RFR: 8331572: Allow using OopMapCache outside of STW GC phases [v2]

[Aleksey Shipilev]

On Thu, 16 May 2024 14:17:03 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Notify service thread on first enqueue
>
> src/hotspot/share/interpreter/oopMapCache.cpp line 584:
> 
>> 582:         MutexLocker ml(Service_lock, Mutex::_no_safepoint_check_flag);
>> 583:         Service_lock->notify_all();
>> 584:       }
> 
> Yes, you don't want to do this too frequently.

Well, tests pass with this change, but now I am thinking if we would eventually run into any lock ranking problem here. At very least `stackwatermark` is ranked above `service`, so we are safe for concurrent GCs. There are only a few locks that are ranked below `service`, so maybe I am overthinking this?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19229#discussion_r1603597310