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

Tue Feb 25 23:22:56 UTC 2025

On Tue, 25 Feb 2025 23:01:24 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Did we really see `propagate_gc_state_to_all_threads` taking a long time? Or was it exiting the safepoint (i.e., after the state had been propagated) that took a long time?

No, we discussed it last week in Slack channel, `propagate_gc_state_to_all_threads` usually takes less than 10 ns for ~1k threads in our test, it not a problem. 

it was the `futex` call when exit the safepoint took long time, we have a [fix](https://bugs.openjdk.org/browse/JDK-8350285) in ShenandoahLock for mitigation since the change we made last year affects scheduler, meanwhile Aleksey is working on https://bugs.openjdk.org/browse/JDK-8350324 which should improve the time to leave safepoint which has much broader impact.

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PR Comment: https://git.openjdk.org/jdk/pull/23759#issuecomment-2683510642