jdk/src/hotspot/share/gc/g1/g1RegionMarkStatsCache.hpp

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#ifndef SHARE_GC_G1_G1REGIONMARKSTATSCACHE_HPP
#define SHARE_GC_G1_G1REGIONMARKSTATSCACHE_HPP

#include "memory/allocation.hpp"
#include "oops/oop.hpp"
#include "runtime/atomic.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/pair.hpp"

// Per-Region statistics gathered during marking.
//
// These include:
// * the number of live words gathered during marking for the area from bottom
//   to tams. This is an exact measure. The code corrects later for the live data
//   between tams and top.
// * the number of incoming references found during marking. This is an approximate
//   value because we do not mark through all objects.
struct G1RegionMarkStats {
  Atomic<size_t> _live_words;
  Atomic<size_t> _incoming_refs;

  // Clear all members.
  void clear() {
    _live_words.store_relaxed(0);
    _incoming_refs.store_relaxed(0);
  }
  // Clear all members after a marking overflow. Only needs to clear the number of
  // incoming references as all objects will be rescanned, while the live words are
  // gathered whenever a thread can mark an object, which is synchronized.
  void clear_during_overflow() {
    _incoming_refs.store_relaxed(0);
  }

  size_t live_words() const { return _live_words.load_relaxed(); }
  size_t incoming_refs() const { return _incoming_refs.load_relaxed(); }
};

// Per-marking thread cache for the region mark statistics.
//
// Each cache is a larg'ish map of region-idx -> G1RegionMarkStats entries that cache
// currently gathered statistics; entries are evicted to the global statistics array
// on every collision. This minimizes synchronization overhead which would be required
// every time statistics change, as marking is very localized.
// The map entry number is a power of two to allow simple and fast hashing using
// logical and.
class G1RegionMarkStatsCache {
private:
  // The array of statistics entries to evict to; the global array.
  G1RegionMarkStats* _target;

  // An entry of the statistics cache.
  struct G1RegionMarkStatsCacheEntry {
    uint _region_idx;
    G1RegionMarkStats _stats;

    void clear(uint idx = 0) {
      _region_idx = idx;
      _stats.clear();
    }
  };

  // The actual cache and its number of entries.
  G1RegionMarkStatsCacheEntry* _cache;
  uint _num_cache_entries;

  // Cache hits/miss counters.
  size_t _cache_hits;
  size_t _cache_misses;

  // Evict a given element of the statistics cache.
  void evict(uint idx);

  size_t _num_cache_entries_mask;

  uint hash(uint idx) {
    return idx & _num_cache_entries_mask;
  }

  G1RegionMarkStatsCacheEntry* find_for_add(uint region_idx);
public:
  // Number of entries in the per-task stats entry. This value seems enough
  // to have a very low cache miss rate.
  static const uint RegionMarkStatsCacheSize = 1024;

  // Initialize cache. Does not reset the cache immediately to avoid the cost
  // during startup.
  G1RegionMarkStatsCache(G1RegionMarkStats* target, uint num_cache_entries);

  ~G1RegionMarkStatsCache();

  void add_live_words(oop obj);
  void add_live_words(uint region_idx, size_t live_words) {
    G1RegionMarkStatsCacheEntry* const cur = find_for_add(region_idx);
    // This method is only ever called single-threaded, so we do not need atomic
    // update here.
    cur->_stats._live_words.store_relaxed(cur->_stats.live_words() + live_words);
  }

  void inc_incoming_refs(uint region_idx) {
    G1RegionMarkStatsCacheEntry* const cur = find_for_add(region_idx);
    // This method is only ever called single-threaded, so we do not need atomic
    // update here.
    cur->_stats._incoming_refs.store_relaxed(cur->_stats.incoming_refs() + 1u);
  }

  void reset(uint region_idx) {
    uint const cache_idx = hash(region_idx);
    G1RegionMarkStatsCacheEntry* cur = &_cache[cache_idx];
    if (cur->_region_idx == region_idx) {
      _cache[cache_idx].clear();
    }
  }

  // Evict all remaining statistics, returning cache hits and misses.
  Pair<size_t, size_t> evict_all();

  // Reset liveness of all cache entries to their default values,
  // initialize _region_idx to avoid initial cache miss.
  void reset();

  size_t hits() const { return _cache_hits; }
  size_t misses() const { return _cache_misses; }
};

#endif // SHARE_GC_G1_G1REGIONMARKSTATSCACHE_HPP