RFR: 8294775: Shenandoah: reduce contention on _threads_in_evac [v5]

[Aleksey Shipilev]

On Mon, 14 Nov 2022 17:28:17 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

>> The idea here is to reduce contention on the shared `_threads_in_evac` counter by splitting its state over multiple independent cache lines. Each thread hashes to one particular counter based on its `Thread*`. This helps improve throughput of concurrent evacuation where many Java threads may be attempting to update this counter on the load barrier slow path.
>> 
>> See this earlier thread for details and SPECjbb results: https://mail.openjdk.org/pipermail/shenandoah-dev/2022-October/017494.html
>> 
>> Also tested `hotspot_gc_shenandoah` on x86 and AArch64.
>
> Nick Gasson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Formatting fixes

Marked as reviewed by shade (Reviewer).

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PR: https://git.openjdk.org/jdk/pull/10573