Spark源码阅读(1): Stage划分
Spark中job由action动作生成，那么stage是如何划分的呢？一般的解答是根据宽窄依赖划分。那么我们深入源码看看吧
一个action 例如count，会在多次runJob中传递，最终会到一个函数
dagScheduler.runJob(rdd, cleanedFunc, partitions, callSite, resultHandler, localProperties.get)
dagScheduler是DAGScheduler的一个实例，因此，后面的工作都交给DAGScheduler进行。
在dagScheduler.runJob主要是调用submitJob函数
submitJob的主要工作是向job scheduler 提交一个job并且创建一个JobWaiter对象。 这个JobWaiter对象一直阻塞到job完成或者取消。
其中提交动作实际上是将Job提交给一个DAGSchedulerEventProcessLoop，在这个里面处理job的运行。主要代码如下那么DAGSchedulerEventProcessLoop是如何处理的呢？
val waiter = new JobWaiter(this, jobId, partitions.size, resultHandler) eventProcessLoop.post(JobSubmitted( jobId, rdd, func2, partitions.toArray, callSite, waiter, SerializationUtils.clone(properties)))
DAGSchedulerEventProcessLoop中回去接受，然后去查看是哪种情况。从这里可以看出，
还有很多操作会被塞进这个Loop中。这么做的原因呢？（解耦？）
override def onReceive(event: DAGSchedulerEvent): Unit = {
val timerContext = timer.time()
try {
doOnReceive(event)
} finally {
timerContext.stop()
}
}
private def doOnReceive(event: DAGSchedulerEvent): Unit = event match {
case JobSubmitted(jobId, rdd, func, partitions, callSite, listener, properties) =>
dagScheduler.handleJobSubmitted(jobId, rdd, func, partitions, callSite, listener, properties)
case StageCancelled(stageId) =>
dagScheduler.handleStageCancellation(stageId)
case JobCancelled(jobId) =>
dagScheduler.handleJobCancellation(jobId)
...
}
DAGSchedulerEventProcessLoop的父类EventLoop有开线程，因此上述处理工作会在另一个线程中进行。
private val eventThread = new Thread(name) {
setDaemon(true)
override def run(): Unit = {
try {
while (!stopped.get) {
...
}
另外，其实所有的处理工作还是在DAGScheduler中进行。接下来深入handleJobSubmitted
这个给出全部代码。主要逻辑是利用最后一个RDD去生成ResultStage。生成之后创建ActiveJob并记录相关信息，并通过submitStage(finalStage)处理
private[scheduler] def handleJobSubmitted(jobId: Int,
finalRDD: RDD[_],
func: (TaskContext, Iterator[_]) => _,
partitions: Array[Int],
callSite: CallSite,
listener: JobListener,
properties: Properties) {
var finalStage: ResultStage = 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 = newResultStage(finalRDD, partitions.length, jobId, callSite)
} catch {
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".format(
job.jobId, callSite.shortForm, partitions.length))
logInfo("Final stage: " + finalStage + "(" + finalStage.name + ")")
logInfo("Parents of final stage: " + finalStage.parents)
logInfo("Missing parents: " + getMissingParentStages(finalStage))
val jobSubmissionTime = clock.getTimeMillis()
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))
submitStage(finalStage)
}
submitWaitingStages()
}
接下来分两个脉络进行(1)newResultStage 和 (2)submitStage
(1) newResultStage
在这里要生成一个ResultStage，这个stage的创建是需要其父stage的信息的，所以通过getParentStagesAndId获取
private def newResultStage(
rdd: RDD[_],
numTasks: Int,
jobId: Int,
callSite: CallSite): ResultStage = {
val (parentStages: List[Stage], id: Int) = getParentStagesAndId(rdd, jobId)
val stage: ResultStage = new ResultStage(id, rdd, numTasks, parentStages, jobId, callSite)
stageIdToStage(id) = stage
updateJobIdStageIdMaps(jobId, stage)
stage
}
这个里面由getParentStages进行。利用一个栈来处理未访问的rdd，首先是末尾的rdd，然后看其依赖。
如果是一个ShuffleDependency
private def getParentStages(rdd: RDD[_], firstJobId: 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, firstJobId)
case _ =>
waitingForVisit.push(dep.rdd)
}
}
}
}
waitingForVisit.push(rdd)
while (waitingForVisit.nonEmpty) {
visit(waitingForVisit.pop())
}
parents.toList
}
这里面通过判断一个RDD的依赖是不是Shuffle的来进行。像reduceByKey这样的操作。RDD1是转换前的，RDD2为转换后的，
那么RDD2的依赖就是ShuffleDependency, 这个ShuffleDependency对象中也有RDD，其RDD就是RDD1.
如果是ShuffleDependency的话就通过getShuffleDependency来获得。
那么这段代码的大致原理就是，先把末尾的RDD加入stack中，也就是waitingForVisit, 然后获取是ShuffleDependency的stage。
所以根据ShuffleDependency来划分stage。但是好像还没有看到如何将中间的那些RDD放到一个stage中。
继续深入getShuffleMapStage，
private def getShuffleMapStage(
shuffleDep: ShuffleDependency[_, _, _],
firstJobId: Int): ShuffleMapStage = {
shuffleToMapStage.get(shuffleDep.shuffleId) match {
case Some(stage) => stage
case None =>
// We are going to register ancestor shuffle dependencies
registerShuffleDependencies(shuffleDep, firstJobId)
// Then register current shuffleDep
val stage = newOrUsedShuffleStage(shuffleDep, firstJobId)
shuffleToMapStage(shuffleDep.shuffleId) = stage
stage
}
}
如果这个ShuffleDependency已经被生成过ShuffleMapStage， 那么直接获取，如果没有则需要注册。
深入registerShuffleDependencies这个函数，
private def registerShuffleDependencies(shuffleDep: ShuffleDependency[_, _, _], firstJobId: Int) {
val parentsWithNoMapStage = getAncestorShuffleDependencies(shuffleDep.rdd) # 获取这个ShuffleDependency前面的还没在
# shuffleToMapStage中注册的ShuffleDependency
while (parentsWithNoMapStage.nonEmpty) {
val currentShufDep = parentsWithNoMapStage.pop()
val stage = newOrUsedShuffleStage(currentShufDep, firstJobId)
shuffleToMapStage(currentShufDep.shuffleId) = stage
}
}
那么newOrUsedShuffleStage这个函数是干嘛的呢？用于生成一个stage，并注册到shuffleToMapStage
(2) submitStage
