RFR: 8335480: Only deoptimize threads if needed when closing shared arena [v3]

[Maurizio Cimadamore]

On Mon, 15 Jul 2024 12:34:37 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This is the baseline if I change `closing` to use a confined arena:
> 
> ```
> Benchmark                                        Mode  Cnt   Score    Error  Units
> ConcurrentClose.sharedClose                      avgt   10   8.089 ±  0.006  us/op
> ConcurrentClose.sharedClose:closing              avgt   10   0.001 ±  0.001  us/op
> ConcurrentClose.sharedClose:memorySegmentAccess  avgt   10  20.046 ±  0.019  us/op
> ConcurrentClose.sharedClose:otherAccess          avgt   10   4.220 ±  0.002  us/op
> ```

This is promising. Effectively, once all the issues surrounding reachability fences will be addressed, we should be able to achieve numbers similar to above even in the case of shared close. The only thing being slower in that case would be the closing thread itself.

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PR Comment: https://git.openjdk.org/jdk/pull/20158#issuecomment-2228448722