RFR: 8350314: Shenandoah: Capture thread state sync times in GC timings [v3]

[Xiaolong Peng]

On Wed, 26 Feb 2025 02:13:07 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> Changes are fine.
> 
> This jumped out in yr sample output:
> 
> ```
> ...
> [11.057s][info][gc,stats    ]   Finish Mark                       129 us, parallelism: 0.01x
> ...
> [11.057s][info][gc,stats    ] Concurrent Weak References           67 us, parallelism: 0.25x
> ```
> 
> which seemed kinda interesting. I assume this is just a consequence of the very little work (and extremely brief time in this phase) here, and can be ignored in this sample output from likely a toy GC, or one where you may have artificially boosted the number of worker threads. Still I thought I'd ask in case you've seen this with bigger timings or more work in any of these phases with low fractional speed-ups.

I'm not sure how parallelism is calculated, but I think it is caused by the test I was running, the test is very simple and there are only small number of live objects after mark.

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PR Comment: https://git.openjdk.org/jdk/pull/23759#issuecomment-2685925735