RFR: 8327388: GenShen: census during marking is partial [v4]

[Y. Srinivas Ramakrishna]

On Tue, 9 Apr 2024 00:45:35 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> There was a bug in the placement of the call to clear stale census data before starting a fresh one for a new marking cycle that marks through the younger generation. This bug resulted in the use of a co-terminal suffix of the census collection, losing all data from the earlier iterations of an iterative collection process that may run up to 5 times.
>> 
>> We stumbled upon the defect when working on a refactoring task involving separation of generational extensions of Shenandoah from its non-generational version. The (performance) defect has existed since day zero of the adaptive tenuring code in GenShen.
>> 
>> Along with fixing the defect, an assertion has been added to check the "reasonable completeness" of the census, which came in useful to detect a reset call inadvertently left behind in one place.
>> 
>> Some ShenandoahAgeCensus APIs have been narrowed and cleaned up a bit, and documentation clarified a bit more.
>> 
>> **Testing**:
>> - [x] GHA 
>> - [x] Code pipeline testing : one intermittent stress dacapo failure, ascribed to an existing bug in coalesce-and-fill
>> - [x] SPECjbb
>> 
>> **Performance**: 
>> - [x] SPECjbb : the variance in tests fails any significant change under 2-tailed Mann-Whitney
>
> Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:
> 
>  - Merge branch 'master' into clear_census
>  - Merge branch 'master' into clear_census
>  - Merge branch 'master' into clear_census
>  - Remove local_reset of age_census object inadvertently left behind in the
>    GLOBAL gen concurrent mark, which was triggering the newly added
>    reasonableness assert (yay!).
>  - Avoid divide-by-zero.
>  - Merge branch 'master' into clear_census
>  - Fix word and byte size unit confusion in comparison/assert.
>  - Loose verification of "reasonable completeness" of census.
>  - Fix release build.
>  - Support for verification of census' reasonableness, but no verification
>    itself yet.
>  - ... and 6 more: https://git.openjdk.org/shenandoah/compare/11295a6a...d5fb72cc

No performance differences were noted; however, the previous census was partial and therefore potentially incorrect. A correct census should allow for future performance tuning.

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PR Comment: https://git.openjdk.org/shenandoah/pull/403#issuecomment-2052708265