RFR: 8348594: Shenandoah: Do not penalize for degeneration when not the fault of triggering heuristic [v5]

[Paul Hohensee]

On Tue, 11 Feb 2025 18:21:09 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains nine additional commits since the last revision:
> 
>  - Merge tag 'jdk-25+9' into eliminate-no-fault-degen-penalties
>    
>    Added tag jdk-25+9 for changeset 30f71622
>  - Revert "Use generation size to determine expected free"
>    
>    This reverts commit 94a32ebfe5fefcc0e899e09e6fbfc0585c62b4e0.
>  - Respond to reviewer feedback
>  - Use generation size to determine expected free
>  - Respond to reviewer feedback
>  - Fix white space
>  - Remove debug instrumentation
>  - Only penalize heuristic if heuristic responsible
>    
>    If we degenerate through no fault of "late triggering", then do not
>    penalize the heuristic.
>  - Eliminate no-fault degen penalties
>    
>    As originally implemented, we apply penalties to the triggering
>    heuristic every time we experience a degenerated cycle.  This has the
>    effect of forcing GC triggers to spiral out of control.  This commit
>    changes the penalty mechanism.  When a degen happens through no fault of
>    the heuristic triggering mechanism, we do not pile on additional
>    penalties.  Specifically, we consider that heuristic triggering is not
>    responsible for a degenerated cycle that is associated with a GC that
>    began immediately following the end of the previous GC cycle.

Marked as reviewed by phh (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/23305#pullrequestreview-2609684439