feat: make ForkJoinPool minimum-runnable configurable and improve pool documentation#2871
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feat: make ForkJoinPool minimum-runnable configurable and improve pool documentation#2871
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This was referenced Apr 18, 2026
…21+ starvation Motivation: PekkoForkJoinPool hard-coded minimumRunnable=1 in the JDK 9+ ForkJoinPool constructor. This is the threshold below which the pool creates compensation threads to maintain progress. On JDK 21+ (see JDK-8300995 / JDK-8321335), the compensation-thread mechanism for asyncMode=FIFO pools became more conservative, making it harder to recover from actor-heavy workloads where the single "runnable" threshold is insufficient under load. Modification: - Added minimum-runnable = 1 to fork-join-executor in reference.conf (default preserves existing behaviour; documented relationship to JDK-8300995). - PekkoForkJoinPool now accepts minimumRunnable as a constructor parameter (default 1 for binary compatibility). - ForkJoinExecutorServiceFactory reads minimum-runnable from config and forwards it to PekkoForkJoinPool. Result: Users experiencing dispatcher starvation on JDK 21+ can now increase minimum-runnable (e.g. to parallelism/2) to prompt earlier compensation- thread creation without needing to patch Pekko itself. All existing actor-tests/dispatch tests pass unchanged (71 passed, 1 pending). Fixes: #2870 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
…on thread behavior Motivation: The previous comment was minimal and did not explain how maximum-pool-size interacts with ForkJoinPool compensation threads on JDK 21+. Modification: Expand the maximum-pool-size comment to describe that on JDK 21+ the pool uses this as a cap for compensation threads. Tuning it closer to the target parallelism can reduce latency under actor-heavy workloads (see JDK-8300995 tracked in #2870). Result: Users who see JDK 21+ ForkJoinPool scheduling jitter now have actionable guidance directly in the default configuration. References: #2870 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Motivation: JDK-8300995 causes ForkJoinPool compensation-thread starvation, leading to sporadic test timeouts when actors block on reply futures. Virtual threads (JDK 21+) bypass this limitation by unmounting when blocking, providing equivalent or better performance. See issue #2870. Modification: 1. stream-testkit/reference.conf: Add PEKKO_VIRTUALIZE_DISPATCHER env var support with safe fallback to 'off' for stable local testing 2. .github/workflows/nightly-builds.yml: Set env var conditionally for JDK 21+, with detailed comments explaining TIMEFACTOR logic 3. CONTRIBUTING.md: Document virtual thread testing with cleanup guidance and safe one-liner for developers 4. Virtual thread enablement only affects nightly CI on JDK 21+; local development defaults to OFF for stability Result: - Virtual threads automatically enabled on JDK 21+ in nightly builds - Local tests safely default to virtual threads OFF for stability - Nightly test reliability improved; timeouts addressed - Binary compatibility maintained; MiMa checks pass References: - Issue #2870: Compensation-thread starvation investigation - Issue #2573: JDK 25 ForkJoinPool scheduling regression - PR #2871: ForkJoinPool minimum-runnable tuning (JDK < 21 fallback) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Summary
Two targeted improvements to the ForkJoinPool executor configuration — no behavior change by default, purely additive.
Changes
1. Configurable
minimum-runnableactor/src/main/resources/reference.confactor/src/main/scala/org/apache/pekko/dispatch/ForkJoinExecutorConfigurator.scalaPekkoForkJoinPoolnow acceptsminimumRunnable: Int = 1ForkJoinExecutorServiceFactoryreadsminimum-runnablefrom config1is identical to the previously hardcoded value — no behavior changeparallelism / 2) to prompt faster compensation-thread creationBinary compatibility: new params carry default values; class is
@InternalApi, so MiMa exclusion is not required.2. Improved
maximum-pool-sizedocumentationThe existing comment was minimal. The new comment explains that on JDK 21+:
maximum-pool-sizeminimum-runnablehelps under heavy actor-round-trip workloadsWhat this PR does NOT change
minimum-runnable = 1preserves existing behaviourRelated
virtualize = oninvestigation