jdk/src/hotspot/share/gc/z/zLiveMap.cpp

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 * version 2 for more details (a copy is included in the LICENSE file that
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#include "gc/z/zGeneration.inline.hpp"
#include "gc/z/zHeap.inline.hpp"
#include "gc/z/zLiveMap.inline.hpp"
#include "gc/z/zStat.hpp"
#include "gc/z/zUtils.hpp"
#include "logging/log.hpp"
#include "runtime/atomic.hpp"
#include "utilities/debug.hpp"
#include "utilities/powerOfTwo.hpp"

static const ZStatCounter ZCounterMarkSeqNumResetContention("Contention", "Mark SeqNum Reset Contention", ZStatUnitOpsPerSecond);
static const ZStatCounter ZCounterMarkSegmentResetContention("Contention", "Mark Segment Reset Contention", ZStatUnitOpsPerSecond);

static size_t bitmap_size(uint32_t size, size_t NumSegments) {
  // We need at least one bit per segment
  return MAX2<size_t>(size, NumSegments) * 2;
}

ZLiveMap::ZLiveMap(uint32_t size)
  : _seqnum(0),
    _live_objects(0),
    _live_bytes(0),
    _segment_live_bits(0),
    _segment_claim_bits(0),
    _bitmap_size(bitmap_size(size, NumSegments)),
    _bitmap(0),
    _segment_shift(log2i_exact(segment_size())) {}

void ZLiveMap::allocate_bitmap() {
  if (_bitmap.size() != _bitmap_size) {
    _bitmap.initialize(_bitmap_size, false /* clear */);
  }
}

void ZLiveMap::reset(ZGenerationId id) {
  ZGeneration* const generation = ZGeneration::generation(id);
  const uint32_t seqnum_initializing = (uint32_t)-1;
  bool contention = false;

  // Multiple threads can enter here, make sure only one of them
  // resets the marking information while the others busy wait.
  for (uint32_t seqnum = Atomic::load_acquire(&_seqnum);
       seqnum != generation->seqnum();
       seqnum = Atomic::load_acquire(&_seqnum)) {
    if ((seqnum != seqnum_initializing) &&
        (Atomic::cmpxchg(&_seqnum, seqnum, seqnum_initializing) == seqnum)) {
      // Reset marking information
      _live_bytes = 0;
      _live_objects = 0;

      // We lazily initialize the bitmap the first time the page is
      // marked, i.e. a bit is about to be set for the first time.
      allocate_bitmap();

      // Clear segment claimed/live bits
      segment_live_bits().clear();
      segment_claim_bits().clear();

      assert(_seqnum == seqnum_initializing, "Invalid");

      // Make sure the newly reset marking information is ordered
      // before the update of the page seqnum, such that when the
      // up-to-date seqnum is load acquired, the bit maps will not
      // contain stale information.
      Atomic::release_store(&_seqnum, generation->seqnum());
      break;
    }

    // Mark reset contention
    if (!contention) {
      // Count contention once
      ZStatInc(ZCounterMarkSeqNumResetContention);
      contention = true;

      log_trace(gc)("Mark seqnum reset contention, thread: " PTR_FORMAT " (%s), map: " PTR_FORMAT,
                    p2i(Thread::current()), ZUtils::thread_name(), p2i(this));
    }
  }
}

void ZLiveMap::reset_segment(BitMap::idx_t segment) {
  bool contention = false;

  if (!claim_segment(segment)) {
    // Already claimed, wait for live bit to be set
    while (!is_segment_live(segment)) {
      // Mark reset contention
      if (!contention) {
        // Count contention once
        ZStatInc(ZCounterMarkSegmentResetContention);
        contention = true;

        log_trace(gc)("Mark segment reset contention, thread: " PTR_FORMAT " (%s), map: " PTR_FORMAT ", segment: %zu",
                      p2i(Thread::current()), ZUtils::thread_name(), p2i(this), segment);
      }
    }

    // Segment is live
    return;
  }

  // Segment claimed, clear it
  const BitMap::idx_t start_index = segment_start(segment);
  const BitMap::idx_t end_index   = segment_end(segment);
  if (segment_size() / BitsPerWord >= 32) {
    _bitmap.clear_large_range(start_index, end_index);
  } else {
    _bitmap.clear_range(start_index, end_index);
  }

  // Set live bit
  const bool success = set_segment_live(segment);
  assert(success, "Should never fail");
}

void ZLiveMap::resize(uint32_t size) {
  const size_t new_bitmap_size = bitmap_size(size, NumSegments);
  _bitmap_size = new_bitmap_size;
  _segment_shift = log2i_exact(segment_size());

  if (_bitmap.size() != 0 && _bitmap.size() != new_bitmap_size) {
    _bitmap.reinitialize(new_bitmap_size, false /* clear */);
  }
}