Github user kiszk commented on a diff in the pull request:
https://github.com/apache/spark/pull/21898#discussion_r207743412
--- Diff: core/src/main/scala/org/apache/spark/BarrierCoordinator.scala ---
@@ -0,0 +1,230 @@
+/*
+ * 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}
+
+/**
+ * For each barrier stage attempt, only at most one barrier() call can be
active at any time, thus
+ * 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
+ * all the requests for a group of `barrier()` calls are received. If the
coordinator is unable to
+ * collect enough global sync requests within a configured time, fail all
the requests and return
+ * an Exception with timeout message.
+ */
+private[spark] class BarrierCoordinator(
+ timeoutInSecs: Long,
+ 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.
+ cleanupBarrierStage(barrierId)
+ }
+ }
+
+ // Record 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)
+ }
+
+ override def onStop(): Unit = {
+ try {
+ states.forEachValue(1, clearStateConsumer)
+ states.clear()
+ listenerBus.removeListener(listener)
+ } finally {
+ super.onStop()
+ }
+ }
+
+ /**
+ * Provide the current state of a barrier() call. A state is created
when a new stage attempt
+ * sends 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 = synchronized {
+ // Timeout current barrier() call, fail all the sync requests.
+ requesters.foreach(_.sendFailure(new SparkException("The
coordinator didn't get all " +
+ s"barrier sync requests for barrier epoch $barrierEpoch from
$barrierId within " +
+ s"$timeoutInSecs second(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 {
+ val taskId = request.taskAttemptId
+ val epoch = request.barrierEpoch
+
+ // Require the number of tasks is correctly set from the
BarrierTaskContext.
+ require(request.numTasks == numTasks, s"Number of tasks of
$barrierId is " +
+ s"${request.numTasks} from Task $taskId, previously it was
$numTasks.")
+
+ // Check whether the epoch from the barrier tasks matches current
barrierEpoch.
+ logInfo(s"Current barrier epoch for $barrierId is $barrierEpoch.")
+ if (epoch != barrierEpoch) {
+ requester.sendFailure(new SparkException(s"The request to sync of
$barrierId with " +
+ s"barrier epoch $barrierEpoch has already finished. Maybe task
$taskId is not " +
+ "properly killed."))
+ } else {
+ // If this is the first sync message received for a barrier()
call, start timer to ensure
+ // we may timeout for the sync.
+ if (requesters.isEmpty) {
+ initTimerTask()
+ timer.schedule(timerTask, timeoutInSecs * 1000)
+ }
+ // Add the requester to array of RPCCallContexts pending for reply.
+ requesters += requester
+ logInfo(s"Barrier sync epoch $barrierEpoch from $barrierId
received update from Task " +
+ s"$taskId, current progress: ${requesters.size}/$numTasks.")
+ if (maybeFinishAllRequesters(requesters, numTasks)) {
+ // Finished current barrier() call successfully, clean up
ContextBarrierState and
+ // increase the barrier epoch.
+ logInfo(s"Barrier sync epoch $barrierEpoch from $barrierId
received all updates from " +
+ s"tasks, finished successfully.")
+ barrierEpoch += 1
+ requesters.clear()
+ cancelTimerTask()
+ }
+ }
+ }
+
+ // Finish all the blocking barrier sync requests from a stage attempt
successfully if we
+ // have received all the sync requests.
+ private def maybeFinishAllRequesters(
+ requesters: ArrayBuffer[RpcCallContext],
+ numTasks: Int): Boolean = {
+ if (requesters.size == numTasks) {
+ requesters.foreach(_.reply(()))
+ true
+ } else {
+ false
+ }
+ }
+
+ // Cleanup the internal state of a barrier stage attempt.
+ def clear(): Unit = synchronized {
+ // The global sync fails so the stage is expected to retry another
attempt, all sync
+ // messages come from current stage attempt shall fail.
+ barrierEpoch = -1
+ requesters.clear()
+ cancelTimerTask()
+ }
+ }
+
+ // Clean up the [[ContextBarrierState]] that correspond to a specific
stage attempt.
+ private def cleanupBarrierStage(barrierId: ContextBarrierId): Unit = {
+ val barrierState = states.remove(barrierId)
+ if (barrierState != null) {
+ barrierState.clear()
+ }
+ }
+
+ override def receiveAndReply(context: RpcCallContext):
PartialFunction[Any, Unit] = {
+ case request @ RequestToSync(numTasks, stageId, stageAttemptId, _, _)
=>
+ // Get or init the ContextBarrierState correspond to the stage
attempt.
+ val barrierId = ContextBarrierId(stageId, stageAttemptId)
+ states.putIfAbsent(barrierId, new ContextBarrierState(barrierId,
numTasks))
--- End diff --
Is it better to use `computeIfAbsent` to avoid allocating
`ContextBarrierState` when an entry with `barrierId` exists?
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