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Posted to notifications@pekko.apache.org by "He-Pin (via GitHub)" <gi...@apache.org> on 2026/04/18 18:52:37 UTC

[PR] feat: enable virtualize=on in stream test dispatcher to bypass JDK 21+ FJP scheduling regression [pekko]

He-Pin opened a new pull request, #2872:
URL: https://github.com/apache/pekko/pull/2872

   ## Summary
   
   Enables `virtualize = on` in the Pekko stream test dispatcher so that on JDK 21+ each dispatcher task runs in its own virtual thread (Project Loom). This **bypasses** the ForkJoinPool compensation-thread scheduling regression (JDK-8300995) at the root rather than working around it.
   
   > ⚠️ **Depends on #2869** (test stability fixes). Retarget this PR to `main` after #2869 merges.
   
   ---
   
   ## Root Cause Recap
   
   ForkJoinPool with `asyncMode=FIFO` on JDK 21+ places response tasks at the back of the deque, causing actor reply futures to queue behind unrelated tasks. Compensation threads that should keep worker count stable became more conservative in JDK-8321335 (JDK 21) and have not improved in JDK 25. This manifests as cascading latency spikes in any test with tight actor round-trips (MergeHub, HubSpec, etc.).
   
   ## How `virtualize = on` Fixes It
   
   With `virtualize = on`, each dispatcher `Runnable` is scheduled as a virtual thread (JDK 21+ Project Loom). Virtual threads **unmount** their carrier when they block (e.g., awaiting an actor reply), freeing the carrier for other work. The FJP pool's FIFO-ordering starvation cannot happen because:
   
   - There are no blocked workers holding carriers
   - Compensation threads are never needed for actor-reply waits
   - The FJP pool is used only as a **carrier scheduler**, not to queue Runnables
   
   ## Changes
   
   `stream-testkit/src/test/resources/reference.conf`
   
   ```hocon
   pekko.test.stream-dispatcher {
     fork-join-executor {
       # Enable virtual threads on JDK 21+. Silently ignored on JDK < 21.
       virtualize = on
     }
   }
   ```
   
   ## Safety Notes
   
   - On JDK < 21: `VirtualThreadSupport.isSupported = false` → flag silently ignored, pool behaves identically to today
   - `--add-opens` flags required by Loom are already supplied by `JdkOptions.scala` for JDK 9+
   - Mailbox ordering is unaffected (UnboundedMailbox is always FIFO)
   - Verified: 48/48 HubSpec tests pass in ~20 seconds on JDK 25 (was 2+ minutes timeout previously)
   
   ## Relation to Other PRs
   
   | PR | What it does |
   |----|-------------|
   | #2869 | Test stability fixes (patience scaling, element counts, TCK timefactor) |
   | #2871 | FJP config improvements (LIFO, configurable minimum-runnable, max-pool-size docs) |
   | **this PR** | Root-cause fix: virtual threads bypass FJP scheduling entirely |
   | #2870 | Tracking issue documenting the JDK-8300995 root cause |
   


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