RFR: 8348594: Shenandoah: Do not penalize for degeneration when not the fault of triggering heuristic [v2]

[William Kemper]

On Mon, 27 Jan 2025 02:05:02 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Shenandoah heuristics use a penalty mechanism to cause earlier GC triggers when recent concurrent GC cycles degenerate.  Degeneration is a stop-the-world remediation that allows GC to catch up when mutator allocations fail during concurrent GC.  The fact that we needed to degenerate indicates that we were overly optimistic in delaying the trigger that starts concurrent GC.
>> 
>> We have observed that it is common for degenerated GC cycles to cascade upon each other.  The condition that caused an initial degenerated cycle is often not fully resolved by the end of that degenerated cycle.  For example, the application may be experiencing a phase change and the GC heuristics are not yet attuned to the new behavior.  Furthermore, a degenerated GC may exacerbate the problem condition.  During the stop-the-world pause imposed by the first degenerated GC, work continues to accumulate in the form of new client requests that are buffered in network sockets until the end of that degenerated GC.
>> 
>> As originally implemented, each degeneration would "pile on" additional penalties.  These penalties cause the GC frequency to continue to increase.  And the expanding CPU load of GC makes it increasingly difficult for mutator threads to catchup.  The large penalties accumulated while we are trying to resolve the problem linger long after the problem condition has been resolved.
>> 
>> This change does not add further to the degeneration penalties if a new degenerated cycle occurs through no fault of the triggering mechanism.  We only add the degeneration penalty if the reason we are now degenerating can be attributed to a consciously late trigger by the heuristic.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Respond to reviewer feedback

Thanks for the changes. If I may paraphrase the change as I understand it: the heuristic penalties will only be applied if the heuristic has declined more than `Penalty_Free_Declinations` (because we believe that means the heuristic is being too lazy). Back to back degenerated cycles will no longer pile on penalties. Sounds good to me.

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Marked as reviewed by wkemper (Committer).

PR Review: https://git.openjdk.org/jdk/pull/23305#pullrequestreview-2576140980