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8350050: Shenandoah: remove support for ShenandoahPacing feature #24002

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@ysramakrishna ysramakrishna commented Mar 12, 2025

/issue JDK-8350050

Shenandoah's experimental "pacing" feature, currently defaulting to "on", and its set of suboption knobs, have been supported for several releases. The idea is that under increasing allocation rate graceful degradation in application latency is often more desirable to abruptly stalling mutators to collect the heap when the allocators outrun the collector, causing a concurrent mode failure. Unfortunately, our experience with this pacing feature is that it's difficult to easily control the suboption knobs and, for latency sensitive applications, customers often prefer abrupt failure to gradual degradation in latency at the cost of a drop in peak performance, and will almost always size their systems so as to avoid driving the collector into that state. We and, independently, many of our customers noticed that this feature extracted a performance penalty when enabled, and as a result the feature is disabled by default for most production deployments of Shenandoah. For Generational Shenandoah, the feature has always been off by default. Inside Amazon, the feature has defaulted to off for several releases.

Removing support for this feature simplifies some of the allocation and GC control paths allowing further improvements to the allocation paths in the future. Graceful degradation of latency with increasing allocation throughput will be implemented separately using other mechanisms in the near future.

We measured the impact of this feature on specjbb scores on x86, and found that there was a marked performance improvement comparing +ShenandoahPacing vs -ShenandoahPacing (+12% Shen, +25% GenShen). There was a further small performance improvement in the case of GenShen with the feature completely removed. Actual performance delta may vary depending on host size and isa.

SpecJBB

[Critical JOPS, relative scores (previous default, new), average of 5 runs, x64 only]

Collector Shenandoah Generaltional Shenandoah
+ShenandoahPacing 22.1 21.5
-ShenandoahPacing 24.8 26.9
Feature deleted entirely 24.4 27.6

Testing:

  • SPECjbb
  • Code Pipeline Stress
  • Perf Dashboard


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Issue

  • JDK-8350050: Shenandoah: remove support for ShenandoahPacing feature (Task - P4)

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$ git pull https://git.openjdk.org/jdk.git pull/24002/head

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bridgekeeper bot commented Mar 12, 2025

👋 Welcome back ysr! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

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openjdk bot commented Mar 12, 2025

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

@openjdk openjdk bot changed the title DRAFT: Shenandoah: evaluate & potentially deprecate ShenandoahPacing 8350050: Shenandoah: evaluate & potentially deprecate ShenandoahPacing Mar 12, 2025
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openjdk bot commented Mar 12, 2025

@ysramakrishna The primary solved issue for a PR is set through the PR title. Since the current title does not contain an issue reference, it will now be updated.

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openjdk bot commented Mar 12, 2025

@ysramakrishna The following labels will be automatically applied to this pull request:

  • hotspot-gc
  • shenandoah

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

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bridgekeeper bot commented May 7, 2025

@ysramakrishna This pull request has been inactive for more than 8 weeks and will be automatically closed if another 8 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

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Getting some performance numbers and will open up for review.

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@ysramakrishna This pull request has been inactive for more than 8 weeks and will be automatically closed if another 8 weeks passes without any activity. To avoid this, simply issue a /touch or /keepalive command to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

@ysramakrishna ysramakrishna marked this pull request as ready for review July 11, 2025 02:03
@openjdk openjdk bot added the rfr Pull request is ready for review label Jul 11, 2025
@ysramakrishna ysramakrishna changed the title 8350050: Shenandoah: evaluate & potentially deprecate ShenandoahPacing 8350050: Shenandoah: remove support for ShenandoahPacing feature Jul 11, 2025
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Webrevs

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