.
      //
      // Enqueue the card
      _into_cset_dcq->enqueue(card_ptr);
    }
    return true;
  }
};

void G1RemSet::updateRS(DirtyCardQueue* into_cset_dcq, uint worker_i) {
  double start = os::elapsedTime();
  // Apply the given closure to all remaining log entries.
  RefineRecordRefsIntoCSCardTableEntryClosure into_cset_update_rs_cl(_g1, into_cset_dcq);

  _g1->iterate_dirty_card_closure(&into_cset_update_rs_cl, into_cset_dcq, false, worker_i);

  // Now there should be no dirty cards.
  if (G1RSLogCheckCardTable) {
    CountNonCleanMemRegionClosure cl(_g1);
    _ct_bs->mod_card_iterate(&cl);
    // XXX This isn't true any more: keeping cards of young regions
    // marked dirty broke it.  Need some reasonable fix.
    guarantee(cl.n() == 0, "Card table should be clean.");
  }

  _g1p->phase_times()->record_update_rs_time(worker_i, (os::elapsedTime() - start) * 1000.0);
}

void G1RemSet::cleanupHRRS() {
  HeapRegionRemSet::cleanup();
}

void G1RemSet::oops_into_collection_set_do(OopsInHeapRegionClosure* oc,
                                           CodeBlobClosure* code_root_cl,
                                           uint worker_i) {
#if CARD_REPEAT_HISTO
  ct_freq_update_histo_and_reset();
#endif

  // We cache the value of 'oc' closure into the appropriate slot in the
  // _cset_rs_update_cl for this worker
  assert(worker_i < n_workers(), "sanity");
  _cset_rs_update_cl[worker_i] = oc;

  // A DirtyCardQueue that is used to hold cards containing references
  // that point into the collection set. This DCQ is associated with a
  // special DirtyCardQueueSet (see g1CollectedHeap.hpp).  Under normal
  // circumstances (i.e. the pause successfully completes), these cards
  // are just discarded (there's no need to update the RSets of regions
  // that were in the collection set - after the pause these regions
  // are wholly 'free' of live objects. In the event of an evacuation
  // failure the cards/buffers in this queue set are passed to the
  // DirtyCardQueueSet that is used to manage RSet updates
  DirtyCardQueue into_cset_dcq(&_g1->into_cset_dirty_card_queue_set());

  assert((ParallelGCThreads > 0) || worker_i == 0, "invariant");

  updateRS(&into_cset_dcq, worker_i);
  scanRS(oc, code_root_cl, worker_i);

  // We now clear the cached values of _cset_rs_update_cl for this worker
  _cset_rs_update_cl[worker_i] = NULL;
}

void G1RemSet::prepare_for_oops_into_collection_set_do() {
  cleanupHRRS();
  _g1->set_refine_cte_cl_concurrency(false);
  DirtyCardQueueSet& dcqs = JavaThread::dirty_card_queue_set();
  dcqs.concatenate_logs();

  guarantee( _cards_scanned == NULL, "invariant" );
  _cards_scanned = NEW_C_HEAP_ARRAY(size_t, n_workers(), mtGC);
  for (uint i = 0; i < n_workers(); ++i) {
    _cards_scanned[i] = 0;
  }
  _total_cards_scanned = 0;
}

void G1RemSet::cleanup_after_oops_into_collection_set_do() {
  guarantee( _cards_scanned != NULL, "invariant" );
  _total_cards_scanned = 0;
  for (uint i = 0; i < n_workers(); ++i) {
    _total_cards_scanned += _cards_scanned[i];
  }
  FREE_C_HEAP_ARRAY(size_t, _cards_scanned, mtGC);
  _cards_scanned = NULL;
  // Cleanup after copy
  _g1->set_refine_cte_cl_concurrency(true);
  // Set all cards back to clean.
  _g1->cleanUpCardTable();

  DirtyCardQueueSet& into_cset_dcqs = _g1->into_cset_dirty_card_queue_set();
  int into_cset_n_buffers = into_cset_dcqs.completed_buffers_num();

  if (_g1->evacuation_failed()) {
    double restore_remembered_set_start = os::elapsedTime();

    // Restore remembered sets for the regions pointing into the collection set.
    // We just need to transfer the completed buffers from the DirtyCardQueueSet
    // used to hold cards that contain references that point into the collection set
    // to the DCQS used to hold the deferred RS updates.
    _g1->dirty_card_queue_set().merge_bufferlists(&into_cset_dcqs);
    _g1->g1_policy()->phase_times()->record_evac_fail_restore_remsets((os::elapsedTime() - restore_remembered_set_start) * 1000.0);
  }

  // Free any completed buffers in the DirtyCardQueueSet used to hold cards
  // which contain references that point into the collection.
  _g1->into_cset_dirty_card_queue_set().clear();
  assert(_g1->into_cset_dirty_card_queue_set().completed_buffers_num() == 0,
         "all buffers should be freed");
  _g1->into_cset_dirty_card_queue_set().clear_n_completed_buffers();
}

class ScrubRSClosure: public HeapRegionClosure {
  G1CollectedHeap* _g1h;
  BitMap* _region_bm;
  BitMap* _card_bm;
  CardTableModRefBS* _ctbs;
public:
  ScrubRSClosure(BitMap* region_bm, BitMap* card_bm) :
    _g1h(G1CollectedHeap::heap()),
    _region_bm(region_bm), _card_bm(card_bm),
    _ctbs(_g1h->g1_barrier_set()) {}

  bool doHeapRegion(HeapRegion* r) {
    if (!r->continuesHumongous()) {
      r->rem_set()->scrub(_ctbs, _region_bm, _card_bm);
    }
    return false;
  }
};

void G1RemSet::scrub(BitMap* region_bm, BitMap* card_bm) {
  ScrubRSClosure scrub_cl(region_bm, card_bm);
  _g1->heap_region_iterate(&scrub_cl);
}

void G1RemSet::scrub_par(BitMap* region_bm, BitMap* card_bm,
                                uint worker_num, int claim_val) {
  ScrubRSClosure scrub_cl(region_bm, card_bm);
  _g1->heap_region_par_iterate_chunked(&scrub_cl,
                                       worker_num,
                                       n_workers(),
                                       claim_val);
}

G1TriggerClosure::G1TriggerClosure() :
  _triggered(false) { }

G1InvokeIfNotTriggeredClosure::G1InvokeIfNotTriggeredClosure(G1TriggerClosure* t_cl,
                                                             OopClosure* oop_cl)  :
  _trigger_cl(t_cl), _oop_cl(oop_cl) { }

G1Mux2Closure::G1Mux2Closure(OopClosure *c1, OopClosure *c2) :
  _c1(c1), _c2(c2) { }

G1UpdateRSOrPushRefOopClosure::
G1UpdateRSOrPushRefOopClosure(G1CollectedHeap* g1h,
                              G1RemSet* rs,
                              OopsInHeapRegionClosure* push_ref_cl,
                              bool record_refs_into_cset,
                              uint worker_i) :
  _g1(g1h), _g1_rem_set(rs), _from(NULL),
  _record_refs_into_cset(record_refs_into_cset),
  _push_ref_cl(push_ref_cl), _worker_i(worker_i) { }

// Returns true if the given card contains references that point
// into the collection set, if we're checking for such references;
// false otherwise.

bool G1RemSet::refine_card(jbyte* card_ptr, uint worker_i,
                           bool check_for_refs_into_cset) {
  assert(_g1->is_in_exact(_ct_bs->addr_for(card_ptr)),
         err_msg("Card at "PTR_FORMAT" index "SIZE_FORMAT" representing heap at "PTR_FORMAT" (%u) must be in committed heap",
                 p2i(card_ptr),
                 _ct_bs->index_for(_ct_bs->addr_for(card_ptr)),
                 _ct_bs->addr_for(card_ptr),
                 _g1->addr_to_region(_ct_bs->addr_for(card_ptr))));

  // If the card is no longer dirty, nothing to do.
  if (*card_ptr != CardTableModRefBS::dirty_card_val()) {
    // No need to return that this card contains refs that point
    // into the collection set.
    return false;
  }

  // Construct the region representing the card.
  HeapWord* start = _ct_bs->addr_for(card_ptr);
  // And find the region containing it.
  HeapRegion* r = _g1->heap_region_containing(start);

  // Why do we have to check here whether a card is on a young region,
  // given that we dirty young regions and, as a result, the
  // post-barrier is supposed to filter them out and never to enqueue
  // them? When we allocate a new region as the "allocation region" we
  // actually dirty its cards after we release the lock, since card
  // dirtying while holding the lock was a performance bottleneck. So,
  // as a result, it is possible for other threads to actually
  // allocate objects in the region (after the acquire the lock)
  // before all the cards on the region are dirtied. This is unlikely,
  // and it doesn't happen often, but it can happen. So, the extra
  // check below filters out those cards.
  if (r->is_young()) {
    return false;
  }

  // While we are processing RSet buffers during the collection, we
  // actually don't want to scan any cards on the collection set,
  // since we don't want to update remembered sets with entries that
  // point into the collection set, given that live objects from the
  // collection set are about to move and such entries will be stale
  // very soon. This change also deals with a reliability issue which
  // involves scanning a card in the collection set and coming across
  // an array that was being chunked and looking malformed. Note,
  // however, that if evacuation fails, we have to scan any objects
  // that were not moved and create any missing entries.
  if (r->in_collection_set()) {
    return false;
  }

  // The result from the hot card cache insert call is either:
  //   * pointer to the current card
  //     (implying that the current card is not 'hot'),
  //   * null
  //     (meaning we had inserted the card ptr into the "hot" card cache,
  //     which had some headroom),
  //   * a pointer to a "hot" card that was evicted from the "hot" cache.
  //

  G1HotCardCache* hot_card_cache = _cg1r->hot_card_cache();
  if (hot_card_cache->use_cache()) {
    assert(!check_for_refs_into_cset, "sanity");
    assert(!SafepointSynchronize::is_at_safepoint(), "sanity");

    card_ptr = hot_card_cache->insert(card_ptr);
    if (card_ptr == NULL) {
      // There was no eviction. Nothing to do.
      return false;
    }

    start = _ct_bs->addr_for(card_ptr);
    r = _g1->heap_region_containing(start);

    // Checking whether the region we got back from the cache
    // is young here is inappropriate. The region could have been
    // freed, reallocated and tagged as young while in the cache.
    // Hence we could see its young type change at any time.
  }

  // Don't use addr_for(card_ptr + 1) which can ask for
  // a card beyond the heap.  This is not safe without a perm
  // gen at the upper end of the heap.
  HeapWord* end   = start + CardTableModRefBS::card_size_in_words;
  MemRegion dirtyRegion(start, end);

#if CARD_REPEAT_HISTO
  init_ct_freq_table(_g1->max_capacity());
  ct_freq_note_card(_ct_bs->index_for(start));
#endif

  OopsInHeapRegionClosure* oops_in_heap_closure = NULL;
  if (check_for_refs_into_cset) {
    // ConcurrentG1RefineThreads have worker numbers larger than what
    // _cset_rs_update_cl[] is set up to handle. But those threads should
    // only be active outside of a collection which means that when they
    // reach here they should have check_for_refs_into_cset == false.
    assert((size_t)worker_i < n_workers(), "index of worker larger than _cset_rs_update_cl[].length");
    oops_in_heap_closure = _cset_rs_update_cl[worker_i];
  }
  G1UpdateRSOrPushRefOopClosure update_rs_oop_cl(_g1,
                                                 _g1->g1_rem_set(),
                                                 oops_in_heap_closure,
                                                 check_for_refs_into_cset,
                                                 worker_i);
  update_rs_oop_cl.set_from(r);

  G1TriggerClosure trigger_cl;
  FilterIntoCSClosure into_cs_cl(NULL, _g1, &trigger_cl);
  G1InvokeIfNotTriggeredClosure invoke_cl(&trigger_cl, &into_cs_cl);
  G1Mux2Closure mux(&invoke_cl, &update_rs_oop_cl);

  FilterOutOfRegionClosure filter_then_update_rs_oop_cl(r,
                        (check_for_refs_into_cset ?
                                (OopClosure*)&mux :
                                (OopClosure*)&update_rs_oop_cl));

  // The region for the current card may be a young region. The
  // current card may have been a card that was evicted from the
  // card cache. When the card was inserted into the cache, we had
  // determined that its region was non-young. While in the cache,
  // the region may have been freed during a cleanup pause, reallocated
  // and tagged as young.
  //
  // We wish to filter out cards for such a region but the current
  // thread, if we're running concurrently, may "see" the young type
  // change at any time (so an earlier "is_young" check may pass or
  // fail arbitrarily). We tell the iteration code to perform this
  // filtering when it has been determined that there has been an actual
  // allocation in this region and making it safe to check the young type.
  bool filter_young = true;

  HeapWord* stop_point =
    r->oops_on_card_seq_iterate_careful(dirtyRegion,
                                        &filter_then_update_rs_oop_cl,
                                        filter_young,
                                        card_ptr);

  // If stop_point is non-null, then we encountered an unallocated region
  // (perhaps the unfilled portion of a TLAB.)  For now, we'll dirty the
  // card and re-enqueue: if we put off the card until a GC pause, then the
  // unallocated portion will be filled in.  Alternatively, we might try
  // the full complexity of the technique used in "regular" precleaning.
  if (stop_point != NULL) {
    // The card might have gotten re-dirtied and re-enqueued while we
    // worked.  (In fact, it's pretty likely.)
    if (*card_ptr != CardTableModRefBS::dirty_card_val()) {
      *card_ptr = CardTableModRefBS::dirty_card_val();
      MutexLockerEx x(Shared_DirtyCardQ_lock,
                      Mutex::_no_safepoint_check_flag);
      DirtyCardQueue* sdcq =
        JavaThread::dirty_card_queue_set().shared_dirty_card_queue();
      sdcq->enqueue(card_ptr);
    }
  } else {
    _conc_refine_cards++;
  }

  // This gets set to true if the card being refined has
  // references that point into the collection set.
  bool has_refs_into_cset = trigger_cl.triggered();

  // We should only be detecting that the card contains references
  // that point into the collection set if the current thread is
  // a GC worker thread.
  assert(!has_refs_into_cset || SafepointSynchronize::is_at_safepoint(),
           "invalid result at non safepoint");

  return has_refs_into_cset;
}

void G1RemSet::print_periodic_summary_info(const char* header) {
  G1RemSetSummary current;
  current.initialize(this);

  _prev_period_summary.subtract_from(&current);
  print_summary_info(&_prev_period_summary, header);

  _prev_period_summary.set(&current);
}

void G1RemSet::print_summary_info() {
  G1RemSetSummary current;
  current.initialize(this);

  print_summary_info(&current, " Cumulative RS summary");
}

void G1RemSet::print_summary_info(G1RemSetSummary * summary, const char * header) {
  assert(summary != NULL, "just checking");

  if (header != NULL) {
    gclog_or_tty->print_cr("%s", header);
  }

#if CARD_REPEAT_HISTO
  gclog_or_tty->print_cr("\nG1 card_repeat count histogram: ");
  gclog_or_tty->print_cr("  # of repeats --> # of cards with that number.");
  card_repeat_count.print_on(gclog_or_tty);
#endif

  summary->print_on(gclog_or_tty);
}

void G1RemSet::prepare_for_verify() {
  if (G1HRRSFlushLogBuffersOnVerify &&
      (VerifyBeforeGC || VerifyAfterGC)
      &&  (!_g1->full_collection() || G1VerifyRSetsDuringFullGC)) {
    cleanupHRRS();
    _g1->set_refine_cte_cl_concurrency(false);
    if (SafepointSynchronize::is_at_safepoint()) {
      DirtyCardQueueSet& dcqs = JavaThread::dirty_card_queue_set();
      dcqs.concatenate_logs();
    }

    G1HotCardCache* hot_card_cache = _cg1r->hot_card_cache();
    bool use_hot_card_cache = hot_card_cache->use_cache();
    hot_card_cache->set_use_cache(false);

    DirtyCardQueue into_cset_dcq(&_g1->into_cset_dirty_card_queue_set());
    updateRS(&into_cset_dcq, 0);
    _g1->into_cset_dirty_card_queue_set().clear();

    hot_card_cache->set_use_cache(use_hot_card_cache);
    assert(JavaThread::dirty_card_queue_set().completed_buffers_num() == 0, "All should be consumed");
  }
}