Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/21898#discussion_r207704089
--- Diff: core/src/main/scala/org/apache/spark/BarrierCoordinator.scala ---
@@ -0,0 +1,233 @@
+/*
+ * 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.spark
+
+import java.util.{Timer, TimerTask}
+import java.util.concurrent.ConcurrentHashMap
+import java.util.function.Consumer
+
+import scala.collection.mutable.ArrayBuffer
+
+import org.apache.spark.internal.Logging
+import org.apache.spark.rpc.{RpcCallContext, RpcEnv, ThreadSafeRpcEndpoint}
+import org.apache.spark.scheduler.{LiveListenerBus, SparkListener,
SparkListenerStageCompleted}
+
+/**
+ * Only one barrier() call shall happen on a barrier stage attempt at each
time, we can use
+ * (stageId, stageAttemptId) to identify the stage attempt where the
barrier() call is from.
+ */
+private case class ContextBarrierId(stageId: Int, stageAttemptId: Int) {
+ override def toString: String = s"Stage $stageId (Attempt
$stageAttemptId)"
+}
+
+/**
+ * A coordinator that handles all global sync requests from
BarrierTaskContext. Each global sync
+ * request is generated by `BarrierTaskContext.barrier()`, and identified
by
+ * stageId + stageAttemptId + barrierEpoch. Reply all the blocking global
sync requests upon
+ * received all the requests for a group of `barrier()` calls. If the
coordinator doesn't collect
+ * enough global sync requests within a configured time, fail all the
requests due to timeout.
+ */
+private[spark] class BarrierCoordinator(
+ timeout: Int,
+ listenerBus: LiveListenerBus,
+ override val rpcEnv: RpcEnv) extends ThreadSafeRpcEndpoint with
Logging {
+
+ private val timer = new Timer("BarrierCoordinator barrier epoch
increment timer")
+
+ // Listen to StageCompleted event, clear corresponding
ContextBarrierState.
+ private val listener = new SparkListener {
+ override def onStageCompleted(stageCompleted:
SparkListenerStageCompleted): Unit = {
+ val stageInfo = stageCompleted.stageInfo
+ val barrierId = ContextBarrierId(stageInfo.stageId,
stageInfo.attemptNumber)
+ // Clear ContextBarrierState from a finished stage attempt.
+ val barrierState = states.remove(barrierId)
+ if (barrierState != null) {
+ barrierState.clear()
+ }
+ }
+ }
+
+ // Remember all active stage attempts that make barrier() call(s), and
the corresponding
+ // internal state.
+ private val states = new ConcurrentHashMap[ContextBarrierId,
ContextBarrierState]
+
+ override def onStart(): Unit = {
+ super.onStart()
+ listenerBus.addToStatusQueue(listener)
+ }
+
+ /**
+ * Provide current state of a barrier() call, the state is created when
a new stage attempt send
+ * out a barrier() call, and recycled on stage completed.
+ *
+ * @param barrierId Identifier of the barrier stage that make a
barrier() call.
+ * @param numTasks Number of tasks of the barrier stage, all barrier()
calls from the stage shall
+ * collect `numTasks` requests to succeed.
+ */
+ private class ContextBarrierState(
+ val barrierId: ContextBarrierId,
+ val numTasks: Int) {
+
+ // There may be multiple barrier() calls from a barrier stage attempt,
`barrierEpoch` is used
+ // to identify each barrier() call. It shall get increased when a
barrier() call succeed, or
+ // reset when a barrier() call fail due to timeout.
+ private var barrierEpoch: Int = 0
+
+ // An array of RPCCallContexts for barrier tasks that are waiting for
reply of a barrier()
+ // call.
+ private val requesters: ArrayBuffer[RpcCallContext] = new
ArrayBuffer[RpcCallContext](numTasks)
+
+ // A timer task that ensures we may timeout for a barrier() call.
+ private var timerTask: TimerTask = null
+
+ // Init a TimerTask for a barrier() call.
+ private def initTimerTask(): Unit = {
+ timerTask = new TimerTask {
+ override def run(): Unit = {
+ // Timeout current barrier() call, fail all the sync requests.
+ failAllRequesters(requesters, "The coordinator didn't get all
barrier sync " +
+ s"requests for barrier epoch $barrierEpoch from $barrierId
within ${timeout}s.")
+ cleanupBarrierStage(barrierId)
+ }
+ }
+ }
+
+ // Cancel the current active TimerTask and release resources.
+ private def cancelTimerTask(): Unit = {
+ if (timerTask != null) {
+ timerTask.cancel()
+ timerTask = null
+ }
+ }
+
+ // Process the global sync request. The barrier() call succeed if
collected enough requests
+ // within a configured time, otherwise fail all the pending requests.
+ def handleRequest(requester: RpcCallContext, request: RequestToSync):
Unit = synchronized {
--- End diff --
why `synchronized`? I think we only access `ContextBarrierState` in the RPC
thread, which is single thread.
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