[PR] fix: configurable ForkJoinPool minimum-runnable and LIFO stream dispatcher for JDK 21+ regression [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin opened a new pull request, #2871:
URL: https://github.com/apache/pekko/pull/2871

   ## Summary
   
   Addresses the JDK-8300995 ForkJoinPool regression on JDK 21+ described in #2870.
   
   This PR contains **two targeted configuration improvements** that mitigate FJP scheduling delays without changing any test assertions or element counts.
   
   ## Root Cause (JDK-8300995)
   
   ForkJoinPool with `asyncMode=true` (FIFO) on JDK 21+ fails to create compensation threads promptly when workers block on actor hand-offs. The FIFO ordering places response tasks at the *back* of the deque, causing cascading delays under load. This is especially visible in stream tests that involve MergeHub (`buffer=1`) or tight actor round-trips.
   
   ## Changes
   
   ### 1. Stream test dispatcher: LIFO task-peeking-mode
   
   `stream-testkit/src/test/resources/reference.conf`
   
   ```hocon
   pekko.test.stream-dispatcher {
     fork-join-executor {
       task-peeking-mode = "LIFO"  # response tasks go to front → resolved immediately
     }
   }
   ```
   
   **Why safe**: mailbox ordering is always FIFO; LIFO only affects how a *worker's own deque* is polled. No correctness impact. Standard actor dispatchers are unchanged.
   
   ### 2. Configurable `minimum-runnable` in ForkJoinPool
   
   `actor/src/main/resources/reference.conf`
   
   ```hocon
   fork-join-executor {
     minimum-runnable = 1   # JDK 9+ ForkJoinPool constructor param
   }
   ```
   
   `actor/src/main/scala/org/apache/pekko/dispatch/ForkJoinExecutorConfigurator.scala`
   
   - `PekkoForkJoinPool` receives a new `minimumRunnable: Int = 1` parameter
   - `ForkJoinExecutorServiceFactory` reads `minimum-runnable` from config
   - Default `1` preserves existing behaviour; users on JDK 21+ under heavy load can increase it
   
   **Binary compatibility**: Both new parameters carry default values and the class is `@InternalApi`, so there is no MiMa concern.
   
   ## Relation to other PRs
   
   - **#2869** — test stability fixes (timeout scaling, element count reduction) — *separate PR*
   - **#2870** — tracking issue for JDK-8300995; further work (`virtualize = on`) tracked there
   
   Closes #2870 (partially — `virtualize = on` remains future work)
   


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Re: [PR] feat: make ForkJoinPool minimum-runnable configurable and improve pool documentation [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin merged PR #2871:
URL: https://github.com/apache/pekko/pull/2871


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