RFR: 8331572: Allow using OopMapCache outside of STW GC phases

[Coleen Phillimore]

On Tue, 14 May 2024 12:31:08 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> As the reproducer in the issue shows, we would also like to use the `OopMapCache` during the concurrent GC phases. Zhengyu mentions there is also a production problem for stack walking that would benefit from letting `OopMapCache` be used without looking at GC at all.
> 
> This PR unblocks `OopMapCache` uses for everything. Cleanups are nominally done by service thread. But, still appreciating that majority of use cases would be from GCs, we leave the proactive cleanups from the GC ops here as well. It requires the synchronization between readers that might be copying out the entries out of the hashmap and the concurrent reclamation. Handily, `GlobalCounter` can be used for that purpose. 
> 
> After this lands, I think we can go over `OopMapCache::compute_one_oop_map` uses and see if they would instead like to use the cached `lookup` to benefit from this cache too. I think those paths are for OSR and deopts, so their performance is unlikely to be critical. This PR already covers the concurrent GC paths well.
> 
> Additional testing:
>  - [x] Performance test reproducer from the bug improves significantly
>  - [x] Linux AArch64 server fastdebug, `hotspot_gc_shenandoah` (10x)
>  - [x] Linux AArch64 server fastdebug, `all`
>  - [x] Linux x86_64 server fastdebug, `all`

I did have questions (sorry hit approve too soon).

src/hotspot/share/interpreter/oopMapCache.cpp line 545:

> 543: 
> 544:   // First search for an empty slot
> 545:   for (int i = 0; i < _probe_depth; i++) {

Does the GlobalCounter read barrier belong around this too?

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.

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Changes requested by coleenp (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19229#pullrequestreview-2058906401
PR Review Comment: https://git.openjdk.org/jdk/pull/19229#discussion_r1602184993
PR Review Comment: https://git.openjdk.org/jdk/pull/19229#discussion_r1602186516