RFR: 8316180: Thread-local backoff for secondary_super_cache updates [v8]

[Martin Doerr]

On Wed, 27 Sep 2023 19:46:39 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Work in progress, submitting for broader attention.
>> 
>> See more details in the bug and related issues.
>> 
>> This is the attempt to mitigate [JDK-8180450](https://bugs.openjdk.org/browse/JDK-8180450), while the more complex fix that would obviate the need for `secondary_super_cache` is being worked out. The goal for this fix is to improve performance in pathological cases, while keeping non-pathological cases out of extra risk, *and* staying simple enough and reliable for backports to currently supported JDK releases.
>> 
>> This implements the mitigation for AArch64 and x86_64. More platforms can be implemented in this PR, or deferred to later PRs. Port maintainers, feel free to suggest the patches for your arches, I'll be happy to fold them in.
>> 
>> Note that the code is supposed to be rather compact, because it is inlined in generated code. That is why, for example, we cannot easily do x86_32 version: we need a thread, so the easiest way would be to call into VM. But we cannot that easily: the code blowout would make some forward branches in external code non-short. I think we we cannot implement this mitigation on some architectures, so be it, it would be a sensible tradeoff for simplicity.
>> 
>> Setting backoff at `0` effectively disables the mitigation, and gives us safety hatch if something goes wrong.
>> 
>> Current PR deliberately sets backoff at `1000` to simplify testing. The actual value should be chosen by broader experiments.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug, `tier1 tier2 tier3`
>>  - [x] Linux AArch64 fastdebug, `tier1 tier2 tier3`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Correct type for flag

Seems like you have posted the wrong patch version.

> Hi @shipilev,
> 
> Please include s390 patch from here:
> 
> [s390_port.patch](https://github.com/openjdk/jdk/files/12882890/s390_port.patch)
> 
> Benchmark: Z15 Machine, with 64 CPUs
> 
> ```
> -XX:SecondarySuperMissBackoff=0
> Benchmark                        Mode  Cnt     Score       Error  Units
> SecondarySuperCache.contended    avgt   15  10655.113 ? 6148.685  ns/op
> SecondarySuperCache.uncontended  avgt   15    107.9   ?   54.109  ns/op
> 
> -XX:SecondarySuperMissBackoff=10
> Benchmark                        Mode  Cnt    Score       Error  Units
> SecondarySuperCache.contended    avgt   15  2541.671 ? 1048.361  ns/op
> SecondarySuperCache.uncontended  avgt   15   120.107 ?   63.345  ns/op
> -XX:SecondarySuperMissBackoff=100
> 
> Benchmark                        Mode  Cnt   Score      Error  Units
> SecondarySuperCache.contended    avgt   15  662.473 ? 139.208  ns/op
> SecondarySuperCache.uncontended  avgt   15   79.371 ?  40.232  ns/op
> 
> -XX:SecondarySuperMissBackoff=1000
> Benchmark                        Mode  Cnt   Score     Error  Units
> SecondarySuperCache.contended    avgt   15  248.292 ? 77.175  ns/op
> SecondarySuperCache.uncontended  avgt   15   87.333 ? 35.103  ns/op
> 
> -XX:SecondarySuperMissBackoff=10000
> Benchmark                        Mode  Cnt   Score    Error  Units
> SecondarySuperCache.contended    avgt   15  139.901 ? 44.83  ns/op
> SecondarySuperCache.uncontended  avgt   15   90.872 ? 31.791 ns/op
> ```

Seems like you have posted the wrong patch version. Please check.

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PR Comment: https://git.openjdk.org/jdk/pull/15718#issuecomment-1759650990