RFR: 8343704: Bad GC parallelism with processing Cleaner queues [v10]

[Brent Christian]

On Fri, 15 Nov 2024 19:30:06 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:
> 
>   prev is not needed

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

> 308:             }
> 309:             assert phc.node.arr[phc.index] == phc;
> 310:             assert head.size > 0;

I now realize that these assertions in `remove()` won't really "work" because _"exceptions thrown by the cleaning action are ignored"_. :(
When the cleaner thread removes the phc (`ref.clean()` in `run()` on L142 -> `list.remove()` on PhantomCleanable L94 ), any `AssertionError`s will get lost in `run()`'s `catch` statement, L144.

(There is a related RFE [JDK-8305979 : UncaughtExceptionHandler for Cleaner](https://bugs.openjdk.org/browse/JDK-8305979))

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PR Review Comment: https://git.openjdk.org/jdk/pull/22043#discussion_r1844530605