RFR: 8343704: Bad GC parallelism with processing Cleaner queues [v6]

[Oli Gillespie]

On Thu, 14 Nov 2024 10:04:04 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See the bug for more discussion and reproducer. This PR replaces the ad-hoc linked list segmented list of arrays. Arrays are easy targets for GC. There are possible improvements here, most glaring is parallelism that is currently knee-capped by global synchronization. The synchronization scheme follows what we have in original code, and I think it is safer to continue with it right now.
>> 
>> I'll put performance data in a separate comment.
>> 
>> Additional testing:
>>  - [x] Original reproducer improves drastically
>>  - [x] New microbenchmark shows no regression on "churning" tests, which covers insertion/removal perf
>>  - [x] New microbenchmark shows improvement on Full GC times (crude, but repeatable), serves as a proxy for reproducer
>>  - [x] `java/lang/ref` tests in release 
>>  - [x] `all` tests in fastdebug
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Reimplement with segmented linked list of arrays

src/java.base/share/classes/jdk/internal/ref/CleanerImpl.java line 310:

> 308:                Node newHead = head.next;
> 309:                newHead.prev = null;
> 310:                head = newHead;

Does this allow removing the final head node when the list is empty? Seems we need to avoid that (`if (head.size == 0 && head.next != null)`?.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22043#discussion_r1842091029