He-Pin commented on code in PR #3035:
URL: https://github.com/apache/pekko/pull/3035#discussion_r3447514593
##########
stream/src/main/scala/org/apache/pekko/stream/stage/GraphStage.scala:
##########
@@ -275,6 +309,135 @@ object GraphStageLogic {
type Receive = ((ActorRef, Any)) => Unit
}
+ private object StageActor {
+ def localCell(ref: ActorRef, description: String): ActorCell =
+ ref match {
+ case ref: LocalActorRef => ref.underlying
+ case ref: RepointableActorRef =>
+ ref.underlying match {
+ case cell: ActorCell => cell
+ case unknown =>
+ throw new IllegalStateException(s"$description must be a local
actor, was [${unknown.getClass.getName}]")
+ }
+ case unknown =>
+ throw new IllegalStateException(s"$description must be a local
actor, was [${unknown.getClass.getName}]")
+ }
+
+ /**
+ * Reads `pekko.stream.materializer.stage-actor-drain-batch` from the
materializer's ActorSystem config.
+ * Called once per lazy StageActor construction (never on the hot path).
Bounded to `>= 1`.
+ */
+ def drainBatchSize(materializer: Materializer): Int =
+ Math.max(1,
materializer.system.settings.config.getInt("pekko.stream.materializer.stage-actor-drain-batch"))
+
+ private final val SchedStateIdle: Int = 0
+ private final val SchedStateScheduled: Int = 1
+
+ /**
+ * Lazy-path dispatch: producers enqueue into a Vyukov MPSC queue and
elect a single drain via
+ * IDLE -> SCHEDULED CAS; only the elected producer pays a mailbox
enqueue. The drain runs on the
+ * interpreter thread, polls in a tight loop bounded by `drainBatchSize`,
then either publishes IDLE
+ * (with a recheck for the publish-window race) or re-schedules another
envelope to yield to other
+ * BoundaryEvents.
+ *
+ * JIT/GC notes:
+ * - `final class` + monomorphic per-StageActor instance → JIT
devirtualizes the apply at the
+ * FunctionRef call site.
+ * - Extends `AbstractNodeQueue` directly so the queue head atomic and
the dispatch function share one
+ * object (one allocation per StageActor, one fewer field deref on the
producer hot path).
+ * - Implements `Any => Unit` directly — serves as both the producer
callback and the drain callback,
+ * eliminating the separate `drainCallback` lambda allocation.
+ * - `state` is a plain `@volatile var Int` driven by a static
`VarHandle` in the companion object
+ * (via `MethodHandles.privateLookupIn`), same pattern as
`AbstractNodeQueue` itself;
+ * avoids per-instance `AtomicInteger`.
+ * - `drainBatchSize` is read once into a stack-local at the top of
`drain` so the JIT can treat the loop
+ * bound as a constant.
+ * - Per-tell allocation = 1 Node (`AbstractNodeQueue.Node`, ~24 bytes) +
1 Tuple2 (~24 bytes). The
+ * Tuple2 is forced by the public `StageActorRef.Receive` type. No
AsyncInput / Envelope per tell —
+ * those are amortized across the batch.
+ */
+ // Not marked `private` so that `class StageActor`'s aux constructor
(compiled outside of the companion
+ // object on Scala 3) can reference it; the enclosing `object StageActor`
is itself private.
+ final class LazyDispatch(
+ interpreter: GraphInterpreter,
+ logic: GraphStageLogic,
+ handler: Any => Unit,
+ drainBatchSize: Int)
+ extends AbstractNodeQueue[(ActorRef, Any)]
+ with (Any => Unit) {
+
+ // IDLE/SCHEDULED election state. VarHandle avoids per-instance
AtomicInteger;
+ // the handle lives in the companion object (true JVM static), same
pattern as
+ // AbstractNodeQueue._tailDoNotCallMeDirectly.
+ @volatile var state: Int = SchedStateIdle
Review Comment:
Rechecked this and updated #3116. Instead of switching to `AtomicInteger`,
the PR now keeps the VarHandle-backed `@volatile var state`, uses `getVolatile`
/ `setVolatile` / `compareAndSet`, and retains a direct private reference to
the field (`_preventPrivateUnusedErasure`) like the existing Pekko VarHandle
fields. This avoids per-instance `AtomicInteger` allocation while addressing
the private-field retention concern.
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