RFR: 8316180: Thread-local backoff for secondary_super_cache updates [v14]

[Aleksey Shipilev]

On Mon, 8 Jan 2024 19:32:47 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> 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 mitigation on most current architectures:
>>   - ✅ x86_64: implemented
>>   - 🔴 x86_32: considered, abandoned; cannot be easily done without blowing up code size
>>   - ✅ AArch64: implemented
>>   - 🔴 ARM32: considered, abandoned; needs cleanups and testing; see [JDK-8318414](https://bugs.openjdk.org/browse/JDK-8318414)
>>   - ✅ PPC64: implemented, thanks @TheRealMDoerr 
>>   - ✅ S390: implemented, thanks @offamitkumar 
>>   - ✅ RISC-V: implemented, thanks @RealFYang 
>>   - ✅ Zero: does not need implementation
>> 
>> 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.
>> 
>> I believe we can go in with `1000` as the default, given the experimental results mentioned in this PR.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug, `tier1 tier2 tier3`
>>  - [x] Linux AArch64 fastdebug, `tier1 tier2 tier3`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 31 commits:
> 
>  - Merge branch 'master' into JDK-8316180-backoff-secondary-super
>  - AArch64: Trying to backoff only for actual back-to-back updates
>  - Merge branch 'master' into JDK-8316180-backoff-secondary-super
>  - Improve benchmarks
>  - Merge branch 'master' into JDK-8316180-backoff-secondary-super
>  - Editorial cleanups
>  - RISC-V implementation
>  - Mention ARM32 bug
>  - Make sure benchmark runs with C1
>  - Merge branch 'master' into JDK-8316180-backoff-secondary-super
>  - ... and 21 more: https://git.openjdk.org/jdk/compare/387828a3...d4254b67

Now that [JDK-8180450](https://bugs.openjdk.org/browse/JDK-8180450) has a viable path forward, I see no point in continuing with this one. It has observable regressions on some workloads that are hard to overcome in a simple manner. Closing as WNF. Will reopen if JDK-8180450 is not in.

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PR Comment: https://git.openjdk.org/jdk/pull/15718#issuecomment-2009247371