引入
上一篇文章《DAGScheduler源码浅析》中，介绍了handleJobSubmitted函数，它作为生成finalStage的重要函数存在，这一篇文章中，我将就DAGScheduler生成Stage过程继续学习，同时介绍Stage的相关源码。
Stage生成
Stage的调度是由DAGScheduler完成的。由RDD的有向无环图DAG切分出了Stage的有向无环图DAG。Stage的DAG通过最后执行的Stage为根进行广度优先遍历，遍历到最开始执行的Stage执行，如果提交的Stage仍有未完成的父母Stage，则Stage需要等待其父Stage执行完才能执行。同时DAGScheduler中还维持了几个重要的Key-Value集合结构，用来记录Stage的状态，这样能够避免过早执行和重复提交Stage。waitingStages中记录仍有未执行的父母Stage，防止过早执行。runningStages中保存正在执行的Stage，防止重复执行。failedStages中保存执行失败的Stage，需要重新执行，这里的设计是出于容错的考虑。
// Stages we need to run whose parents aren't done
private[scheduler] val waitingStages = new HashSet[Stage]
// Stages we are running right now
private[scheduler] val runningStages = new HashSet[Stage]
// Stages that must be resubmitted due to fetch failures
private[scheduler] val failedStages = new HashSet[Stage]
依赖关系
RDD的窄依赖是指父RDD的所有输出都会被指定的子RDD消费，即输出路径是固定的；宽依赖是指父RDD的输出会由不同的子RDD消费，即输出路径不固定。
调度器会计算RDD之间的依赖关系，将拥有持续窄依赖的RDD归并到同一个Stage中，而宽依赖则作为划分不同Stage的判断标准。
导致窄依赖的Transformation操作：map、flatMap、filter、sample；导致宽依赖的Transformation操作：sortByKey、reduceByKey、groupByKey、cogroupByKey、join、cartensian。
Stage分为两种：
ShuffleMapStage, in which case its tasks' results are input for another stage
其实就是,非最终stage, 后面还有其他的stage, 所以它的输出一定是需要shuffle并作为后续的输入。
这种Stage是以Shuffle为输出边界，其输入边界可以是从外部获取数据，也可以是另一个ShuffleMapStage的输出
其输出可以是另一个Stage的开始。
ShuffleMapStage的最后Task就是ShuffleMapTask。
在一个Job里可能有该类型的Stage，也可以能没有该类型Stage。
ResultStage, in which case its tasks directly compute the action that initiated a job (e.g. count(), save(), etc)
最终的stage, 没有输出, 而是直接产生结果或存储。
这种Stage是直接输出结果，其输入边界可以是从外部获取数据，也可以是另一个ShuffleMapStage的输出。
ResultStage的最后Task就是ResultTask，在一个Job里必定有该类型Stage。
一个Job含有一个或多个Stage，但至少含有一个ResultStage。
Stage类
stage的RDD参数只有一个RDD, final RDD, 而不是一系列的RDD。
因为在一个stage中的所有RDD都是map, partition不会有任何改变, 只是在data依次执行不同的map function所以对于TaskScheduler而言, 一个RDD的状况就可以代表这个stage。
Stage参数说明：
val id: Int //Stage的序号数值越大，优先级越高
val rdd: RDD[], //归属于本Stage的最后一个rdd
val numTasks: Int, //创建的Task数目，等于父RDD的输出Partition数目
val shuffleDep: Option[ShuffleDependency[, _, _]], //是否存在SuffleDependency，宽依赖
val parents: List[Stage], //父Stage列表
val jobId: Int //作业ID
private[spark] class Stage(
val id: Int,
val rdd: RDD[_],
val numTasks: Int,
val shuffleDep: Option[ShuffleDependency[_, _, _]], // Output shuffle if stage is a map stage
val parents: List[Stage],
val jobId: Int,
val callSite: CallSite)
extends Logging {
val isShuffleMap = shuffleDep.isDefined
val numPartitions = rdd.partitions.size
val outputLocs = Array.fill[List[MapStatus]](numPartitions)(Nil)
var numAvailableOutputs = 0
/** Set of jobs that this stage belongs to. */
val jobIds = new HashSet[Int]
/** For stages that are the final (consists of only ResultTasks), link to the ActiveJob. */
var resultOfJob: Option[ActiveJob] = None
var pendingTasks = new HashSet[Task[_]]
private var nextAttemptId = 0
val name = callSite.shortForm
val details = callSite.longForm
/** Pointer to the latest [StageInfo] object, set by DAGScheduler. */
var latestInfo: StageInfo = StageInfo.fromStage(this)
def isAvailable: Boolean = {
if (!isShuffleMap) {
true
} else {
numAvailableOutputs == numPartitions
}
}
def addOutputLoc(partition: Int, status: MapStatus) {
val prevList = outputLocs(partition)
outputLocs(partition) = status :: prevList
if (prevList == Nil) {
numAvailableOutputs += 1
}
}
def removeOutputLoc(partition: Int, bmAddress: BlockManagerId) {
val prevList = outputLocs(partition)
val newList = prevList.filterNot(_.location == bmAddress)
outputLocs(partition) = newList
if (prevList != Nil && newList == Nil) {
numAvailableOutputs -= 1
}
}
/**
* Removes all shuffle outputs associated with this executor. Note that this will also remove
* outputs which are served by an external shuffle server (if one exists), as they are still
* registered with this execId.
*/
def removeOutputsOnExecutor(execId: String) {
var becameUnavailable = false
for (partition <- 0 until numPartitions) {
val prevList = outputLocs(partition)
val newList = prevList.filterNot(_.location.executorId == execId)
outputLocs(partition) = newList
if (prevList != Nil && newList == Nil) {
becameUnavailable = true
numAvailableOutputs -= 1
}
}
if (becameUnavailable) {
logInfo("%s is now unavailable on executor %s (%d/%d, %s)".format(
this, execId, numAvailableOutputs, numPartitions, isAvailable))
}
}
/** Return a new attempt id, starting with 0. */
def newAttemptId(): Int = {
val id = nextAttemptId
nextAttemptId += 1
id
}
def attemptId: Int = nextAttemptId
override def toString = "Stage " + id
override def hashCode(): Int = id
override def equals(other: Any): Boolean = other match {
case stage: Stage => stage != null && stage.id == id
case _ => false
}
}
处理Job，分割Job为Stage，封装Stage成TaskSet，最终提交给TaskScheduler的调用链
dagScheduler.handleJobSubmitted-->dagScheduler.submitStage-->dagScheduler.submitMissingTasks-->taskScheduler.submitTasks。
handleJobSubmitted函数
函数handleJobSubmitted和submitStage主要负责依赖性分析，对其处理逻辑做进一步的分析。
handleJobSubmitted最主要的工作是生成Stage，并根据finalStage来产生ActiveJob。
private[scheduler] def handleJobSubmitted(jobId: Int,
finalRDD: RDD[_],
func: (TaskContext, Iterator[_]) => _,
partitions: Array[Int],
allowLocal: Boolean,
callSite: CallSite,
listener: JobListener,
properties: Properties) {
var finalStage: Stage = null
try {
// New stage creation may throw an exception if, for example, jobs are run on a
// HadoopRDD whose underlying HDFS files have been deleted.
finalStage = newStage(finalRDD, partitions.size, None, jobId, callSite)
} catch {
//错误处理，告诉监听器作业失败，返回....
case e: Exception =>
logWarning("Creating new stage failed due to exception - job: " + jobId, e)
listener.jobFailed(e)
return
}
if (finalStage != null) {
val job = new ActiveJob(jobId, finalStage, func, partitions, callSite, listener, properties)
clearCacheLocs()
logInfo("Got job %s (%s) with %d output partitions (allowLocal=%s)".format(
job.jobId, callSite.shortForm, partitions.length, allowLocal))
logInfo("Final stage: " + finalStage + "(" + finalStage.name + ")")
logInfo("Parents of final stage: " + finalStage.parents)
logInfo("Missing parents: " + getMissingParentStages(finalStage))
val shouldRunLocally =
localExecutionEnabled && allowLocal && finalStage.parents.isEmpty && partitions.length == 1
val jobSubmissionTime = clock.getTimeMillis()
if (shouldRunLocally) {
// 很短、没有父stage的本地操作，比如 first() or take() 的操作本地执行
// Compute very short actions like first() or take() with no parent stages locally.
listenerBus.post(
SparkListenerJobStart(job.jobId, jobSubmissionTime, Seq.empty, properties))
runLocally(job)
} else {
// collect等操作走的是这个过程，更新相关的关系映射，用监听器监听，然后提交作业
jobIdToActiveJob(jobId) = job
activeJobs += job
finalStage.resultOfJob = Some(job)
val stageIds = jobIdToStageIds(jobId).toArray
val stageInfos = stageIds.flatMap(id => stageIdToStage.get(id).map(_.latestInfo))
listenerBus.post(
SparkListenerJobStart(job.jobId, jobSubmissionTime, stageInfos, properties))
// 提交stage
submitStage(finalStage)
}
}
// 提交stage
submitWaitingStages()
}
newStage函数
/**
* Create a Stage -- either directly for use as a result stage, or as part of the (re)-creation
* of a shuffle map stage in newOrUsedStage. The stage will be associated with the provided
* jobId. Production of shuffle map stages should always use newOrUsedStage, not newStage
* directly.
*/
private def newStage(
rdd: RDD[_],
numTasks: Int,
shuffleDep: Option[ShuffleDependency[_, _, _]],
jobId: Int,
callSite: CallSite)
: Stage =
{
val parentStages = getParentStages(rdd, jobId)
val id = nextStageId.getAndIncrement()
val stage = new Stage(id, rdd, numTasks, shuffleDep, parentStages, jobId, callSite)
stageIdToStage(id) = stage
updateJobIdStageIdMaps(jobId, stage)
stage
}
其中，Stage的初始化参数：在创建一个Stage之前，需要知道该Stage需要从多少个Partition读入数据，这个数值直接影响要创建多少个Task。也就是说，创建Stage时，已经清楚该Stage需要从多少不同的Partition读入数据，并写出到多少个不同的Partition中，输入和输出的个数均已明确。
getParentStages函数：
通过不停的遍历它之前的rdd，如果碰到有依赖是ShuffleDependency类型的，就通过getShuffleMapStage方法计算出来它的Stage来。
/**
* Get or create the list of parent stages for a given RDD. The stages will be assigned the
* provided jobId if they haven't already been created with a lower jobId.
*/
private def getParentStages(rdd: RDD[_], jobId: Int): List[Stage] = {
val parents = new HashSet[Stage]
val visited = new HashSet[RDD[_]]
// We are manually maintaining a stack here to prevent StackOverflowError
// caused by recursively visiting
val waitingForVisit = new Stack[RDD[_]]
def visit(r: RDD[_]) {
if (!visited(r)) {
visited += r
// Kind of ugly: need to register RDDs with the cache here since
// we can't do it in its constructor because # of partitions is unknown
for (dep <- r.dependencies) {
dep match {
case shufDep: ShuffleDependency[_, _, _] =>
parents += getShuffleMapStage(shufDep, jobId)
case _ =>
waitingForVisit.push(dep.rdd)
}
}
}
}
waitingForVisit.push(rdd)
while (!waitingForVisit.isEmpty) {
visit(waitingForVisit.pop())
}
parents.toList
}
ActiveJob类
用户所提交的job在得到DAGScheduler的调度后，会被包装成ActiveJob，同时会启动JobWaiter阻塞监听job的完成状况。
同时依据job中RDD的dependency和dependency属性(NarrowDependency，ShufflerDependecy)，DAGScheduler会根据依赖关系的先后产生出不同的stage DAG(result stage, shuffle map stage)。
在每一个stage内部，根据stage产生出相应的task，包括ResultTask或是ShuffleMapTask，这些task会根据RDD中partition的数量和分布，产生出一组相应的task，并将其包装为TaskSet提交到TaskScheduler上去。
/**
* Tracks information about an active job in the DAGScheduler.
*/
private[spark] class ActiveJob(
val jobId: Int,
val finalStage: Stage,
val func: (TaskContext, Iterator[_]) => _,
val partitions: Array[Int],
val callSite: CallSite,
val listener: JobListener,
val properties: Properties) {
val numPartitions = partitions.length
val finished = Array.fill[Boolean](numPartitions)(false)
var numFinished = 0
}
submitStage函数
submitStage函数中会根据依赖关系划分stage，通过递归调用从finalStage一直往前找它的父stage，直到stage没有父stage时就调用submitMissingTasks方法提交改stage。这样就完成了将job划分为一个或者多个stage。
submitStage处理流程：
- 所依赖的Stage是否都已经完成，如果没有完成则先执行所依赖的Stage
- 如果所有的依赖已经完成，则提交自身所处的Stage
- 最后会在submitMissingTasks函数中将stage封装成TaskSet通过taskScheduler.submitTasks函数提交给TaskScheduler处理。
/** Submits stage, but first recursively submits any missing parents. */
private def submitStage(stage: Stage) {
val jobId = activeJobForStage(stage)
if (jobId.isDefined) {
logDebug("submitStage(" + stage + ")")
if (!waitingStages(stage) && !runningStages(stage) && !failedStages(stage)) {
val missing = getMissingParentStages(stage).sortBy(_.id) // 根据final stage发现是否有parent stage
logDebug("missing: " + missing)
if (missing == Nil) {
logInfo("Submitting " + stage + " (" + stage.rdd + "), which has no missing parents")
submitMissingTasks(stage, jobId.get) // 如果没有parent stage需要执行, 则直接submit当前stage
} else {
for (parent <- missing) {
submitStage(parent) // 如果有parent stage,需要先submit parent, 因为stage之间需要顺序执行
}
waitingStages += stage // 当前stage放到waitingStages中
}
}
} else {
abortStage(stage, "No active job for stage " + stage.id)
}
}
getMissingParentStages
getMissingParentStages通过图的遍历，来找出所依赖的所有父Stage。
private def getMissingParentStages(stage: Stage): List[Stage] = {
val missing = new HashSet[Stage]
val visited = new HashSet[RDD[_]]
// We are manually maintaining a stack here to prevent StackOverflowError
// caused by recursively visiting
val waitingForVisit = new Stack[RDD[_]]
def visit(rdd: RDD[_]) {
if (!visited(rdd)) {
visited += rdd
if (getCacheLocs(rdd).contains(Nil)) {
for (dep <- rdd.dependencies) {
dep match {
case shufDep: ShuffleDependency[_, _, _] => // 如果发现ShuffleDependency, 说明遇到新的stage
val mapStage = getShuffleMapStage(shufDep, stage.jobId)
// check shuffleToMapStage, 如果该stage已经被创建则直接返回, 否则newStage
if (!mapStage.isAvailable) {
missing += mapStage
}
case narrowDep: NarrowDependency[_] => // 对于NarrowDependency, 说明仍然在这个stage中
waitingForVisit.push(narrowDep.rdd)
}
}
}
}
}
waitingForVisit.push(stage.rdd)
while (!waitingForVisit.isEmpty) {
visit(waitingForVisit.pop())
}
missing.toList
}
submitMissingTasks
可见无论是哪种stage，都是对于每个stage中的每个partitions创建task，并最终封装成TaskSet，将该stage提交给taskscheduler。
/** Called when stage's parents are available and we can now do its task. */
private def submitMissingTasks(stage: Stage, jobId: Int) {
logDebug("submitMissingTasks(" + stage + ")")
// Get our pending tasks and remember them in our pendingTasks entry
stage.pendingTasks.clear()
// First figure out the indexes of partition ids to compute.
val partitionsToCompute: Seq[Int] = {
if (stage.isShuffleMap) {
(0 until stage.numPartitions).filter(id => stage.outputLocs(id) == Nil)
} else {
val job = stage.resultOfJob.get
(0 until job.numPartitions).filter(id => !job.finished(id))
}
}
val properties = if (jobIdToActiveJob.contains(jobId)) {
jobIdToActiveJob(stage.jobId).properties
} else {
// this stage will be assigned to "default" pool
null
}
runningStages += stage
// SparkListenerStageSubmitted should be posted before testing whether tasks are
// serializable. If tasks are not serializable, a SparkListenerStageCompleted event
// will be posted, which should always come after a corresponding SparkListenerStageSubmitted
// event.
stage.latestInfo = StageInfo.fromStage(stage, Some(partitionsToCompute.size))
outputCommitCoordinator.stageStart(stage.id)
listenerBus.post(SparkListenerStageSubmitted(stage.latestInfo, properties))
// TODO: Maybe we can keep the taskBinary in Stage to avoid serializing it multiple times.
// Broadcasted binary for the task, used to dispatch tasks to executors. Note that we broadcast
// the serialized copy of the RDD and for each task we will deserialize it, which means each
// task gets a different copy of the RDD. This provides stronger isolation between tasks that
// might modify state of objects referenced in their closures. This is necessary in Hadoop
// where the JobConf/Configuration object is not thread-safe.
var taskBinary: Broadcast[Array[Byte]] = null
try {
// For ShuffleMapTask, serialize and broadcast (rdd, shuffleDep).
// For ResultTask, serialize and broadcast (rdd, func).
val taskBinaryBytes: Array[Byte] =
if (stage.isShuffleMap) {
closureSerializer.serialize((stage.rdd, stage.shuffleDep.get) : AnyRef).array()
} else {
closureSerializer.serialize((stage.rdd, stage.resultOfJob.get.func) : AnyRef).array()
}
taskBinary = sc.broadcast(taskBinaryBytes)
} catch {
// In the case of a failure during serialization, abort the stage.
case e: NotSerializableException =>
abortStage(stage, "Task not serializable: " + e.toString)
runningStages -= stage
return
case NonFatal(e) =>
abortStage(stage, s"Task serialization failed: $e\n${e.getStackTraceString}")
runningStages -= stage
return
}
val tasks: Seq[Task[_]] = if (stage.isShuffleMap) {
partitionsToCompute.map { id =>
val locs = getPreferredLocs(stage.rdd, id)
val part = stage.rdd.partitions(id)
new ShuffleMapTask(stage.id, taskBinary, part, locs)
}
} else {
val job = stage.resultOfJob.get
partitionsToCompute.map { id =>
val p: Int = job.partitions(id)
val part = stage.rdd.partitions(p)
val locs = getPreferredLocs(stage.rdd, p)
new ResultTask(stage.id, taskBinary, part, locs, id)
}
}
if (tasks.size > 0) {
logInfo("Submitting " + tasks.size + " missing tasks from " + stage + " (" + stage.rdd + ")")
stage.pendingTasks ++= tasks
logDebug("New pending tasks: " + stage.pendingTasks)
taskScheduler.submitTasks(
new TaskSet(tasks.toArray, stage.id, stage.newAttemptId(), stage.jobId, properties))
stage.latestInfo.submissionTime = Some(clock.getTimeMillis())
} else {
// Because we posted SparkListenerStageSubmitted earlier, we should mark
// the stage as completed here in case there are no tasks to run
markStageAsFinished(stage, None)
logDebug("Stage " + stage + " is actually done; %b %d %d".format(
stage.isAvailable, stage.numAvailableOutputs, stage.numPartitions))
}
}
参考资料
转载请注明作者Jason Ding及其出处
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