RFR: 8320969: Shenandoah: Enforce stable number of GC workers

[Y. Srinivas Ramakrishna]

On Wed, 29 Nov 2023 11:01:32 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See bug for rationale.
> 
> Additional testing:
>  - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`
>  - [x] Linux x86_64 server fastdebug, `tier{1,2,3}` with `-XX:+UseShenandoahGC`
>  - [x] Linux AArch64 server fastdebug, `tier{1,2,3}` with `-XX:+UseShenandoahGC`

@shipilev can you share any performance numbers with this change? The infra to modify the number of threads per phase would seem to be useful for adaptively changing the number of threads, specializing the per-GC/collector/phase policy for doing so, instead of removing it entirely. The problem you point out with the existing implementation was that it deferred completely to the shared policy; couldn't we just specialize that for Shenandoah or Generational Shenandoah (and for now just fix the number of workers at the values you chose here, but allow this to be more intelligently adapted in the future)?

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PR Comment: https://git.openjdk.org/jdk/pull/16878#issuecomment-1832388065