RFR: 8300926: Several startup regressions ~6-70% in 21-b6 all platforms [v4]

[Daniel D. Daugherty]

On Tue, 21 Feb 2023 08:08:00 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi all, please consider.
>> 
>> The original issue was when thread 1 asked to deopt nmethod set X and thread 2 asked for the same or a subset of X.
>> All method will already be marked, but the actual deoptimizing, not entrant, patching PC on stacks and patching post call nops, was not done yet. Which meant thread 2 could 'pass' thread 1.
>> Most places did deopt under Compile_lock, thus this is not an issue, but WB and clearCallSiteContext do not.
>> 
>> Since a handshakes may take long before completion and Compile_lock is used for so much more than deopts.
>> The fix in https://bugs.openjdk.org/browse/JDK-8299074 instead always emitted a handshake even when everything was already marked. (instead of adding Compile_lock to all places)
>> 
>> This turnout to be problematic in the startup, for example the number of deopt handshakes in jetty dry run startup went from 5 to 39 handshakes.
>> 
>> This fix first adds a barrier for which you do not pass until the requested deopts have happened and it coalesces the handshakes.
>> Secondly it moves handshakes part out of the Compile_lock where it is possible.
>> 
>> Which means we fix the performance bug and we reduce the contention on Compile_lock, meaning higher throughput in compiler and things such as class-loading.
>> 
>> It passes t1-t7 with flying colours! t8 still not completed and I'm redoing some testing due to last minute simplifications.
>> 
>> Thanks, Robbin
>
> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Coleen fix

src/hotspot/share/classfile/systemDictionary.cpp line 1632:

> 1630:     k->process_interfaces();                        // handle all "implements" declarations
> 1631: 
> 1632:     // Now flush all code that depended on old class hierarchy.

nit typo: s/flush/mark/

since you've changed that elsewhere...

src/hotspot/share/classfile/systemDictionaryShared.cpp line 856:

> 854:   // Add to class hierarchy, and do possible deoptimizations.
> 855:   SystemDictionary::add_to_hierarchy(loaded_lambda, THREAD);
> 856:   // But, do not add to dictionary.

nit comment location: move above L855 like other places?

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PR: https://git.openjdk.org/jdk/pull/12585