He-Pin opened a new pull request, #3092:
URL: https://github.com/apache/pekko/pull/3092
### Motivation
The "must be able to send messages with actorSelection concurrently
preserving order" test flakes on CI: 4 sender actors each drive 1000
round-trips via `ActorSelection` (4004 messages total). With multi-lane artery
config (`outbound-lanes > 1`), **all ActorSelection messages are routed to the
same outbound queue** because `selectQueue` uses the anchor's UID as the
distribution key:
```scala
OrdinaryQueueIndex + (math.abs(r.path.uid % outboundLanes))
```
The anchor for `ActorSelection` is the root guardian (`RootActorPath`),
whose UID is always `0` (`ActorCell.undefinedUid`). So `math.abs(0 % N) = 0`
for any N — all ActorSelection traffic concentrates on lane 0 while other lanes
sit idle.
In contrast, the ActorRef variant distributes across lanes because each echo
actor has a distinct non-zero UID.
This is **not a recent regression** — the `selectQueue` logic has been
unchanged since the Pekko fork from Akka. PR #3090 (timeout widening) masks the
symptom but doesn't fix the structural bottleneck.
### Modification
Add a dedicated `case sel: ActorSelectionMessage` (non-PriorityMessage) in
`Association.send` that computes the queue index from the selection's **target
path elements hash** instead of the anchor's UID:
```scala
case sel: ActorSelectionMessage =>
val queueIndex =
if (outboundLanes == 1) OrdinaryQueueIndex
else OrdinaryQueueIndex + (math.abs(sel.elements.hashCode()) %
outboundLanes)
val queue = queues(queueIndex)
if (!queue.offer(outboundEnvelope))
dropped(queueIndex, queueSize, outboundEnvelope)
```
This distributes ActorSelection messages across lanes by their target path
while preserving **per-path message ordering** (same target path → same hash →
same lane).
PriorityMessage ActorSelection (used by cluster heartbeats) continues to go
through the control queue unchanged.
### Result
ActorSelection messages are now distributed across all outbound lanes based
on target path, eliminating the single-lane throughput bottleneck. The existing
ActorRef-based test and PriorityMessage routing are unaffected.
### Tests
- `sbt "remote / Test / compile"` — compile check
- CI will exercise the artery variants (1-lane and 3-lane configs)
### References
Refs #3041 (previous timeout widening), supersedes #3090
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