RFR: 8331411: Shenandoah: Reconsider spinning duration in ShenandoahLock [v7]

[Xiaolong Peng]

On Wed, 26 Jun 2024 17:05:01 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Are we trying to minimize TTSP or SP Polls? Does SP mean Safepoint or SpinPause here? If we're trying to minimize TTSP, The result table suggests 0x0F would be better?

Thank you William for looking reviewing this.
SP means Safepoint here.

I have discussed the reason for the choice in the description. Yes lower spin pauses does give much better result for the designed scenarios w/ TLAB disabled causing extremely heavily contended heap lock, less than 0xFF spin pauses seem to be overly optimized and causing regression for the regular/normal case in which TLAB is likely/always enabled. 

We also observed some regressions in the metrics from shenandoah's test farm with 0x1F spin pauses, e.g. Spring latency, which is gone after I increased the spin pauses to 0xFF.

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PR Comment: https://git.openjdk.org/jdk/pull/19570#issuecomment-2192249413