Spark源码知识讲解之SchedulerBackend分析

TaskScheduler是一个接口，DAGScheduler在提交TaskSet给底层调度器的时候是面向接口TaskScheduler。

TaskSchduler的核心任务是提交Taskset到集群运算并汇报结果

# 为TaskSet创建和维护一个TaskSetManager并追踪任务的本地性以及错误信息

# 遇到Straggle任务会放到其他的节点进行重试

# 向DAGScheduler汇报执行情况，包括在Shuffle输出lost的时候报告fetch failed错误等信息

在Standalone模式下StandaloneSchedulerBackend在启动的时候构造AppClient实例并在该实例start的时候启动了ClientEndpoint这个消息循环体。ClientEndpoint在启动的时候会向Master注册当前程序。而StandaloneSchedulerBackend的父类CoarseGrainedSchedulerBackend在start的时候会实例化类型为DriverEndpoint（这就是我们程序运行时候的经典对象的Driver）的消息循环体，StandaloneSchedulerBackend专门负责收集Worker上的资源信息，当ExecutorBackend启动的时候会发送RegisteredExecutor信息向DriverEndpoint注册，此时StandaloneSchedulerBackend就掌握了当前应用程序拥有的计算资源，TaskScheduler就是通过StandaloneSchedulerBackend拥有的计算资源来具体运行Task

一 核心属性

Int maxTaskFailures: task最多失败次数

Boolean isLocal： 是否本地运行

AtomicLong nextTaskId：递增的task id

SPECULATION_INTERVAL_MS : 多久检查一次推测任务

CPUS_PER_TASK: 每一个任务需要的cpu核数

HashMap[Long, TaskSetManager]taskIdToTaskSetManager: 为TaskSet创建和维护一个TaskSetManager并追踪任务的本地性以及错误信息

HashMap[Long, String] taskIdToExecutorId: 维护的taskId和executorId的映射

HashMap[String, HashSet[Long]]executorIdToRunningTaskIds:每一个execuotor上运行的task集合的映射

HashMap[String, HashSet[String]] hostToExecutors: 主机名和executors之间的映射

HashMap[String, HashSet[String]] hostsByRack：机架和主机名的映射

HashMap[String, String] executorIdToHost： executorID和主机名映射

DAGScheduler dagScheduler：

SchedulerBackend backend：调度器的通信终端

SchedulableBuilder schedulableBuilder：调度模式，比如FIFO或者Fair

schedulingModeConf：所配置的调度模式，默认FIFO

Pool rootPool: 用于调度TaskManager

TaskResultGetter taskResultGetter: Task结果获取器

二 重要方法

2.1 初始化和启动方法

我们知道，在SparkContext初始化的时候，就会初始化TaskScheduler以及SchedulerBackend，并且会初始化和启动TaskScheduler。

definitialize(backend:SchedulerBackend) {
// 初始化SchedulerBackend
this.backend= backend
// 创建一个Pool用于调度TasksetManager
rootPool = new Pool("",schedulingMode, 0, 0)
// 通过配置的调度模式，构建SchedulableBuilder
schedulableBuilder= {
schedulingModematch{
case SchedulingMode.FIFO=>
new FIFOSchedulableBuilder(rootPool)
case SchedulingMode.FAIR=>
new FairSchedulableBuilder(rootPool,conf)
case _ =>
throw new IllegalArgumentException(s"Unsupportedspark.scheduler.mode:$schedulingMode")
}
}
// 开始构建pool
schedulableBuilder.buildPools()
}

override def start() {
 // 启动SchedulerBackend的start方法，StandaloneSchedulerBackend在
 // 启动的时候构造AppClient实例并在该实例start的时候启动了ClientEndpoint
 // 这个消息循环体。ClientEndpoint在启动的时候会向Master注册当前程序。
 backend.start()
 // 如果非本地执行，则检查是否需要推测
 if (!isLocal && conf.getBoolean("spark.speculation", false)) {
 logInfo("Starting speculative execution thread")
 // 如果可以推测则调用speculationSchedule定时调度checkSpeculatableTasks方法
 speculationScheduler.scheduleAtFixedRate(new Runnable {
 override def run(): Unit = Utils.tryOrStopSparkContext(sc) {
 checkSpeculatableTasks()
 }
 }, SPECULATION_INTERVAL_MS, SPECULATION_INTERVAL_MS, TimeUnit.MILLISECONDS)
 }
}

2.2 submitTasks 提交task

override def submitTasks(taskSet: TaskSet) {
 // 获取task集合，TaskSet是对Task的封装
 val tasks = taskSet.tasks
 logInfo("Adding task set " + taskSet.id + " with " + tasks.length + " tasks")
 this.synchronized {
 // 创建TaskSetManager，用于跟踪每一个Task，task失败进行重试等
 val manager = createTaskSetManager(taskSet, maxTaskFailures)
 // 获取该TaskSet所对应的stageId
 val stage = taskSet.stageId
 // 构建一个<>映射
 val stageTaskSets =
 taskSetsByStageIdAndAttempt.getOrElseUpdate(stage, new HashMap[Int, TaskSetManager])
 // 将这个创建TaskSetManager放入到映射中
 stageTaskSets(taskSet.stageAttemptId) = manager
 // 如果有冲突的TaskSet，则抛异常
 val conflictingTaskSet = stageTaskSets.exists { case (_, ts) =>
 ts.taskSet != taskSet && !ts.isZombie
 }
 if (conflictingTaskSet) {
 throw new IllegalStateException(s"more than one active taskSet for stage $stage:" +
 s" ${stageTaskSets.toSeq.map{_._2.taskSet.id}.mkString(",")}")
 }
 // 申请任务调度，有FIFO和FAIR两种策略。根据executor的空闲资源状态
 // 及locality策略将task分配给executor。调度的数据结构封装为Pool类，
 // 对于FIFO，Pool就是TaskSetManager的队列；对于Fair，则是TaskSetManager
 // 组成的树。Pool维护TaskSet的优先级，等待executor接受资源offer(resourceOffer)
 // 的时候出列并提交executor计算
 schedulableBuilder.addTaskSetManager(manager, manager.taskSet.properties)
 // 不是本地且没有接收task，启动一个timer定时调度，如果一直没有task就警告，直到有task
 if (!isLocal && !hasReceivedTask) {
 starvationTimer.scheduleAtFixedRate(new TimerTask() {
 override def run() {
 if (!hasLaunchedTask) {
 logWarning("Initial job has not accepted any resources; " +
 "check your cluster UI to ensure that workers are registered " +
 "and have sufficient resources")
 } else {
 this.cancel()
 }
 }
 }, STARVATION_TIMEOUT_MS, STARVATION_TIMEOUT_MS)
 }
 hasReceivedTask = true
 }
 // SchedulerBackend向driver发送ReviveOffers消息
 backend.reviveOffers()
}

2.3 DriverEndPoint主要用于接受消息

# receive

override def receive: PartialFunction[Any, Unit] = {
 // 如果接收StatusUpdate消息，用于状态更新
 case StatusUpdate(executorId, taskId, state, data) =>
 // 调用TaskSchedulerImpl#statusUpdate进行更新
 scheduler.statusUpdate(taskId, state, data.value)
 // 如果Task处于完成状态
 if (TaskState.isFinished(state)) {
 // 通过executor id获取ExecutorData
 executorDataMap.get(executorId) match {
 // 如果存在数据
 case Some(executorInfo) =>
 // 则更新executor的cpu核数
 executorInfo.freeCores += scheduler.CPUS_PER_TASK
 // 获取集群中可用的executor列表,发起task
 makeOffers(executorId)
 case None =>
 logWarning(s"Ignored task status update ($taskId state $state) " +
 s"from unknown executor with ID $executorId")
 }
 }
 // 如果发送ReviveOffers消息
 case ReviveOffers =>
 // 获取集群中可用的executor列表,发起task
 makeOffers()
 // 如果是KillTask消息，表示kill掉这个task
 case KillTask(taskId, executorId, interruptThread) =>
 executorDataMap.get(executorId) match {
 // 向Executor发送KillTask的消息
 case Some(executorInfo) =>
 executorInfo.executorEndpoint.send(KillTask(taskId, executorId, interruptThread))
 case None =>
 // Ignoring the task kill since the executor is not registered.
 logWarning(s"Attempted to kill task $taskId for unknown executor $executorId.")
 }
}

# receiveAndReply

override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
 // 接收RegisterExecutor表示向Executor注册
 case RegisterExecutor(executorId, executorRef, hostname, cores, logUrls) =>
 // 如果已经注册过，则会返回RegisterExecutorFailed向executor注册失败的消息
 if (executorDataMap.contains(executorId)) {
 executorRef.send(RegisterExecutorFailed("Duplicate executor ID: " + executorId))
 context.reply(true)
 } else {
 // 获取executor的地址
 val executorAddress = if (executorRef.address != null) {
 executorRef.address
 } else {
 context.senderAddress
 }
 logInfo(s"Registered executor $executorRef ($executorAddress) with ID $executorId")
 // 更新集合
 addressToExecutorId(executorAddress) = executorId
 // 重新计算现在的总的CPU核数
 totalCoreCount.addAndGet(cores)
 // 计算现在已经注册executor数量
 totalRegisteredExecutors.addAndGet(1)
 // 构建一个Executor数据
 val data = new ExecutorData(executorRef, executorRef.address, hostname,
 cores, cores, logUrls)
 // 然后开始注册executor
 CoarseGrainedSchedulerBackend.this.synchronized {
 executorDataMap.put(executorId, data)
 if (currentExecutorIdCounter < executorId.toInt) {
 currentExecutorIdCounter = executorId.toInt
 }
 if (numPendingExecutors > 0) {
 numPendingExecutors -= 1
 logDebug(s"Decremented number of pending executors ($numPendingExecutors left)")
 }
 }
 // 然后返回消息RegisteredExecutor
 executorRef.send(RegisteredExecutor)
 context.reply(true)
 listenerBus.post(
 SparkListenerExecutorAdded(System.currentTimeMillis(), executorId, data))
 // 获取有效的executor，开始发起任务
 makeOffers()
 }
 // 接收StopDriver消息,表示停止Driver
 case StopDriver =>
 context.reply(true)
 stop()
 // 接收StopExecutors消息,表示停止Executor
 case StopExecutors =>
 logInfo("Asking each executor to shut down")
 // 遍历注册所有的executor,然后向Executor终端发送StopExecutor消息
 for ((_, executorData) <- executorDataMap) {
 executorData.executorEndpoint.send(StopExecutor)
 }
 context.reply(true)
 // 接收RemoveExecutor消息,表示删除Executor
 case RemoveExecutor(executorId, reason) =>
 executorDataMap.get(executorId).foreach(_.executorEndpoint.send(StopExecutor))
 removeExecutor(executorId, reason)
 context.reply(true)
 // 接收RetrieveSparkAppConfig消息，表示获取application相关的配置信息
 case RetrieveSparkAppConfig =>
 val reply = SparkAppConfig(sparkProperties,
 SparkEnv.get.securityManager.getIOEncryptionKey())
 context.reply(reply)
}

# makeOffers获取有效的executor，开始发起任务

private def makeOffers() {
 val activeExecutors = executorDataMap.filterKeys(executorIsAlive)
 val workOffers = activeExecutors.map { case (id, executorData) =>
 new WorkerOffer(id, executorData.executorHost, executorData.freeCores)
 }.toIndexedSeq
 launchTasks(scheduler.resourceOffers(workOffers))
}

private def makeOffers(executorId: String) {
 // 获取集群中可用的executor列表
 if (executorIsAlive(executorId)) {
 val executorData = executorDataMap(executorId)
 // 创建WorkerOffer，只是表示executor上有可用的空闲资源
 val workOffers = IndexedSeq(
 new WorkerOffer(executorId, executorData.executorHost, executorData.freeCores))
 // 发起task
 launchTasks(scheduler.resourceOffers(workOffers))
 }
}

# launchTasks

发起task,会把任务一个个发送到worker节点上的CoarseGrainedExecutorBackend,由其内部的executor来执行

private def launchTasks(tasks: Seq[Seq[TaskDescription]]) {
 for (task <- tasks.flatten) {
 // 将每一个task序列化
 val serializedTask = ser.serialize(task)
 // 检查task序列化之后是否超过所允许的rpc消息的最大值
 if (serializedTask.limit >= maxRpcMessageSize) {
 scheduler.taskIdToTaskSetManager.get(task.taskId).foreach { taskSetMgr =>
 try {
 var msg = "Serialized task %s:%d was %d bytes, which exceeds max allowed: " +
 "spark.rpc.message.maxSize (%d bytes). Consider increasing " +
 "spark.rpc.message.maxSize or using broadcast variables for large values."
 msg = msg.format(task.taskId, task.index, serializedTask.limit, maxRpcMessageSize)
 taskSetMgr.abort(msg)
 } catch {
 case e: Exception => logError("Exception in error callback", e)
 }
 }
 }
 else {
 // 获取对应的ExecutorData数据
 val executorData = executorDataMap(task.executorId)
 // Executor的剩余核数就需要减少一个task需要的cpu核数
 executorData.freeCores -= scheduler.CPUS_PER_TASK

 logDebug(s"Launching task ${task.taskId} on executor id: ${task.executorId} hostname: " +
 s"${executorData.executorHost}.")
 // 然后向Executor终端发送LaunchTask，发起task
 executorData.executorEndpoint.send(LaunchTask(new SerializableBuffer(serializedTask)))
 }
 }
}

# removeExecutor

private def removeExecutor(executorId: String, reason: ExecutorLossReason): Unit = {
 logDebug(s"Asked to remove executor $executorId with reason $reason")
 // 获取对应的ExecutorData
 executorDataMap.get(executorId) match {
 case Some(executorInfo) =>
 // 从相关集合或者列表移除该executorId
 val killed = CoarseGrainedSchedulerBackend.this.synchronized {
 addressToExecutorId -= executorInfo.executorAddress
 executorDataMap -= executorId
 executorsPendingLossReason -= executorId
 executorsPendingToRemove.remove(executorId).getOrElse(false)
 }
 // 重新计算CPU核数
 totalCoreCount.addAndGet(-executorInfo.totalCores)
 totalRegisteredExecutors.addAndGet(-1)
 scheduler.executorLost(executorId, if (killed) ExecutorKilled else reason)
 listenerBus.post(
 SparkListenerExecutorRemoved(System.currentTimeMillis(), executorId, reason.toString))
 case None =>
 scheduler.sc.env.blockManager.master.removeExecutorAsync(executorId)
 logInfo(s"Asked to remove non-existent executor $executorId")
 }
}

2.4 resourceOffers 为executor分配task

计算每一个TaskSetMangaer的本地化级别(locality_level);并且对task set尝试使用最小的本地化级别(locality_level), 将task set的task在executor上启动;如果启动不了，放大本地化级别，以此类推直到某种本地化级别尝试成功

defresourceOffers(offers:IndexedSeq[WorkerOffer]):Seq[Seq[TaskDescription]] = synchronized {
// 标记每一个slave是可用的且记住主机名
var newExecAvail= false
// 遍历有可用资源的Executor
for (o <- offers) {
// 如果没有包含了这个executor的host，初始化一个集合，存放host
if (!hostToExecutors.contains(o.host)) {
hostToExecutors(o.host) =new HashSet[String]()
}
// 如果不包含这个executorId
if (!executorIdToRunningTaskIds.contains(o.executorId)) {
hostToExecutors(o.host)+= o.executorId
// 通知DAGScheduler添加executor
executorAdded(o.executorId, o.host)
executorIdToHost(o.executorId) = o.host
executorIdToRunningTaskIds(o.executorId) =HashSet[Long]()
newExecAvail = true
}
// 遍历主机所在机架
for (rack <- getRackForHost(o.host)) {
// 更新hosts和机架的映射
hostsByRack.getOrElseUpdate(rack,new HashSet[String]())+= o.host
}
}

// 将WorkerOffer打乱,做到负载均衡
val shuffledOffers= Random.shuffle(offers)
// 构建一个task列表，然后分配给每一个worker
val tasks= shuffledOffers.map(o=> new ArrayBuffer[TaskDescription](o.cores))
// 有效可用的CPU核数
val availableCpus= shuffledOffers.map(o=> o.cores).toArray
// 从调度池获取按照调度策略排序好的TaskSetManager
val sortedTaskSets= rootPool.getSortedTaskSetQueue
// 如果有新加入的executor，需要重新计算数据本地性
for (taskSet <- sortedTaskSets) {
logDebug("parentName: %s, name: %s, runningTasks: %s".format(
taskSet.parent.name, taskSet.name, taskSet.runningTasks))
if (newExecAvail) {
taskSet.executorAdded()
}
}
// 为排好序的TaskSetManager列表分配资源，分配原则是就近原则，按照顺序为
// PROCESS_LOCAL, NODE_LOCAL, NO_PREF,RACK_LOCAL, ANY
for (taskSet <- sortedTaskSets) {
var launchedAnyTask= false
var launchedTaskAtCurrentMaxLocality=false
// 计算每一个TaskSetMangaer的本地化级别(locality_level),
// 并且对task set尝试使用最小的本地化级别(locality_level),将task set的task在executor上启动
// 如果启动不了，放大本地化级别，以此类推直到某种本地化级别尝试成功
for (currentMaxLocality <- taskSet.myLocalityLevels) {
do {
launchedTaskAtCurrentMaxLocality= resourceOfferSingleTaskSet(
taskSet, currentMaxLocality, shuffledOffers,availableCpus, tasks)
launchedAnyTask|= launchedTaskAtCurrentMaxLocality
} while (launchedTaskAtCurrentMaxLocality)
}
// 如果这个task在任何本地化级别都启动不了，有可能在黑名单
if (!launchedAnyTask) {
taskSet.abortIfCompletelyBlacklisted(hostToExecutors)
}
}
if (tasks.size> 0) {
hasLaunchedTask= true
}
return tasks
}

2.5 resourceOfferSingleTaskSet 分配单个TaskSet里的task到executor

调用resourceOffer方法找到在executor上，哪些TaskSet的task可以通过当前本地化级别启动；遍历在该executor上当前本地化级别可以运行的task

private defresourceOfferSingleTaskSet(
taskSet: TaskSetManager,
maxLocality: TaskLocality,
shuffledOffers: Seq[WorkerOffer],
availableCpus: Array[Int],
tasks: IndexedSeq[ArrayBuffer[TaskDescription]]) : Boolean = {
// 默认发起task为false
var launchedTask= false
// 遍历所有executor
for (i <- 0 until shuffledOffers.size) {
// 获取executorId和host
val execId= shuffledOffers(i).executorId
val host= shuffledOffers(i).host
// 必须要有每一个task可供分配的的CPU核数，否则直接返回
if (availableCpus(i) >=CPUS_PER_TASK) {
try {
// 调用resourceOffer方法找到在executor上，哪些TaskSet的task可以通过当前本地化级别启动
// 遍历在该executor上当前本地化级别可以运行的task
for (task <- taskSet.resourceOffer(execId,host, maxLocality)) {
// 如果存在，则把每一个task放入要在当前executor运行的task数组里面
// 即指定executor要运行的task
tasks(i) += task
// 将相应的分配信息加入内存缓存
val tid= task.taskId
taskIdToTaskSetManager(tid) =taskSet
taskIdToExecutorId(tid) =execId
executorIdToRunningTaskIds(execId).add(tid)
availableCpus(i) -= CPUS_PER_TASK
assert(availableCpus(i) >=0)
launchedTask = true
}
} catch {
case e:TaskNotSerializableException =>
logError(s"Resource offer failed, task set${taskSet.name} was not serializable")
// Do notoffer resources for this task, but don't throw an error to allow other
// task sets to be submitted.
return launchedTask
}
}
}
return launchedTask
}