RFR: 8261501: Shenandoah: reconsider heap statistics memory ordering

[Roman Kennke]

On Wed, 10 Feb 2021 11:10:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> ShenandoahHeap collects heap-wide statistics (used, committed, etc). It does so by atomically updating them with default CASes. Unfortunately, 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 is excessive for statistics gathering, and "relaxed" should be just as good.
> 
> Additional testing:
>  - [x] Linux x86_64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 tier1 with Shenandoah

Looks good to me! Thanks!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2504