RFR: 8361099: Shenandoah: Improve heap lock contention by using CAS for memory allocation [v20]

[Kelvin Nilsen]

On Wed, 7 Jan 2026 00:04:41 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Xiaolong Peng has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 265 commits:
>> 
>>  - Merge branch 'openjdk:master' into cas-alloc-1
>>  - Fix build error after merging from tip
>>  - Merge branch 'master' into cas-alloc-1
>>  - Merge branch 'master' into cas-alloc-1
>>  - Some comments updates as suggested in PR review
>>  - Fix build failure after merge
>>  - Expend promoted from ShenandoahOldCollectorAllocator
>>  - Merge branch 'master' into cas-alloc-1
>>  - Address PR comments
>>  - Merge branch 'openjdk:master' into cas-alloc-1
>>  - ... and 255 more: https://git.openjdk.org/jdk/compare/de81d389...cf13b7b5
>
> src/hotspot/share/gc/shenandoah/shenandoahAllocator.cpp line 428:
> 
>> 426: }
>> 427: 
>> 428: HeapWord* ShenandoahOldCollectorAllocator::allocate(ShenandoahAllocRequest& req, bool& in_new_region) {
> 
> Confer with William Kemper about this.  He is working on a change that may simplify the handling of PLABs, in which case ShenandoahOldCollectorAllocator can behave the same as ShenandoahCollector.

Alternatively, I don't think it would be too terribly difficult to implement try_allocate_aligned() function to support fast (CAS) allocation of aligned PLABs.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26171#discussion_r2668774560