RFR: 8251158: Implementation of JEP 387: Elastic Metaspace [v11]

[Thomas Stuefe]

On Mon, 5 Oct 2020 16:04:16 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Review Feedback Richard 1
>
> src/hotspot/share/memory/metaspace/metaspaceArena.cpp line 81:
> 
>> 79:            METACHUNK_FULL_FORMAT_ARGS(c));
>> 80:   }
>> 81: }
> 
> Have you thought about splitting the chunk if possible and returning the free chunk to the ChunkManager? If you return
> it to the free list of this class loader then only this loader can make use of it. If you return it to the CM then
> every loader can make use of it and, potentially, it can even be uncommitted.

Yes, but in practice, the benefit would be slim:
- it is very rare that you retire a chunk which is not even halfway used. For that to happen a metaspace allocation must
  be requested larger than half of the current chunk. Does not happen that often.
- This only affects the committed portion of the chunk. So, for larger chunks - where this could hurt - the waste (waste
  in the sense as "unnecessarily earmarked for one loader only") mostly consists of address space only, not committed
  space.

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PR: https://git.openjdk.java.net/jdk/pull/336