RFR: 8343704: Bad GC parallelism with processing Cleaner queues [v14]

[Brent Christian]

On Wed, 20 Nov 2024 09:19:39 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See the bug for more discussion and reproducer. This PR replaces the ad-hoc linked list with 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:
> 
>   Check all elements are removable after random test

test/micro/org/openjdk/bench/java/lang/ref/CleanerGC.java line 45:

> 43: 
> 44:     // Deliberately a linked list to avoid exposing external parallelism to GC.
> 45:     Target prev;

Is `CleanerGC.prev` used?

test/micro/org/openjdk/bench/java/lang/ref/CleanerGC.java line 57:

> 55:     @Benchmark
> 56:     public void test() {
> 57:         System.gc();

How confident are we that most/all/(any?) `Target` objects are being collected?

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PR Review Comment: https://git.openjdk.org/jdk/pull/22043#discussion_r1851391803
PR Review Comment: https://git.openjdk.org/jdk/pull/22043#discussion_r1851402011