[jdk17u-dev] RFR: 8280784: VM_Cleanup unnecessarily processes all thread oops

[Aleksey Shipilev]

On Wed, 16 Nov 2022 18:05:36 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Clean backport of performance enhancement for GCs that do concurrent thread roots (in JDK 17u those are Shenandoah and Z). On my machine, with Shenandoah running 20K threads, the safepoint cleanup time for "Cleanup" VM ops dropped from 2ms to nearly zero. A possible follow-up would be [JDK-8280817](https://bugs.openjdk.org/browse/JDK-8280817), which would extend this to other "empty" ops.
> 
> Additional testing:
>  - [x] Ad-hoc benchmarks
>  - [x] Linux x86_64 fastdebug `tier1`, `tier2` 
>  - [x] Linux x86_64 fastdebug `tier1`, `tier2`  with `-XX:+DeoptimizeALot`
>  - [x] Linux x86_64 fastdebug `tier1`, `tier2`  with `-XX:+UseShenandoahGC`

@stefank, would you like to ack this?

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PR: https://git.openjdk.org/jdk17u-dev/pull/889