RFR: JDK-8180450: secondary_super_cache does not scale well [v4]

[Andrew Haley]

On Tue, 19 Mar 2024 02:54:36 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>>> I'm unclear why you are presenting this as a Diagnostic feature? I would expect either Experimental if you consider it early days and want more feedback; or else a full Product option that people can opt-in to using, and which eventually becomes the default.
>> 
>> The code for x86 and AArch64 is product ready, but other platforms aren't yet done. I think it should be enabled by default, but YMMV.
>> 
>> The new -XX options will surely be needed by the maintainers of other platforms during porting. They may also be useful during the review phase of this patch, for reviewers to test before/after performance on their own systems.
>> 
>> `UseSecondarySuperCache` and `HashSecondarySupers` perhaps make sense as (rather esoteric) product flags, but`VerifySecondarySupers` and `StressSecondarySuperHash` are strictly for port maintainers.
>
>> I'm unclear why you are presenting this as a Diagnostic feature?
> 
> From a user perspective, there should be no reason to specify `HashSecondarySupers` or `UseSecondarySuperCache` unless you diagnose a performance regression or looking for a workaround for a crash. There should be no other reason to override default value and switch to the old implementation once `HashSecondarySupers` is supported on a platform.
> 
> IMO diagnostic flags are well justified here. Experimental flag doesn't cut it since the feature is turned on by default and making the flags product puts unnecessary obligations on the VM for something so obscure (from a user perspective). 
> 
> Some other VM flags followed the same practice (e.g., `UseVtableBasedCHA`) and so far it worked well.

Temporarily converted this to Draft status while @iwanowww and I are working on it. Back soon.

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PR Comment: https://git.openjdk.org/jdk/pull/18309#issuecomment-2023198100