RFR: JDK-8307314: Implementation: Generational Shenandoah (Experimental) [v4]

[Y. Srinivas Ramakrishna]

On Fri, 2 Jun 2023 02:49:25 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> OpenJDK Colleagues:
>> 
>> Please review this proposed integration of Generational mode for Shenandoah GC under https://bugs.openjdk.org/browse/JDK-8307314.
>> 
>> Generational mode of Shenandoah is enabled by adding `-XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=generational` to a command line that already specifies ` -XX:+UseShenandoahGC`.  The implementation automatically adjusts the sizes of old generation and young generation to efficiently utilize the entire heap capacity.  Generational mode of Shenandoah resembles G1 in the following regards:
>> 
>> 1. Old-generation marking runs concurrently during the time that multiple young generation collections run to completion.
>> 2. After old-generation marking completes, we perform a sequence of mixed collections.  Each mixed collection combines collection of young generation with evacuation of a portion of the old-generation regions identified for collection based on old-generation marking information.
>> 3. Unlike G1, young-generation collections and evacuations are entirely concurrent, as with single-generation Shenandoah.
>> 4. As with single-generation Shenandoah, there is no explicit notion of eden and survivor space within the young generation.  In practice, regions that were most recently allocated tend to have large amounts of garbage and these regions tend to be collected with very little effort.  Young-generation objects that survive garbage collection tend to accumulate in regions that hold survivor objects.  These regions tend to have smaller amounts of garbage, and are less likely to be collected.  If they survive a sufficient number of young-generation collections, the “survivor” regions are promoted into the old generation.
>> 
>> We expect to refine heuristics as we gain experience with more production workloads.  In the future, we plan to remove the “experimental” qualifier from generational mode, at which time we expect that generational mode will become the default mode for Shenandoah.
>> 
>> **Testing**: We continuously run jtreg tiers 1-4 + hotspot_gc_shenandoah, gcstress, jck compiler, jck runtime, Dacapo, SpecJBB, SpecVM, Extremem, HyperAlloc, and multiple AWS production workload simulators. We test on Linux x64 and aarch64, Alpine x64 and aarch64, macOS x64 and aarch64, and Windows x64.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Force PLAB sizes to align on card-table size

Thanks for issue reports, for which we are filing JBS tickets as we receive reports, and will be resolving them as we go. The issues reported to us have been filed as issues linked to the JBS ticket for this PR. Reviewers and testers should please feel free to file tickets as they find other issues, given testing notes in the PR, and platforms on which we have inadequate or no testing or coverage. We appreciate the help!

We will be incrementally resolving as many of the reported issues as we are able to, including in follow up PRs as bug fixes as appropriate.

There were some issues reported with copyright headers, and those will be fixed soon in this PR.

Please follow the tickets for further updates.

Many thanks to all the reviewers so far.

I have been a reviewer at the project level for several of the commits that comprise this PR, so I am happy to review and approve this pull request as a reviewer, as well as a project participant and partial implementor for some fixes.

Onwards to the brave new generation of Shenandoah! ;-)

🛳       🚀

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Marked as reviewed by ysr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14185#pullrequestreview-1458425985