He-Pin commented on code in PR #3035:
URL: https://github.com/apache/pekko/pull/3035#discussion_r3345692957
##########
stream/src/main/scala/org/apache/pekko/stream/stage/GraphStage.scala:
##########
@@ -275,6 +308,123 @@ 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).
+ * - All hot-path state is `private[this]` → direct field access, no
accessor methods.
+ * - `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 (((ActorRef, Any)) => Unit) {
+
+ // IDLE/SCHEDULED election state. AtomicInteger gives us volatile read +
CAS without the cross-Scala
+ // VarHandle / field-updater access fuss; the wrapper costs one extra
reference per StageActor, which
+ // is negligible against the per-tell mailbox traffic we are saving.
+ private[this] val state = new AtomicInteger(SchedStateIdle)
Review Comment:
Updated the PR description to match the implementation. The `@volatile var
Int` + `VarHandle` design lived in `AbstractMpscDispatch.java` in an earlier
revision, but the helper hit cross-Scala polymorphic-signature inference quirks
for `VarHandle.getAcquire` between Scala 2 and Scala 3, so commit 1b533f8
dropped it in favour of a single `AtomicInteger` field. The extra reference per
`StageActor` is negligible against the per-tell mailbox traffic this PR already
saves. Inline comment at GraphStage.scala:364 also explains the rationale.
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