RFR: 8325857: G1 Full GC flushes mark stats cache too early

[Thomas Schatzl]

On Fri, 23 Feb 2024 12:57:26 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this fix that flushes mark stats cache after reference processing because that phase can mark some objects. Apart from a minor performance issue (the amount live data is used to determine whether a region is considered for the dead-wood optimization only, so at most g1 full gc compacted too much) there has been no correctness issue.
>> 
>> In this patch the work is performed serially because even on not-so-current processors this phase takes < 1ms for up to 256 threads (~0.004ms per parallel gc thread).
>> 
>> There is a prototype for a parallel version at https://github.com/openjdk/jdk/compare/master...tschatzl:jdk:submit/8325857-wrong-mark-cache-flush?expand=1 (ugly code) that costs, depending on the machine <= 0,1ms (slightly older machine, 36 threads) or <= 0.03ms (current machine, 256 threads), but introducing like 30 LOC + some magic constant.
>> 
>> I assume that one does not use/force such large #threads to run tiny applications where the other part of g1 full gc only takes time where even that 1ms matter, so saving a bit on code complexity. If somebody disagrees with that analysis, I can change the code to use the parallel version.
>> Additionally g1 full gc is kind of an error condition anyway.
>> 
>> Testing: gha
>> 
>> Hth,
>>   Thomas
>
> Marked as reviewed by iwalulya (Reviewer).

Thanks @walulyai @albertnetymk for your reviews

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PR Comment: https://git.openjdk.org/jdk/pull/17979#issuecomment-1963471066