RFR: 8361099: Shenandoah: Improve heap lock contention by using CAS for memory allocation [v17]

[Kelvin Nilsen]

On Tue, 9 Dec 2025 21:03:21 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Some comments updates as suggested in PR review
>
> src/hotspot/share/gc/shenandoah/shenandoahAllocator.cpp line 41:
> 
>> 39:   _alloc_region_count(alloc_region_count), _free_set(free_set), _alloc_partition_name(ShenandoahRegionPartitions::partition_name(ALLOC_PARTITION)) {
>> 40:   if (alloc_region_count > 0) {
>> 41:     _alloc_regions = PaddedArray<ShenandoahAllocRegion, mtGC>::create_unfreeable(alloc_region_count);
> 
> Rethinking about the the PaddedArray used here, we may not really need it.
> Allocator has multiple shared alloc regions for CAS, and only refreshes them when all of them run out of usable memory, so _alloc_regions won't be frequently updated, the PaddedArray here should have a negative performance impact.

Are you running any experiments (on different hardware configurations) to test your assumptions about this?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26171#discussion_r2663185400