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

[Thomas Stuefe]

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.

Hi @kdnilsen,

I see that following settings changed default values for all of Shenandoah:

- ShenandoahLearningSteps (was 10, now 5)
- ShenandoahImmediateThreshold (was 90, now 70)
- ShenandoahAdaptiveDecayFactor (was 0.5, now 0.1)
- ShenandoahFullGCThreshold (was 3, now 64)

Assuming that the behavior of legacy Shenandoah remains unchanged, I assume the switches are now handled differently to arrive at the same behavior.

I see that we now have ShenandoahOOMGCRetries. Does the changed default for ShenandoahFullGCThreshold and this new ShenandoahOOMGCRetries switch mean the degeneration behavior of legacy Shenandoah did change?

I think the general thrust of my questions is, you assured us that legacy Shenandoah will show the same behavior post-patch, but since the settings changed, I assume that the meaning of these settings did change. We will need to document these effects for users of legacy Shenandoah, in case they need to translate existing settings in their environment. A release note would be really helpful.

Cheers, Thomas

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

PR Comment: https://git.openjdk.org/jdk/pull/14185#issuecomment-1578693858