src/hotspot/share/gc/g1/g1SurvRateGroup.hpp

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#ifndef SHARE_GC_G1_G1SURVRATEGROUP_HPP
#define SHARE_GC_G1_G1SURVRATEGROUP_HPP

#include "gc/g1/g1Predictions.hpp"
#include "utilities/numberSeq.hpp"

// A survivor rate group tracks survival ratios of objects allocated in the
// heap regions associated to a set of regions (a "space", i.e. eden or survivor)
// on a time basis to predict future survival rates of regions of the same "age".
//
// Every time a new heap region associated with a survivor rate group is retired
// (i.e. the time basis), it gets associated the next "age" entry in that group.
//
// During garbage collection G1 keeps track how much of total data is copied out
// of a heap region (i.e. survives), to update the survivor rate predictor of that age.
//
// This information is used to predict, given a particular age of a heap region,
// how much of its contents will likely survive to determine young generation sizes.
//
// The age index associated with a heap region is incremented from 0 (retired first)
// to N (retired just before the GC).
//
// To avoid copying around data all the time when the total amount of regions in
// a survivor rate group changes, this class organizes the arrays containing the
// predictors in reverse chronological order as returned by age_in_group(). I.e.
// index 0 contains the rate information for the region retired most recently.
class G1SurvRateGroup : public CHeapObj<mtGC> {
  uint _stats_arrays_length;
  uint _num_added_regions;   // The number of regions in this survivor rate group.

  // The initial survivor rate for predictors. Somewhat random value.
  const double InitialSurvivorRate = 0.4;

  double* _accum_surv_rate_pred;
  double  _last_pred;
  TruncatedSeq** _surv_rate_predictors;

  void fill_in_last_surv_rates();
  void finalize_predictions(const G1Predictions& predictor);

public:
  static const uint InvalidAgeIndex = UINT_MAX;
  bool is_valid_age_index(uint age_index) const {
    return age_index >= 1 && age_index <= _num_added_regions;
  }
  bool is_valid_age(uint age) const { return age < _num_added_regions; }

  G1SurvRateGroup();
  void reset();
  void start_adding_regions();
  void stop_adding_regions();
  void record_surviving_words(uint age, size_t surv_words);
  void all_surviving_words_recorded(const G1Predictions& predictor, bool update_predictors);

  double accum_surv_rate_pred(uint age) const;

  double surv_rate_pred(G1Predictions const& predictor, uint age) const {
    assert(is_valid_age(age), "must be");

    // _stats_arrays_length might not be in sync with _num_added_regions in Cleanup pause.
    age = MIN2(age, _stats_arrays_length - 1);

    return predictor.predict_in_unit_interval(_surv_rate_predictors[age]);
  }

  uint next_age_index() {
    return ++_num_added_regions;
  }

  uint age_in_group(uint age_index) const {
    assert(is_valid_age_index(age_index), "invariant" );
    return _num_added_regions - age_index;
  }
};

#endif // SHARE_GC_G1_G1SURVRATEGROUP_HPP