RFR: 8361099: Shenandoah: Improve heap lock contention by using CAS for memory allocation [v14]

[Xiaolong Peng]

On Tue, 2 Dec 2025 22:24:55 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 255 commits:
>> 
>>  - Add missing header for ShenandoahFreeSetPartitionId
>>  - Declare ShenandoahFreeSetPartitionId as enum instead of enum class
>>  - Fix a typo
>>  - Remove unnecessary `enum class ShenandoahFreeSetPartitionId : uint8_t` in shenandoahAllocator.php
>>  - Make ShenandoahAllocator as template class to make compiled code more efficient for each alloc partition
>>  - Port the fix of JDK-8372566
>>  - Merge branch 'master' into cas-alloc-1
>>  - Merge remote-tracking branch 'origin/master' into cas-alloc-1
>>  - Remove junk code
>>  - Remove unnecessary change and tidy up
>>  - ... and 245 more: https://git.openjdk.org/jdk/compare/79e99bb0...7980c039
>
> src/hotspot/share/gc/shenandoah/shenandoahAllocator.cpp line 121:
> 
>> 119: template <ShenandoahFreeSetPartitionId ALLOC_PARTITION>
>> 120: HeapWord* ShenandoahAllocator<ALLOC_PARTITION>::attempt_allocation_slow(ShenandoahAllocRequest& req, bool& in_new_region) {
>> 121:   ShenandoahHeapLocker locker(ShenandoahHeap::heap()->lock(), _yield_to_safepoint);
> 
> I think this is an error.  We don't want to acquire the lock here.  We also don't want to introduce accounting_update here.  Instead, I think these belong before line 130, in case we need to refresh the alloc regions.

It is not an error, before calling into attempt_allocation_slow, it already called attempt_allocation_in_alloc_regions once and failed to allocate, slow path is always with heap lock.

After taking the lock, we should try the attempt_allocation_in_alloc_regions right away, because other mutator thread may have refreshed the alloc regions while holding the lock.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26171#discussion_r2583295858