RFR: 8261495: Shenandoah: reconsider update references memory ordering [v4]

[Roman Kennke]

On Tue, 16 Feb 2021 13:21:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Shenandoah update heap references code uses default Atomic::cmpxchg to avoid races with mutator updates. Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64.
>> 
>> This seems to be excessive for Shenandoah update references code, and "release" is enough. We do not seem to piggyback on update-references memory effects anywhere (in fact, if not for mutator, we would not even need a CAS). But, there is an interplay with concurrent evacuation and updates from self-healing.
>> 
>> Average time goes down, the number of GC cycles go up, since the cycles are shorter.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 hotspot_gc_shenandoah
>>  - [x] Linux AArch64 hotspot_gc_shenandoah
>>  - [x] Linux AArch64 tier1 with Shenandoah
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:
> 
>  - Comment touchup
>  - Specialize out witness-checking methods, drop acquire again
>  - Even more explanation
>  - Move the comment
>  - Also handle clearing the oops
>  - Minor touchups to the comment
>  - Merge branch 'master' into JDK-8261495-shenandoah-updaterefs-memord
>  - Use release only
>  - Do acq_rel instead
>  - 8261495: Shenandoah: reconsider update references memory ordering

Looks good to me.

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2498