RFR: 8343704: Bad GC parallelism with processing Cleaner queues [v7]

[Axel Boldt-Christmas]

On Thu, 14 Nov 2024 13:24:26 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:
> 
>   Avoid NPE on empty list, add tests, touchups

This seem to handle excessive allocations when churning around an empty list by keeping the head node always allocated. I wonder if there is any worth adding some hysteresis if it churns around a multiple of the `NODE_CAPACITY`, by for example pooling one node.

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PR Comment: https://git.openjdk.org/jdk/pull/22043#issuecomment-2476360805