8350050: Shenandoah: remove support for ShenandoahPacing feature

[ysramakrishna]

/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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[bridgekeeper]

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[openjdk]

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[openjdk]

@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.

[openjdk]

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

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[ysramakrishna]

Getting some performance numbers and will open up for review.

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