RFR: JDK-8307314: Implementation: Generational Shenandoah (Experimental) [v5]

[Paul Hohensee]

On Sun, 4 Jun 2023 21:39:58 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:
> 
>   Remove three asserts making comparisons between atomic volatile variables
>   
>   Though changes to the volatile variables are individually protected by
>   Atomic load and store operations, these asserts were not assuring
>   atomic access to multiple volatile variables, each of which could be
>   modified independently of the others.  The asserts were therefore not
>   trustworthy, as has been confirmed by more extensive testing.

Thanks for finding the single-gen regression, we're very happy you took the time to run it and write up your results. We're very concerned about single-gen regressions too because we have single-gen Shen in production for several critical services.

We'd like to propose to push now, and tackle/fix the single-gen issue you identified during RDP1, as well as any other significant single-gen regressions that may come up. We have four Shen experts on board, Roman, Aleksey, Kelvin, and William, so believe it's doable before RDP2 in July. In the worst case that we fail, we'd emulate ZGC and move GenShen to it's own directory as an entirely separate collector before RDP2. Make sense?

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PR Comment: https://git.openjdk.org/jdk/pull/14185#issuecomment-1579351993