RFR: 8335480: Only deoptimize threads if needed when closing shared arena [v2]

[Rémi Forax]

On Fri, 12 Jul 2024 20:59:26 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> This PR limits the number of cases in which we deoptimize frames when closing a shared Arena. The initial intent of this was to improve the performance of shared arena closure in cases where a lot of threads are accessing and closing shared arenas at the same time (see attached benchmark), but unfortunately even disabling deoptimization altogether does not have a great effect on that benchmark.
>> 
>> Nevertheless, I think the extra logging/testing/benchmark code, and comments I've written, together with reducing the number of cases where we deoptimize (which makes it clearer exactly why we need to deoptimize in the first place), will be useful going forward. So, I've a create this PR out of them.
>> 
>> In this PR:
>> - Deoptimizing is now only done in cases where it's needed, instead of always. Which is in cases where we are not inside an `@Scoped` method, but are inside a compiled frame that has a scoped access somewhere inside of it.
>> - I've separated the stack walking code (`for_scope_method`) from the code that checks for a reference to the arena being closed (`is_accessing_session`), and added logging code to the former. That also required changing vframe code to accept an `ouputStream*` rather than always printing to `tty`.
>> - Added a new test (`TestConcurrentClose`), that tries to close many shared arenas at the same time, in order to stress that use case.
>> - Added a new benchmark (`ConcurrentClose`), that stresses the cases where many threads are accessing and closing shared arenas.
>> 
>> I've done several benchmark runs with different amounts of threads. The confined case stays much more consistent, while the shared cases balloons up in time spent quickly when there are more than 4 threads:
>> 
>> 
>> Benchmark                     Threads   Mode  Cnt     Score     Error  Units
>> ConcurrentClose.sharedAccess       32   avgt   10  9017.397 ± 202.870  us/op
>> ConcurrentClose.sharedAccess       24   avgt   10  5178.214 ± 164.922  us/op
>> ConcurrentClose.sharedAccess       16   avgt   10  2224.420 ± 165.754  us/op
>> ConcurrentClose.sharedAccess        8   avgt   10   593.828 ±   8.321  us/op
>> ConcurrentClose.sharedAccess        7   avgt   10   470.700 ±  22.511  us/op
>> ConcurrentClose.sharedAccess        6   avgt   10   386.697 ±  59.170  us/op
>> ConcurrentClose.sharedAccess        5   avgt   10   291.157 ±   7.023  us/op
>> ConcurrentClose.sharedAccess        4   avgt   10   209.178 ±   5.802  us/op
>> ConcurrentClose.sharedAccess        1   avgt   10  ...
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   track has_scoped_access for compiled methods

Knowing that all the segments are freed during close() is something you may want.
But having the execution time of close() be linear with the number of threads is also problematic. Maybe, it means that we need another kind of Arena that works like shared() but allow the freed to be done asynchronously (ofSharedAsyncFree ?).

Note that the semantics of ofSharedAsyncFree() is different from ofAuto(), ofAuto() relies on the GC to free a segment so the delay before a segment is freed is not time bounded if the application has enough memory, the memory of the segment may never be reclaimed. With ofSharedAsyncFree(), the segments are freed by the last thread, so while this mechanism is not deterministic, it is time bounded.

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PR Comment: https://git.openjdk.org/jdk/pull/20158#issuecomment-2226992713