RFR: 8261495: Shenandoah: reconsider update references memory ordering [v3]

[Zhengyu Gu]

On Thu, 11 Feb 2021 06:37:56 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 incrementally with one additional commit since the last revision:
> 
>   Use release only

Looks good to me.

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Marked as reviewed by zgu (Reviewer).

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