Github user jiangxb1987 commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21898#discussion_r207827294
  
    --- 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 lazy val timer = new Timer("BarrierCoordinator barrier epoch 
increment timer")
    --- End diff --
    
    I opened https://issues.apache.org/jira/browse/SPARK-25030 to track the 
issue.


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