RFR: 8261503: Shenandoah: reconsider verifier memory ordering

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RFR: 8261503: Shenandoah: reconsider verifier memory ordering

Aleksey Shipilev-5
Shenandoah verifier uses lots of atomic operations. 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.

In most cases, that is excessive for verifier, and "relaxed" would do.

Additional testing:
 - [x] Linux x86_64 hotspot_gc_shenandoah
 - [x] Linux AArch64 hotspot_gc_shenandoah
 - [x] Linux AArch64 tier1 with Shenandoah

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Commit messages:
 - 8261503: Shenandoah: reconsider verifier memory ordering

Changes: https://git.openjdk.java.net/jdk/pull/2505/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2505&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261503
  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2505.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2505/head:pull/2505

PR: https://git.openjdk.java.net/jdk/pull/2505
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Re: RFR: 8261503: Shenandoah: reconsider verifier memory ordering

Zhengyu Gu-3
On Wed, 10 Feb 2021 11:41:45 GMT, Aleksey Shipilev <[hidden email]> wrote:

> Shenandoah verifier uses lots of atomic operations. 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.
>
> In most cases, that is excessive for verifier, and "relaxed" would do.
>
> Additional testing:
>  - [x] Linux x86_64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 tier1 with Shenandoah

Looks good.

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Marked as reviewed by zgu (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2505
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Integrated: 8261503: Shenandoah: reconsider verifier memory ordering

Aleksey Shipilev-5
In reply to this post by Aleksey Shipilev-5
On Wed, 10 Feb 2021 11:41:45 GMT, Aleksey Shipilev <[hidden email]> wrote:

> Shenandoah verifier uses lots of atomic operations. 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.
>
> In most cases, that is excessive for verifier, and "relaxed" would do.
>
> Additional testing:
>  - [x] Linux x86_64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 tier1 with Shenandoah

This pull request has now been integrated.

Changeset: 7c931591
Author:    Aleksey Shipilev <[hidden email]>
URL:       https://git.openjdk.java.net/jdk/commit/7c931591
Stats:     8 lines in 1 file changed: 0 ins; 0 del; 8 mod

8261503: Shenandoah: reconsider verifier memory ordering

Reviewed-by: zgu

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PR: https://git.openjdk.java.net/jdk/pull/2505