RFR: 8344009: Improve compiler memory statistics [v4]

[Thomas Stuefe]

On Tue, 25 Feb 2025 06:03:21 GMT, Ashutosh Mehra <asmehra at openjdk.org> wrote:

>> Thomas Stuefe has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
>> 
>>   avoid Thread::current in high traffic chunk alloc path
>
> src/hotspot/share/memory/arena.hpp line 48:
> 
>> 46:   const size_t _len;      // Size of this Chunk
>> 47:   // Used for Compilation Memory Statistic
>> 48:   uint64_t _stamp;
> 
> This is wasted space if compilation memory stats is not enabled. One way to avoid this is to subclass `Chunk` as a `StampedChunk` and use that if compilation memory stats is enabled. Is this complexity worth the space saving?

I'd like to avoid that complexity. Arena coding is already needlessly complex. Note that a chunk is variable-sized and typically 1Kb in size or larger, and we should not have that many chunks in live arenas at any given moment (some hundred maybe). Note also that using 8 bytes seems wasteful, but the payload section has to be aligned to void* anyway.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23530#discussion_r1970128193