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

[Aleksey Shipilev]

On Wed, 15 May 2024 20:07:57 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 593:
> 
>> 591: bool OopMapCache::has_cleanup_work() {
>> 592:   return Atomic::load(&_old_entries) != nullptr;
>> 593: }
> 
> Does this need to notify the ServiceThread?  Since the ServiceThread is now a timed wait, maybe this is fine.

Right. I have not realized the service thread timed wait addition was recent. So it would become a problem if we pull this patch to the JDK release where service thread needs to be explicitly notified. We need to do this carefully, though, since we don't want to acquire `ServiceLock` all that often or inundate the thread with requests. So I added a simple notification when queue is populated with the first element.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19229#discussion_r1602795482