src/hotspot/share/memory/metaspace/commitLimiter.hpp

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#ifndef SHARE_MEMORY_METASPACE_COMMITLIMITER_HPP
#define SHARE_MEMORY_METASPACE_COMMITLIMITER_HPP

#include "memory/allocation.hpp"
#include "memory/metaspace/counters.hpp"

namespace metaspace {

// The CommitLimiter encapsulates a limit we may want to impose on how much
//  memory can be committed. This is a matter of separation of concerns:
//
// In metaspace, we have two limits to committing memory: the absolute limit,
//  MaxMetaspaceSize; and the GC threshold. In both cases an allocation should
//  fail if it would require committing memory and hit one of these limits.
//
// However, the actual Metaspace allocator is a generic one and this
//  GC- and classloading specific logic should be kept separate. Therefore
//  it is hidden inside this interface.
//
// This allows us to:
//  - more easily write tests for metaspace, by providing a different implementation
//    of the commit limiter, thus keeping test logic separate from VM state.
//  - (potentially) use the metaspace for things other than class metadata,
//    where different commit rules would apply.
//
class CommitLimiter : public CHeapObj<mtMetaspace> {

  // Counts total words committed for metaspace
  SizeCounter _cnt;

  // Purely for testing purposes: cap, in words.
  const size_t _cap;

public:

  // Create a commit limiter. This is only useful for testing, with a cap != 0,
  // since normal code should use the global commit limiter.
  // If cap != 0 (word size), the cap replaces the internal logic of limiting.
  CommitLimiter(size_t cap = 0) : _cnt(), _cap(cap) {}

  // Returns the size, in words, by which we may expand the metaspace committed area without:
  // - _cap == 0: hitting GC threshold or the MaxMetaspaceSize
  // - _cap > 0: hitting cap (this is just for testing purposes)
  size_t possible_expansion_words() const;

  void increase_committed(size_t word_size)   { _cnt.increment_by(word_size); }
  void decrease_committed(size_t word_size)   { _cnt.decrement_by(word_size); }

  size_t committed_words() const              { return _cnt.get(); }

  // Returns the global metaspace commit counter
  static CommitLimiter* globalLimiter();

};

} // namespace metaspace

#endif // SHARE_MEMORY_METASPACE_COMMITLIMITER_HPP