RFR: 8337511: Implement JEP-404: Generational Shenandoah (Experimental) [v3]

[Liang Mao]

On Tue, 8 Oct 2024 17:20:31 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This PR merges JEP 404, a generational mode for the Shenandoah garbage collector. The JEP can be viewed here: https://openjdk.org/jeps/404. We would like to target JDK24 with this PR.
>
> William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 478 commits:
> 
>  - Fix merge error
>  - Merge remote-tracking branch 'jdk/master' into great-genshen-pr-redux
>  - Merge remote-tracking branch 'jdk/master' into great-genshen-pr-redux
>  - Merge branch 'shenandoah/master' into great-genshen-pr-redux
>  - Merge
>  - 8341099: GenShen: assert(HAS_FWD == _heap->has_forwarded_objects()) failed: Forwarded object status is sane
>    
>    Reviewed-by: kdnilsen
>  - 8341485: GenShen: Make evac tracker a non-product feature and confine it to generational mode
>    
>    Reviewed-by: kdnilsen, ysr
>  - Merge
>  - 8341042: GenShen: Reset mark bitmaps for unaffiliated regions when preparing for a cycle
>    
>    Reviewed-by: kdnilsen
>  - 8339616: GenShen: Introduce new state to distinguish promote-in-place phase as distinct from concurrent evacuation
>    
>    Reviewed-by: kdnilsen, shade, ysr
>  - ... and 468 more: https://git.openjdk.org/jdk/compare/b9db74a6...4db1e0e1

Congratulations! Good to see the great progress. I just built this PR for some testing and found something unexpected. I ran the genshen VS shenandoah(default) with jbb2015 on aarch64 N2 with 8 cores and Xmx8g. The critical-jOPS of genshen(5373) is behind shenandoah(6027). Do I miss something on the options?
```java -Xmx8g -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCHeuristics=adaptive -XX:ShenandoahGCMode=generational -Xlog:gc*  -XX:MetaspaceSize=1g   -jar specjbb2015.jar -m COMPOSITE```

-------------

PR Comment: https://git.openjdk.org/jdk/pull/21273#issuecomment-2401194597