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Metaspace implementation is divided into separate sub systems, each of which is isolated from its peers and has a small number of tasks.

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The Virtual Memory Subsystem

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• "Allocate new root chunk"

  Metachunk* VirtualSpaceList::allocate_root_chunk();

  This carves out a new root chunk from the underlying reserved space and hands it to the caller (nothing is committed yet, this is purely reserved memory).

• "commit this range"

  bool VirtualSpaceNode::ensure_range_is_committed(MetaWord* p, size_t word_size);

  Upper layers request that a given arbitrary address range should be committed. Subsystem figures out which granules would be affected and makes sure those are committed (which may be a noop if they had been committed before).

  When committing, subsystem honors VM limits (MaxMetaspaceSize resp. the commit gc threshold) via the commit limiter.

• "uncommit this range"

  void VirtualSpaceNode::uncommit_range(MetaWord* p, size_t word_size);

  Similar to committing. Subsystem figures out which commit granules are affected, and uncommits those.

   

• "purge" 

  void VirtualSpaceList::purge()

  This unmaps all completely empty memory regions, and uncommits all unused commit granules.

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base                                                                    end
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 v                                                                       v

 | root chunk area | root chunk area | root chunk area | root chunk area |

 |x| |x|x|x| | | | |x|x|x| | | |x|x| | | |x|x|x|x| | | | | |x| |x| | | | |    <-- commit granules (x=committed)

CommitMask

(commitMask.hpp) 

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