RFR: 8312116: GenShen: make instantaneous allocation rate triggers more timely [v4]

[William Kemper]

On Wed, 7 Jan 2026 20:56:51 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> After studying large numbers of GC logs with degenerated cycles that have resulted from "late" triggers, we propose the following general improvements:
>> 
>> 1. Track trends in GC times rather than always using the average GC time plus standard deviation.  In many situations, GC times trend upward due to, for example, increasing amounts of live data that must be marked as a workload builds up its working set of memory.
>> 2. Sample allocation rates more frequently than once every 100 ms.
>> 3. Track trends in allocation rates.  In some situations, the allocation rate trends upwards due to, for example, the start of a new phase of execution or a spike in client workload.
>> 4. When we detect acceleration of allocation rate, predict consumption of memory based on accelerated allocation rates rather than assuming constant allocation rate.
>
> Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 65 commits:
> 
>  - Fix comment
>  - Use PROPERFMT macros
>  - Simplify code flow: reviewer suggestion
>  - Merge remote-tracking branch 'jdk/master' into accelerated-triggers
>  - Remove develop/debug instrumentation
>  - add another override
>  - Change type of command-line args
>  - fix white space
>  - Add override to virtual methods
>  - Fix race between allocation reporting and querying
>  - ... and 55 more: https://git.openjdk.org/jdk/compare/dd20e915...7f3a6d1e

Changes requested by wkemper (Reviewer).

src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp line 643:

> 641:                     future_accelerated_planned_gc_time * 1000);
> 642:       } else {
> 643:         log_trigger("Momentary spike consumption (%zu%s) exceeds free headroom (%zu%s) at "

Should the 'Momentary spike' trigger replace the 'instantaneous spike' trigger? It seems like we now have two spike detecting triggers?

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PR Review: https://git.openjdk.org/jdk/pull/29039#pullrequestreview-3645745671
PR Review Comment: https://git.openjdk.org/jdk/pull/29039#discussion_r2677779429