src/hotspot/share/gc/z/zTLABUsage.hpp

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#ifndef SHARE_GC_Z_ZTLABUSAGE_HPP
#define SHARE_GC_Z_ZTLABUSAGE_HPP

#include "utilities/globalDefinitions.hpp"
#include "utilities/numberSeq.hpp"

// ZGC is retiring TLABs concurrently with the application running when
// processing the stack watermarks. For the common TLAB heuristic to work we
// need to return consistent TLAB usage information when a TLAB is retired.
// We snapshot the TLAB usage in the mark start pause for the young generation
// and use this information until the next garbage collection cycle.
//
// ZGC does not have set generation sizes unlike most other GCs and because of
// this there is no fixed TLAB capacity. For the common TLAB sizing heuristic
// to work properly ZGC estimates the current capacity by using a weighted
// average of the last 10 used values. ZGC uses the last snapshotted value as
// the value returned as tlab_used().

class ZTLABUsage {
private:
  // Accounting TLAB used until the next GC cycle
  volatile size_t _used;
  // Sequence of historic used values
  TruncatedSeq    _used_history;

public:
  ZTLABUsage();

  void increase_used(size_t size);
  void decrease_used(size_t size);
  void reset();

  size_t tlab_used() const;
  size_t tlab_capacity() const;
};

#endif // SHARE_GC_Z_ZTLABUSAGE_HPP