RFR: 8261495: Shenandoah: reconsider update references memory ordering [v7]

[Roman Kennke]

On Tue, 29 Jun 2021 12:33:43 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. For example, `SPECjvm2008:compiler.sunflow`:
>> 
>> 
>> # Before
>> [172.931s][info][gc,stats] Concurrent Update Refs         =    9.866 s (a =    60161 us)
>> [283.225s][info][gc,stats] Concurrent Update Refs         =    8.274 s (a =    57859 us)
>> [232.514s][info][gc,stats] Concurrent Update Refs         =    9.321 s (a =    58991 us)
>> [215.495s][info][gc,stats] Concurrent Update Refs         =    9.233 s (a =    58434 us)
>> [217.557s][info][gc,stats] Concurrent Update Refs         =    9.153 s (a =    58672 us)
>> 
>> # After
>> [169.752s][info][gc,stats] Concurrent Update Refs         =    8.150 s (a =    49697 us)
>> [168.798s][info][gc,stats] Concurrent Update Refs         =    8.430 s (a =    51092 us)
>> [166.353s][info][gc,stats] Concurrent Update Refs         =    8.812 s (a =    54395 us)
>> [236.542s][info][gc,stats] Concurrent Update Refs         =    8.215 s (a =    51026 us)
>> [214.006s][info][gc,stats] Concurrent Update Refs         =    8.244 s (a =    52175 us)
>> 
>> 
>> 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 three additional commits since the last revision:
> 
>  - Comment tune-ups
>  - Merge branch 'master' into JDK-8261495-shenandoah-updaterefs-memord
>  - 8261495: Shenandoah: reconsider update references memory ordering

Marked as reviewed by rkennke (Reviewer).

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