chore: Use array list for better performance in BroadcastHub (#2262)

* chore: Use array list for better performance in BroadcastHub

* chore: add benchmark

bench-jmh/src/main/scala/org/apache/pekko/stream/BroadcastHubBenchmark.scala

@@ -0,0 +1,94 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.pekko.stream
+
+import com.typesafe.config.ConfigFactory
+import org.apache.pekko.NotUsed
+import org.apache.pekko.actor.ActorSystem
+import org.apache.pekko.remote.artery.{ BenchTestSource, LatchSink }
+import org.apache.pekko.stream.scaladsl._
+import org.apache.pekko.stream.testkit.scaladsl.StreamTestKit
+import org.openjdk.jmh.annotations._
+
+import java.util.concurrent.{ CountDownLatch, TimeUnit }
+import scala.concurrent.Await
+import scala.concurrent.duration._
+
+object BroadcastHubBenchmark {
+  final val OperationsPerInvocation = 100000
+}
+
+@State(Scope.Benchmark)
+@OutputTimeUnit(TimeUnit.SECONDS)
+@BenchmarkMode(Array(Mode.Throughput))
+class BroadcastHubBenchmark {
+  import BroadcastHubBenchmark._
+
+  val config = ConfigFactory.parseString("""
+    pekko.actor.default-dispatcher {
+      executor = "fork-join-executor"
+      fork-join-executor {
+        parallelism-factor = 1
+      }
+    }
+    """)
+
+  implicit val system: ActorSystem = ActorSystem("BroadcastHubBenchmark", config)
+  import system.dispatcher
+
+  var testSource: Source[java.lang.Integer, NotUsed] = _
+
+  @Param(Array("64", "256"))
+  var parallelism = 0
+
+  @Setup
+  def setup(): Unit = {
+    // eager init of materializer
+    SystemMaterializer(system).materializer
+    testSource = Source.fromGraph(new BenchTestSource(OperationsPerInvocation))
+  }
+
+  @TearDown
+  def shutdown(): Unit = {
+    Await.result(system.terminate(), 5.seconds)
+  }
+
+  @Benchmark
+  @OperationsPerInvocation(OperationsPerInvocation)
+  def broadcast(): Unit = {
+    val latch = new CountDownLatch(parallelism)
+    val broadcastSink =
+      BroadcastHub.sink[java.lang.Integer](bufferSize = parallelism, startAfterNrOfConsumers = parallelism)
+    val sink = new LatchSink(OperationsPerInvocation, latch)
+    val source = testSource.runWith(broadcastSink)
+    var idx = 0
+    while (idx < parallelism) {
+      source.runWith(sink)
+      idx += 1
+    }
+    awaitLatch(latch)
+  }
+
+  private def awaitLatch(latch: CountDownLatch): Unit = {
+    if (!latch.await(30, TimeUnit.SECONDS)) {
+      StreamTestKit.printDebugDump(SystemMaterializer(system).materializer.supervisor)
+      throw new RuntimeException("Latch didn't complete in time")
+    }
+  }
+
+}