上一篇文章我们着重分析了Task的提交过程，本文中我们将对Task的运行进行详细的分析。
我们从CoarseGrainedExecutorBackend接收到CoarseGrainedSchedulerBackend发过来的LaunchTask消息开始：
case LaunchTask(data) => if (executor == null) {
logError("Received LaunchTask command but executor was null") System.exit(1)
} else { // 反序列化
val taskDesc = ser.deserialize[TaskDescription](data.value)
logInfo("Got assigned task " + taskDesc.taskId) // 调用Executor的launchTask来运行Task
executor.launchTask(this, taskId = taskDesc.taskId, attemptNumber = taskDesc.attemptNumber,
taskDesc.name, taskDesc.serializedTask)
}接着进入Executor的launchTask方法：
def launchTask(
context: ExecutorBackend,
taskId: Long,
attemptNumber: Int,
taskName: String,
serializedTask: ByteBuffer): Unit = { // 实例化TaskRunner
val tr = new TaskRunner(context, taskId = taskId, attemptNumber = attemptNumber, taskName,
serializedTask) // 放入ConcurrentHashMap[Long, TaskRunner]的数据结构中
runningTasks.put(taskId, tr) // 在线程池中运行刚才实例化的TaskRunner，也就是执行其中的run()方法
threadPool.execute(tr)
}Executor的launchTask方法首先实例化一个TaskRunner(实现了Runnable接口)，然后使用线程池中的线程执行实例化的TaskRunner中的run()方法，下面就进入到TaskRunner的run()方法中，为了便于大家阅读我们将该方法分成几个部分：
// 实例化TaskMemoryManager，即内存管理val taskMemoryManager = new TaskMemoryManager(env.memoryManager, taskId)// 记录反序列化的开始事件val deserializeStartTime = System.currentTimeMillis()// 设置ClassLoaderThread.currentThread.setContextClassLoader(replClassLoader)// 序列化器val ser = env.closureSerializer.newInstance()// 打印日志信息logInfo(s"Running $taskName (TID $taskId)")// 通过ExecutorBackend的statusUpdate方法向Driver发消息，汇报Task的状态为RUNNING状态execBackend.statusUpdate(taskId, TaskState.RUNNING, EMPTY_BYTE_BUFFER)var taskStart: Long = 0// GC事件startGCTime = computeTotalGcTime()
Driver(DriverEndpoint)接收到消息后的处理不是我们关注的重点，我们聚焦于Task是怎样运行的，继续阅读下面的源码：
try { // 反序列化成Task的依赖关系，包括taskBytes
val (taskFiles, taskJars, taskBytes) = Task.deserializeWithDependencies(serializedTask) // 更新依赖关系，也就是下载依赖(文件、jar)，下载的时候使用了synchronized关键字
// 因为对于每个Executor中的Tasks而言，这些依赖是共享资源
updateDependencies(taskFiles, taskJars) // 将taskBytes反序列化成Task
task = ser.deserialize[Task[Any]](taskBytes, Thread.currentThread.getContextClassLoader) // 设置内存管理器
task.setTaskMemoryManager(taskMemoryManager) // If this task has been killed before we deserialized it, let's quit now. Otherwise,
// continue executing the task.
if (killed) { // Throw an exception rather than returning, because returning within a try{} block
// causes a NonLocalReturnControl exception to be thrown. The NonLocalReturnControl
// exception will be caught by the catch block, leading to an incorrect ExceptionFailure
// for the task.
throw new TaskKilledException
}
logDebug("Task " + taskId + "'s epoch is " + task.epoch)
env.mapOutputTracker.updateEpoch(task.epoch) // 调用task的run()方法来执行任务并获得执行结果
// Run the actual task and measure its runtime.
taskStart = System.currentTimeMillis() var threwException = true
val (value, accumUpdates) = try { val res = task.run(
taskAttemptId = taskId,
attemptNumber = attemptNumber,
metricsSystem = env.metricsSystem)
threwException = false
res
} finally {
...
}
... // 后面是对Task运行完成后返回结果进行的处理首先就是反序列化依赖关系，关于序列化和反序列化我们会在本文的最统一的进行总结。然后将taskBytes反序列化成Task，最后调用Task的run()方法来执行具体的Task并获得执行结果，后面就是对Task运行完成后返回结果的处理，我们在Task运行完成后再进行分析，接下来我们进入Task的run()方法：
final def run(
taskAttemptId: Long,
attemptNumber: Int,
metricsSystem: MetricsSystem)
: (T, AccumulatorUpdates) = {
context = new TaskContextImpl(
stageId,
partitionId,
taskAttemptId,
attemptNumber,
taskMemoryManager,
metricsSystem,
internalAccumulators,
runningLocally = false) TaskContext.setTaskContext(context)
context.taskMetrics.setHostname(Utils.localHostName())
context.taskMetrics.setAccumulatorsUpdater(context.collectInternalAccumulators)
taskThread = Thread.currentThread() if (_killed) {
kill(interruptThread = false)
} try {
(runTask(context), context.collectAccumulators())
} catch {
...
} finally {
...
}
}可以看到内部实际上调用的是Task的runTask方法，而根据不同的Task类型运行的就是ShuffleMapTask或者ResultTask的runTask方法，下面我们就分别进行说明：
ShuffleMapTask
override def runTask(context: TaskContext): MapStatus = { // Deserialize the RDD using the broadcast variable.
// 记录反序列化开始的时间
val deserializeStartTime = System.currentTimeMillis() // 获取序列化/反序列化器
val ser = SparkEnv.get.closureSerializer.newInstance() // 反序列化RDD及其ShuffleDependency
val (rdd, dep) = ser.deserialize[(RDD[_], ShuffleDependency[_, _, _])]( ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader) // 计算出反序列化所需要的时间
_executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
metrics = Some(context.taskMetrics) var writer: ShuffleWriter[Any, Any] = null
try { // 获得ShuffleManager，分成Hash和Sort的方式，默认是Sort的方式
// ShuffleManager是在SparkEnv中创建的(包括Driver和Executor)
// Driver使用它注册shuffles，而Executors可以向他读取和写入数据
val manager = SparkEnv.get.shuffleManager
writer = manager.getWriter[Any, Any](dep.shuffleHandle, partitionId, context)
writer.write(rdd.iterator(partition, context).asInstanceOf[Iterator[_ <: Product2[Any, Any]]])
writer.stop(success = true).get
} catch { case e: Exception => try { if (writer != null) {
writer.stop(success = false)
}
} catch { case e: Exception =>
log.debug("Could not stop writer", e)
} throw e
}
}因为Shuffle是影响整个Spark应用程序运行的关键所在，所以关于Shuffle的部分我们会单独用文章分析，现在关心的是Task的具体计算，可以看出最后执行的是RDD的iterator方法，该方法就是我们针对当前Task所对应的Partition进行计算的关键所在，在具体的处理内部会迭代Partition的元素并交给我们自定义的function进行处理。
final def iterator(split: Partition, context: TaskContext): Iterator[T] = { if (storageLevel != StorageLevel.NONE) { SparkEnv.get.cacheManager.getOrCompute(this, split, context, storageLevel)
} else {
computeOrReadCheckpoint(split, context)
}
}第一次肯定是没有缓存的，所以直接调用compute，而具体的RDD实现不同的compute逻辑，我们这里以MapPartitionsRDD的compute方法为例：
override def compute(split: Partition, context: TaskContext): Iterator[U] = f(context, split.index, firstParent[T].iterator(split, context))
可以清楚的看见直接执行了我们编写的函数f，这里注意第二个参数，同样也是调用的父RDD的iterator方法，这样就将同一个Stage内的函数进行展开计算，形如：
// RDD1x = 1 + y // 这里的y就可以代表从HDFS中读取的数据// RDD2z = x + 3// 展开之后z = (1 + y) + 3// 这里只是打个比方，方便大家理解
ResultTask
override def runTask(context: TaskContext): U = { // Deserialize the RDD and the func using the broadcast variables.
// 记录反序列化事件
val deserializeStartTime = System.currentTimeMillis() // 获取序列化/反序列化器
val ser = SparkEnv.get.closureSerializer.newInstance() // 执行反序列化，和Shuffle不同返回的是RDD和我们编写的业务逻辑
val (rdd, func) = ser.deserialize[(RDD[T], (TaskContext, Iterator[T]) => U)]( ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
_executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
metrics = Some(context.taskMetrics) // 执行我们编写的业务逻辑代码
func(context, rdd.iterator(partition, context))
}我们再来看ResultTask，和Shuffle不同的是ResultTask会直接产生最后的计算结果。
接下来我们回过头来看一下Task的run()方法对计算结果的处理：
override def run(): Unit = {
...
try {
...
// 记录task运行结束的时间
val taskFinish = System.currentTimeMillis() // If the task has been killed, let's fail it.
if (task.killed) { throw new TaskKilledException
} // 序列化器
val resultSer = env.serializer.newInstance() // 记录序列化开始时间
val beforeSerialization = System.currentTimeMillis() // 对返回的结果进行序列化
val valueBytes = resultSer.serialize(value) // 记录序列化结束的时间
val afterSerialization = System.currentTimeMillis() // 记录一系列统计信息
for (m <- task.metrics) { // Deserialization happens in two parts: first, we deserialize a Task object, which
// includes the Partition. Second, Task.run() deserializes the RDD and function to be run
m.setExecutorDeserializeTime(
(taskStart - deserializeStartTime) + task.executorDeserializeTime) // We need to subtract Task.run()'s deserialization time to avoid double-counting
m.setExecutorRunTime((taskFinish - taskStart) - task.executorDeserializeTime)
m.setJvmGCTime(computeTotalGcTime() - startGCTime)
m.setResultSerializationTime(afterSerialization - beforeSerialization)
m.updateAccumulators()
} // 使用DirectTaskResult对结果等信息进行封装
val directResult = new DirectTaskResult(valueBytes, accumUpdates, task.metrics.orNull) // 对DirectTaskResult进行序列化
val serializedDirectResult = ser.serialize(directResult) // 获取序列化后的大小
val resultSize = serializedDirectResult.limit // directSend = sending directly back to the driver
val serializedResult: ByteBuffer = { // 判断序列化后的大小是否大于maxResultSize的限制(默认大小为1GB)
if (maxResultSize > 0 && resultSize > maxResultSize) {
logWarning(s"Finished $taskName (TID $taskId). Result is larger than maxResultSize " + s"(${Utils.bytesToString(resultSize)} > ${Utils.bytesToString(maxResultSize)}), " + s"dropping it.")
ser.serialize(new IndirectTaskResult[Any](TaskResultBlockId(taskId), resultSize)) // 然后再判断序列化后的大小是否大于等于akkaFrameSize - AkkaUtils.reservedSizeBytes，默认大小为：128MB-200k
} else if (resultSize >= akkaFrameSize - AkkaUtils.reservedSizeBytes) { // 获得blockId
val blockId = TaskResultBlockId(taskId) // 通过blockManager写入，这里是存储级别是MEMORY_AND_DISK_SER
env.blockManager.putBytes(
blockId, serializedDirectResult, StorageLevel.MEMORY_AND_DISK_SER)
logInfo( s"Finished $taskName (TID $taskId). $resultSize bytes result sent via BlockManager)") // 序列化
ser.serialize(new IndirectTaskResult[Any](blockId, resultSize))
} else {
logInfo(s"Finished $taskName (TID $taskId). $resultSize bytes result sent to driver") // 不经过BlockManager，直接返回序列化后的结果
serializedDirectResult
}
}
execBackend.statusUpdate(taskId, TaskState.FINISHED, serializedResult)
} catch {
...
} finally {
runningTasks.remove(taskId)
}
}具体的结果(serializedResult)需要通过判断序列化后的大小resultSize来决定：
如果resultSize的大于maxResultSize(通过“spark.driver.maxResultSize”进行配置)，同时保证maxResultSize的值是大于0的，那么返回的就是对IndirectTaskResult[Any](TaskResultBlockId(taskId), resultSize)序列化后的结果，并打下Warning日志
如果resultSize的小于等于maxResultSize并且大于等于128MB-200k，就通过BlockManager进行存储，存储的级别为MEMORY_AND_DISK_SER，并且最后对封装的IndirectTaskResult进行序列化后的结果
如果resultSize的大小小于128MB-200k，则直接返回序列化后的结果
最后通过调用ExecutorBackend(Standalone下就是CoarseGrainedExecutorBackend)的statusUpdate方法将结果返回给DriverEndpoint，具体就是CoarseGrainedExecutorBackend向DriverEndpoint发送StatusUpdate来传输执行结果：
override def statusUpdate(taskId: Long, state: TaskState, data: ByteBuffer) { // 将信息封装成StatusUpdate
val msg = StatusUpdate(executorId, taskId, state, data)
driver match { case Some(driverRef) => driverRef.send(msg) case None => logWarning(s"Drop $msg because has not yet connected to driver")
}
}DriverEndpoint在接收到statusUpdate消息后进行的操作：
case StatusUpdate(executorId, taskId, state, data) => // 首先调用TaskSchedulerImpl的statusUpdate方法
scheduler.statusUpdate(taskId, state, data.value) // 下面就是释放并重新分配刚才Task使用的计算资源
if (TaskState.isFinished(state)) {
executorDataMap.get(executorId) match { case Some(executorInfo) =>
executorInfo.freeCores += scheduler.CPUS_PER_TASK
makeOffers(executorId) case None => // Ignoring the update since we don't know about the executor.
logWarning(s"Ignored task status update ($taskId state $state) " + s"from unknown executor with ID $executorId")
}
}上面的操作分成两步：首先调用TaskSchedulerImpl的statusUpdate方法；然后就是释放并重新分配刚才Task使用的计算资源，我们直接进入TaskSchedulerImpl的statusUpdate方法：
def statusUpdate(tid: Long, state: TaskState, serializedData: ByteBuffer) { var failedExecutor: Option[String] = None
synchronized { try { if (state == TaskState.LOST && taskIdToExecutorId.contains(tid)) { // We lost this entire executor, so remember that it's gone
val execId = taskIdToExecutorId(tid) if (executorIdToTaskCount.contains(execId)) {
removeExecutor(execId, SlaveLost(s"Task $tid was lost, so marking the executor as lost as well."))
failedExecutor = Some(execId)
}
}
taskIdToTaskSetManager.get(tid) match { case Some(taskSet) => if (TaskState.isFinished(state)) {
taskIdToTaskSetManager.remove(tid)
taskIdToExecutorId.remove(tid).foreach { execId => if (executorIdToTaskCount.contains(execId)) {
executorIdToTaskCount(execId) -= 1
}
}
} if (state == TaskState.FINISHED) {
taskSet.removeRunningTask(tid)
taskResultGetter.enqueueSuccessfulTask(taskSet, tid, serializedData)
} else if (Set(TaskState.FAILED, TaskState.KILLED, TaskState.LOST).contains(state)) {
taskSet.removeRunningTask(tid)
taskResultGetter.enqueueFailedTask(taskSet, tid, state, serializedData)
} case None =>
logError(
("Ignoring update with state %s for TID %s because its task set is gone (this is " + "likely the result of receiving duplicate task finished status updates)")
.format(state, tid))
}
} catch { case e: Exception => logError("Exception in statusUpdate", e)
}
} // 防止产生死锁
// Update the DAGScheduler without holding a lock on this, since that can deadlock
if (failedExecutor.isDefined) {
dagScheduler.executorLost(failedExecutor.get)
backend.reviveOffers()
}
}上面的源码中最主要的部分就是使用TaskResultGetter来处理Successful或是FailedTask，即分别调用了TaskResultGetter的enqueueSuccessfulTask方法和enqueueFailedTask方法，我们现在关注的是Task执行成功的情况(对于失败的情况简单来说就是进行重试)，所以我们进入TaskResultGetter的enqueueSuccessfulTask方法：(注意下面只选取了主要的部分)
// 对结果进行了反序列化处理val (result, size) = serializer.get().deserialize[TaskResult[_]](serializedData) match { // 下面就是匹配受到结果的类型，进而进行不同的处理
case directResult: DirectTaskResult[_] => if (!taskSetManager.canFetchMoreResults(serializedData.limit())) { return
} // deserialize "value" without holding any lock so that it won't block other threads.
// We should call it here, so that when it's called again in
// "TaskSetManager.handleSuccessfulTask", it does not need to deserialize the value.
directResult.value()
(directResult, serializedData.limit()) case IndirectTaskResult(blockId, size) => if (!taskSetManager.canFetchMoreResults(size)) { // dropped by executor if size is larger than maxResultSize
sparkEnv.blockManager.master.removeBlock(blockId) return
}
logDebug("Fetching indirect task result for TID %s".format(tid))
scheduler.handleTaskGettingResult(taskSetManager, tid) val serializedTaskResult = sparkEnv.blockManager.getRemoteBytes(blockId) if (!serializedTaskResult.isDefined) { /* We won't be able to get the task result if the machine that ran the task failed
* between when the task ended and when we tried to fetch the result, or if the
* block manager had to flush the result. */
scheduler.handleFailedTask(
taskSetManager, tid, TaskState.FINISHED, TaskResultLost) return
} val deserializedResult = serializer.get().deserialize[DirectTaskResult[_]](
serializedTaskResult.get)
sparkEnv.blockManager.master.removeBlock(blockId)
(deserializedResult, size)
}// 使用统计系统记录ResultSizeresult.metrics.setResultSize(size)
scheduler.handleSuccessfulTask(taskSetManager, tid, result)具体就是根据发过来的结果的类型进行模式匹配，然后分情况进行处理：
如果接收到的是DirectTaskResult类型的数据，也就是说序列化后的大小小于128MB-200k的话，就返回(directResult, serializedData.limit())给(result, size)；
如果接收到的是IndirectTaskResult，且序列化后的大小大于1GB的话，就dropped掉，否则就通过BlockManager获取上面使用BlcokManager存储的数据，然后进行反序列化处理，处理完成后返回(deserializedResult, size)给(result, size)。
最后调用TaskSchedulerImpl的handleSuccessfulTask方法：
def handleSuccessfulTask(
taskSetManager: TaskSetManager,
tid: Long,
taskResult: DirectTaskResult[_]): Unit = synchronized {
taskSetManager.handleSuccessfulTask(tid, taskResult)
}进而调用TaskSetManager的handleSuccessfulTask方法：
def handleSuccessfulTask(tid: Long, result: DirectTaskResult[_]): Unit = {
...
sched.dagScheduler.taskEnded(
tasks(index), Success, result.value(), result.accumUpdates, info, result.metrics)
...
}最主要的就是调用DAGScheduler的taskEnded方法：
def taskEnded(
task: Task[_],
reason: TaskEndReason,
result: Any,
accumUpdates: Map[Long, Any],
taskInfo: TaskInfo,
taskMetrics: TaskMetrics): Unit = {
eventProcessLoop.post( CompletionEvent(task, reason, result, accumUpdates, taskInfo, taskMetrics))
}通过eventProcessLoop.post将CompletionEvent加入到消息队列中，我们直接看DAGScheduler对该消息的处理：
case completion @ CompletionEvent(task, reason, _, _, taskInfo, taskMetrics) => dagScheduler.handleTaskCompletion(completion)
至此我们就不再往下追踪了，感兴趣的朋友可以继续追踪下去，接下来的文章我们开始对Shuffle部分进行细致的分析。
使用一张图来简单的概括一下上面的流程：
补充：Task的序列化和反序列化的总结：
序列化：
1、对RDD及其ShuffleDependency的序列化：
try { // For ShuffleMapTask, serialize and broadcast (rdd, shuffleDep).
// For ResultTask, serialize and broadcast (rdd, func).
val taskBinaryBytes: Array[Byte] = stage match { case stage: ShuffleMapStage =>
closureSerializer.serialize((stage.rdd, stage.shuffleDep): AnyRef).array() case stage: ResultStage =>
closureSerializer.serialize((stage.rdd, stage.func): AnyRef).array()
}
taskBinary = sc.broadcast(taskBinaryBytes)
} catch {2、TaskSetManager:对Task依赖关系的序列化
val serializedTask: ByteBuffer = try { Task.serializeWithDependencies(task, sched.sc.addedFiles, sched.sc.addedJars, ser)
} catch {
序列化完成后封装成TaskDescription：return Some(new TaskDescription(taskId = taskId, attemptNumber = attemptNum, execId,
taskName, index, serializedTask))3、CoarseGrainedSchedulerBackend中的DriverEndpoint：对TaskDescription的序列化：
// Launch tasks returned by a set of resource offersprivate def launchTasks(tasks: Seq[Seq[TaskDescription]]) { for (task <- tasks.flatten) { val serializedTask = ser.serialize(task)反序列化：
1、CoarseGrainedExecutorBackend接收到LaunchTask消息后：反序列化成TaskDescription
case LaunchTask(data) => if (executor == null) {
logError("Received LaunchTask command but executor was null") System.exit(1)
} else { val taskDesc = ser.deserialize[TaskDescription](data.value)2、Executor在使用线程池中的线程运行TaskRunner的run()方法的时候：反序列化依赖关系
try { val (taskFiles, taskJars, taskBytes) = Task.deserializeWithDependencies(serializedTask)3、Executor在使用线程池中的线程运行TaskRunner的run()方法的时候：反序列化成Task
task = ser.deserialize[Task[Any]](taskBytes, Thread.currentThread.getContextClassLoader)
4、ShuffleMapTask或者ResultTask在执行runTask()方法的时候：反序列化RDD及其ShuffleDependency
ShuffleMapTask：
val ser = SparkEnv.get.closureSerializer.newInstance()val (rdd, dep) = ser.deserialize[(RDD[_], ShuffleDependency[_, _, _])]( ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
ResultTask：
val ser = SparkEnv.get.closureSerializer.newInstance()val (rdd, func) = ser.deserialize[(RDD[T], (TaskContext, Iterator[T]) => U)]( ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
本文参照的是Spark 1.6.3版本的源码，同时给出Spark 2.1.0版本的连接：
本文为原创，欢迎转载，转载请注明出处、作者，谢谢！
作者：sun4lower
链接：https://www.jianshu.com/p/b2d419b3ade6
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