collectionSetChooser.hpp

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_COLLECTIONSETCHOOSER_HPP
#define SHARE_VM_GC_IMPLEMENTATION_G1_COLLECTIONSETCHOOSER_HPP

#include "gc_implementation/g1/heapRegion.hpp"
#include "utilities/growableArray.hpp"

// We need to sort heap regions by collection desirability.
// This sorting is currently done in two "stages". An initial sort is
// done following a cleanup pause as soon as all of the marked but
// non-empty regions have been identified and the completely empty
// ones reclaimed.
// This gives us a global sort on a GC efficiency metric
// based on predictive data available at that time. However,
// any of these regions that are collected will only be collected
// during a future GC pause, by which time it is possible that newer
// data might allow us to revise and/or refine the earlier
// pause predictions, leading to changes in expected gc efficiency
// order. To somewhat mitigate this obsolescence, more so in the
// case of regions towards the end of the list, which will be
// picked later, these pre-sorted regions from the _markedRegions
// array are not used as is, but a small prefix thereof is
// insertion-sorted again into a small cache, based on more
// recent remembered set information. Regions are then drawn
// from this cache to construct the collection set at each
// incremental GC.
// This scheme and/or its implementation may be subject to
// revision in the future.

class CSetChooserCache VALUE_OBJ_CLASS_SPEC {
private:
  enum {
    CacheLength = 16
  } PrivateConstants;

  HeapRegion*  _cache[CacheLength];
  int          _occupancy; // number of regions in cache
  int          _first;     // (index of) "first" region in the cache

  // adding CacheLength to deal with negative values
  inline int trim_index(int index) {
    return (index + CacheLength) % CacheLength;
  }

  inline int get_sort_index(int index) {
    return -index-2;
  }
  inline int get_index(int sort_index) {
    return -sort_index-2;
  }

public:
  CSetChooserCache(void);

  inline int occupancy(void) { return _occupancy; }
  inline bool is_full()      { return _occupancy == CacheLength; }
  inline bool is_empty()     { return _occupancy == 0; }

  void clear(void);
  void insert(HeapRegion *hr);
  HeapRegion *remove_first(void);
  inline HeapRegion *get_first(void) {
    return _cache[_first];
  }

#ifndef PRODUCT
  bool verify (void);
  bool region_in_cache(HeapRegion *hr) {
    int sort_index = hr->sort_index();
    if (sort_index < -1) {
      int index = get_index(sort_index);
      guarantee(index < CacheLength, "should be within bounds");
      return _cache[index] == hr;
    } else
      return 0;
  }
#endif // PRODUCT
};

class CollectionSetChooser: public CHeapObj {

  GrowableArray<HeapRegion*> _markedRegions;
  int _curMarkedIndex;
  int _numMarkedRegions;
  CSetChooserCache _cache;

  // True iff last collection pause ran of out new "age 0" regions, and
  // returned an "age 1" region.
  bool _unmarked_age_1_returned_as_new;

  jint _first_par_unreserved_idx;

public:

  HeapRegion* getNextMarkedRegion(double time_so_far, double avg_prediction);

  CollectionSetChooser();

  void sortMarkedHeapRegions();
  void fillCache();
  void addMarkedHeapRegion(HeapRegion *hr);

  // Must be called before calls to getParMarkedHeapRegionChunk.
  // "n_regions" is the number of regions, "chunkSize" the chunk size.
  void prepareForAddMarkedHeapRegionsPar(size_t n_regions, size_t chunkSize);
  // Returns the first index in a contiguous chunk of "n_regions" indexes
  // that the calling thread has reserved.  These must be set by the
  // calling thread using "setMarkedHeapRegion" (to NULL if necessary).
  jint getParMarkedHeapRegionChunk(jint n_regions);
  // Set the marked array entry at index to hr.  Careful to claim the index
  // first if in parallel.
  void setMarkedHeapRegion(jint index, HeapRegion* hr);
  // Atomically increment the number of claimed regions by "inc_by".
  void incNumMarkedHeapRegions(jint inc_by);

  void clearMarkedHeapRegions();

  void updateAfterFullCollection();

  bool unmarked_age_1_returned_as_new() { return _unmarked_age_1_returned_as_new; }

  // Returns true if the used portion of "_markedRegions" is properly
  // sorted, otherwise asserts false.
#ifndef PRODUCT
  bool verify(void);
  bool regionProperlyOrdered(HeapRegion* r) {
    int si = r->sort_index();
    return (si == -1) ||
      (si > -1 && _markedRegions.at(si) == r) ||
      (si < -1 && _cache.region_in_cache(r));
  }
#endif

};

#endif // SHARE_VM_GC_IMPLEMENTATION_G1_COLLECTIONSETCHOOSER_HPP