spark shuffle流程分析
回到ShuffleMapTask.runTask函数
现在回到ShuffleMapTask.runTask函数中：
override def runTask(context: TaskContext): MapStatus = {
首先得到要reduce的task的个数。
valnumOutputSplits = dep.partitioner.numPartitions
metrics = Some(context.taskMetrics)
valblockManager = SparkEnv.get.blockManager
valshuffleBlockManager = blockManager.shuffleBlockManager
varshuffle: ShuffleWriterGroup = null
varsuccess = false
try {
得到对数据时行serializer操作的类，
// Obtain all the block writers for shuffle blocks.
valser = SparkEnv.get.serializerManager.get(dep.serializerClass, SparkEnv.get.conf)
通过shuffleid与要进行reduce的task个数，生成ShuffleBlockId，
同时根据blockid生成ShuffleWriterGroup.shuffle的实现为DiskBlockObjectWriter。
通过spark.shuffle.consolidateFiles配置是否合并文件的输入。默认的为false,
合并文件设置为true,下次再有task在本机运行时，会直接打开当前输入的文件进行输入。
shuffle = shuffleBlockManager.forMapTask(dep.shuffleId, partitionId, numOutputSplits, ser)
根据rdd的iterator取出数据，根据element的key重新进行partition,重新写入到shuffle的
// Write the map output to its associated buckets.
for (elem <- rdd.iterator(split, context)) {
valpair = elem.asInstanceOf[Product2[Any, Any]]
valbucketId = dep.partitioner.getPartition(pair._1)
每一个partition都对应着一个DiskBlockObjectWriter, 通过此实例的write函数，写入shuffle的数据。
也就是说，这个时候此RDD远行的task个数为core的个数，此时打开的文件个数为corenum*numpartition。
shuffle.writers(bucketId).write(pair)
}
// Commit the writes. Get the size of each bucket block (total block size).
vartotalBytes = 0L
vartotalTime = 0L
把这次打开的所有的文件全部commit,同时关闭文件的输入。
valcompressedSizes: Array[Byte] = shuffle.writers.map { writer: BlockObjectWriter =>
writer.commit()
writer.close()
valsize = writer.fileSegment().length
totalBytes += size
totalTime += writer.timeWriting()
MapOutputTracker.compressSize(size)
}
// Update shuffle metrics.
valshuffleMetrics = new ShuffleWriteMetrics
shuffleMetrics.shuffleBytesWritten = totalBytes
shuffleMetrics.shuffleWriteTime = totalTime
metrics.get.shuffleWriteMetrics = Some(shuffleMetrics)
success = true
new MapStatus(blockManager.blockManagerId, compressedSizes)
} catch { casee: Exception =>
// If there is an exception from running the task, revert the partial writes
// and throw the exception upstream to Spark.
if (shuffle != null && shuffle.writers != null) {
for (writer <- shuffle.writers) {
writer.revertPartialWrites()
writer.close()
}
}
throwe
} finally {
// Release the writers back to the shuffle block manager.
if (shuffle != null && shuffle.writers != null) {
shuffle.releaseWriters(success)
}
// Execute the callbacks on task completion.
context.executeOnCompleteCallbacks()
}
}
关于SparkEnv
在ShuffleMapTask.runTask中开始就通过SparkEnv.get去获取SparkEnv里面的内容，
SparkEnv中主要通过ThreadLocal来存储此实例，
此实例中包含Akka actor,serializer,BlockManager,shuffle使用的MapoutputTracker等。
SparkEnv实例生成包含两部分，master与worker,
master是在sparkcontext生成时生成，worker是在executor生成时生成
因此现在我们来分析下这个类定义
针对每一个Worker中的executor会生成一个SparkEnv实例：
在Executor实例生成时，会执行发下代码：
设置当前executor的属性env为创建一个SparkEnv实例，此实例通过当前的executorId与当前的host生成。
private val env = {
if (!isLocal) {
val_env = SparkEnv.create(conf, executorId, slaveHostname, 0,
isDriver = false, isLocal = false)
SparkEnv.set(_env)
_env.metricsSystem.registerSource(executorSource)
_env
} else {
SparkEnv.get
}
}
针对master启动时生成的SparkEnv实例：
通过在生成SparkContext实例时，生成SparkEnv属性：
private[spark] val env = SparkEnv.create(
conf,
//注意：此处使用的是driver,表示这是一个driver程序(master),worker时这里传入的是具体的executorid
"<driver>",
conf.get("spark.driver.host"),
conf.get("spark.driver.port").toInt,
isDriver = true,
isLocal = isLocal)
SparkEnv.set(env)
生成的env实例，此实例是一个线程本地实例，每一个线程都有自己独立的SparkEnv
private val env = new ThreadLocal[SparkEnv]
声明可变的变量，用来存储最后变化的实例，通过sparkEnv.get时如果env不存在，会拿这个值
@volatileprivatevarlastSetSparkEnv : SparkEnv = _
def set(e: SparkEnv) {
lastSetSparkEnv = e
env.set(e)
}
def get: SparkEnv = {
Option(env.get()).getOrElse(lastSetSparkEnv)
}
下面是sparkenv的create函数：
private[spark] def create(
conf: SparkConf,
executorId: String,
hostname: String,
port: Int,
isDriver: Boolean,
isLocal: Boolean): SparkEnv = {
val (actorSystem, boundPort) = AkkaUtils.createActorSystem("spark", hostname, port,
conf = conf)
// Bit of a hack: If this is the driver and our port was 0 (meaning bind to any free port),
// figure out which port number Akka actually bound to and set spark.driver.port to it.
if (isDriver && port == 0) {
conf.set("spark.driver.port", boundPort.toString)
}
valclassLoader = Thread.currentThread.getContextClassLoader
// Create an instance of the class named by the given Java system property, or by
// defaultClassName if the property is not set, and return it as a T
def instantiateClass[T](propertyName: String, defaultClassName: String): T = {
valname = conf.get(propertyName, defaultClassName)
Class.forName(name, true, classLoader).newInstance().asInstanceOf[T]
}
生成一个Serializermanager实例
valserializerManager = new SerializerManager
得到配置的Serializer实例，这个地方有部分资料建议配置为org.apache.spark.serializer.KryoSerializer.
请参见http://spark.apache.org/docs/0.9.0/tuning.html的说明。
valserializer = serializerManager.setDefault(
conf.get("spark.serializer", "org.apache.spark.serializer.JavaSerializer"), conf)
闭包使用的serializer,如果闭包中函数使用了大量的对象，可修改默认的值
valclosureSerializer = serializerManager.get(
conf.get("spark.closure.serializer", "org.apache.spark.serializer.JavaSerializer"),
conf)
此部分检查是否是driver(也就是是否是master)
def registerOrLookup(name: String, newActor: => Actor): ActorRef = {
如果是master时，生成一个actor的实例，
if (isDriver) {
logInfo("Registering " + name)
actorSystem.actorOf(Props(newActor), name = name)
} else {
否则表示是worker,生成一个actor的引用。对指定的actor进行连接，生成actorref
valdriverHost: String = conf.get("spark.driver.host", "localhost")
valdriverPort: Int = conf.getInt("spark.driver.port", 7077)
Utils.checkHost(driverHost, "Expected hostname")
valurl = s"akka.tcp://spark@$driverHost:$driverPort/user/$name"
valtimeout = AkkaUtils.lookupTimeout(conf)
logInfo(s"Connecting to $name: $url")
Await.result(actorSystem.actorSelection(url).resolveOne(timeout), timeout)
}
}
此处生成BlockManagerMaster实例，如果是driver时，
会生成一个名称为BlockManagerMaster的BlockManagerMasterActor实例。
否则表示是worker,生成BlockManagerMaster,并创建与master中的BlockManagerMasterActor的actorref引用。
BlockManagerMasterActor中通过配置spark.storage.blockManagerTimeoutIntervalMs，,默认值为60000ms
定期检查上面注册的BlockManagerId是否过期。
valblockManagerMaster = new BlockManagerMaster(registerOrLookup(
"BlockManagerMaster",
new BlockManagerMasterActor(isLocal, conf)), conf)
生成BlockManager,BlockManager中会生成ShuffleBlockManager,DiskBlockManager,memory/disk的store.
针对此BlockManager，生成一个BlockManagerId实例，
通过master的actor(BlockManagerMasterActor),向master注册此block,并定期向master发送心跳。
心跳的发送通过spark.storage.blockManagerTimeoutIntervalMs配置的值/4
valblockManager = new BlockManager(executorId,
actorSystem, blockManagerMaster, serializer, conf)
valconnectionManager = blockManager.connectionManager
valbroadcastManager = new BroadcastManager(isDriver, conf)
生成CacheManager,
valcacheManager = new CacheManager(blockManager)
生成MapOutputTracker,如果是master时，生成 MapOutputTrackerMaster，否则生成 MapOutputTracker
// Have to assign trackerActor after initialization as MapOutputTrackerActor
// requires the MapOutputTracker itself
valmapOutputTracker = if (isDriver) {
new MapOutputTrackerMaster(conf)
} else {
new MapOutputTracker(conf)
}
如果是master时，生成 MapOutputTrackerMasterActor实例，否则生成对actor的引用。
mapOutputTracker.trackerActor = registerOrLookup(
"MapOutputTracker",
new MapOutputTrackerMasterActor(mapOutputTracker.asInstanceOf[MapOutputTrackerMaster]))
生成ShuffleFetcher的实例，通过spark.shuffle.fetcher 配置，默认为BlockStoreShuffleFetcher。
valshuffleFetcher = instantiateClass[ShuffleFetcher](
"spark.shuffle.fetcher", "org.apache.spark.BlockStoreShuffleFetcher")
valhttpFileServer = new HttpFileServer()
httpFileServer.initialize()
conf.set("spark.fileserver.uri", httpFileServer.serverUri)
valmetricsSystem = if (isDriver) {
MetricsSystem.createMetricsSystem("driver", conf)
} else {
MetricsSystem.createMetricsSystem("executor", conf)
}
metricsSystem.start()
// Set the sparkFiles directory, used when downloading dependencies. In local mode,
// this is a temporary directory; in distributed mode, this is the executor's current working
// directory.
valsparkFilesDir: String = if (isDriver) {
Utils.createTempDir().getAbsolutePath
} else {
"."
}
// Warn about deprecated spark.cache.class property
if (conf.contains("spark.cache.class")) {
logWarning("The spark.cache.class property is no longer being used! Specify storage " +
"levels using the RDD.persist() method instead.")
}
new SparkEnv(
executorId,
actorSystem,
serializerManager,
serializer,
closureSerializer,
cacheManager,
mapOutputTracker,
shuffleFetcher,
broadcastManager,
blockManager,
connectionManager,
httpFileServer,
sparkFilesDir,
metricsSystem,
conf)
}
ShuffleBlockManager.forMapTask函数
shuffleBlockManager.forMapTask函数是shufflemaptask运行shuffle的核心函数，
此函数中会生成ShuffleWriterGroup实例，
并根据运行的task个数，通常是cpu core个数*reduce的partition个shuffle个文件，每一次的运行都会生成这么多个文件。
因此这部分会同时打开core*reduceparitionnum个file,每一个的maptask运行都会生成这么多个文件。
此部分完成后就会产生大量的map output文件个数，总文件个数为maptasknum*reducetasknum个文件。
同时spark中为了控制文件的生成个数，可通过spark.shuffle.consolidateFiles配置是否重用write文件。默认为false,
如果此值设置为true,每一个worker通常只生成core*reducetasknum个文件。
每一个文件打开通过spark.shuffle.file.buffer.kb配置的缓存大小。默认为100kb，也就是一次运行中
每一个worker中会有core*reducetasknum*100kb的内存buffer的使用。由这部分我个人认为，
这玩意还是不合适maptask的任务太多的分析任务。Mapreduce的shuffle从性能上会比这要慢一些，
但是从对大数据量的支持上还是要好一些。
函数定义
def forMapTask(shuffleId: Int, mapId: Int, numBuckets: Int, serializer: Serializer) = {
生成一个ShuffleWriterGroup实例
new ShuffleWriterGroup {
shuffleStates.putIfAbsent(shuffleId, new ShuffleState(numBuckets))
privatevalshuffleState = shuffleStates(shuffleId)
privatevarfileGroup: ShuffleFileGroup = null
如果spark.shuffle.consolidateFiles配置的值为true,检查是否有上次生成的writer文件，重新打开这个文件。
也就是在文件中进行append操作。
valwriters: Array[BlockObjectWriter] = if (consolidateShuffleFiles) {
fileGroup = getUnusedFileGroup()
Array.tabulate[BlockObjectWriter](numBuckets) { bucketId =>
valblockId = ShuffleBlockId(shuffleId, mapId, bucketId)
blockManager.getDiskWriter(blockId, fileGroup(bucketId), serializer, bufferSize)
}
} else {
否则每一个task都会生成新的writer文件。
Array.tabulate[BlockObjectWriter](numBuckets) { bucketId =>
valblockId = ShuffleBlockId(shuffleId, mapId, bucketId)
此处主要是通过sparkenv中的diskBlockMangaer来在指定的路径下生成文件。
路径通过spark.local.dir配置。默认为java.io.tmpdir。
valblockFile = blockManager.diskBlockManager.getFile(blockId)
// Because of previous failures, the shuffle file may already exist on this machine.
// If so, remove it.
if (blockFile.exists) {
if (blockFile.delete()) {
logInfo(s"Removed existing shuffle file $blockFile")
} else {
logWarning(s"Failed to remove existing shuffle file $blockFile")
}
}
blockManager.getDiskWriter(blockId, blockFile, serializer, bufferSize)
}
}
这个函数在shuffleMapTask执行完成的时候调用。如果上面提到的配置为true时，
会把writer的blockfile放到一个容器中，下一次task运行时，会直接打开这个blockfile文件。
overridedef releaseWriters(success: Boolean) {
if (consolidateShuffleFiles) {
if (success) {
valoffsets = writers.map(_.fileSegment().offset)
fileGroup.recordMapOutput(mapId, offsets)
}
recycleFileGroup(fileGroup)
} else {
shuffleState.completedMapTasks.add(mapId)
}
}
privatedef getUnusedFileGroup(): ShuffleFileGroup = {
valfileGroup = shuffleState.unusedFileGroups.poll()
if (fileGroup != null) fileGroupelse newFileGroup()
}
privatedef newFileGroup(): ShuffleFileGroup = {
valfileId = shuffleState.nextFileId.getAndIncrement()
valfiles = Array.tabulate[File](numBuckets) { bucketId =>
valfilename = physicalFileName(shuffleId, bucketId, fileId)
blockManager.diskBlockManager.getFile(filename)
}
valfileGroup = new ShuffleFileGroup(fileId, shuffleId, files)
shuffleState.allFileGroups.add(fileGroup)
fileGroup
}
privatedef recycleFileGroup(group: ShuffleFileGroup) {
shuffleState.unusedFileGroups.add(group)
}
}
}
DAGShuduler中注册shuffleid与mapStatus
在DAGSheduler的调度中，启动一个stage时，如果是shuffle stage,会执行如下代码：
DAGsheduler.runjob-->submitJob-->JobSubmitted actor-->
newStage传入参数getParentStages-->getShuffleMapStage-->newOrUsedStage
private def newOrUsedStage(
rdd: RDD[_],
numTasks: Int,
shuffleDep: ShuffleDependency[_,_],
jobId: Int,
callSite: Option[String] = None)
: Stage =
{
valstage = newStage(rdd, numTasks, Some(shuffleDep), jobId, callSite)
if (mapOutputTracker.has(shuffleDep.shuffleId)) {
valserLocs = mapOutputTracker.getSerializedMapOutputStatuses(shuffleDep.shuffleId)
vallocs = MapOutputTracker.deserializeMapStatuses(serLocs)
for (i <- 0 until locs.size) stage.outputLocs(i) = List(locs(i))
stage.numAvailableOutputs = locs.size
} else {
在master中注册此shuffleid
// Kind of ugly: need to register RDDs with the cache and map output tracker here
// since we can't do it in the RDD constructor because # of partitions is unknown
logInfo("Registering RDD " + rdd.id + " (" + rdd.origin + ")")
mapOutputTracker.registerShuffle(shuffleDep.shuffleId, rdd.partitions.size)
}
stage
}
回到dagsheduler的调度中，当shuffle的所有的task处理完成后，会调用如下代码：
....
execBackend.statusUpdate(taskId, TaskState.FINISHED, serializedResult)
.....
case smt: ShuffleMapTask =>
valstatus = event.result.asInstanceOf[MapStatus]
valexecId = status.location.executorId
logDebug("ShuffleMapTask finished on " + execId)
if (failedEpoch.contains(execId) && smt.epoch <= failedEpoch(execId)) {
logInfo("Ignoring possibly bogus ShuffleMapTask completion from " + execId)
} else {
第一个task完成后，都会把map返回的MapStatus(记录有location信息)记录到stage的outputloc中。
stage.addOutputLoc(smt.partitionId, status)
}
if (running.contains(stage) && pendingTasks(stage).isEmpty) {
markStageAsFinished(stage)
logInfo("looking for newly runnable stages")
logInfo("running: " + running)
logInfo("waiting: " + waiting)
logInfo("failed: " + failed)
if (stage.shuffleDep != None) {
.........................................
如果所有的shuffle的task都执行完成，把此stage对应的shuffled与所有的location注册到mapOutputTracker中
此处是通过DAGSheculer来完成的，因此，mapoutputtracker是一个MapOutputTrackerMaster的实现。
mapOutputTracker.registerMapOutputs(
stage.shuffleDep.get.shuffleId,
stage.outputLocs.map(list => if (list.isEmpty) nullelse list.head).toArray,
changeEpoch = true)
}
Shuffle的读取计算
此时shuffle的MAP RDD执行完成后，会通过PairRDDFunctions来做处理
回到PairRDDFunctions中的reduceByKey，
reduceByKey-->combineByKey
再次来看这个函数的定义
def combineByKey[C](createCombiner: V => C,
mergeValue: (C, V) => C,
mergeCombiners: (C, C) => C,
partitioner: Partitioner,
mapSideCombine: Boolean = true,
serializerClass: String = null): RDD[(K, C)] = {
if (getKeyClass().isArray) {
if (mapSideCombine) {
thrownew SparkException("Cannot use map-side combining with array keys.")
}
if (partitioner.isInstanceOf[HashPartitioner]) {
thrownew SparkException("Default partitioner cannot partition array keys.")
}
}
valaggregator = new Aggregator[K, V, C](createCombiner, mergeValue, mergeCombiners)
如果当前的RDD的partitioner与传入的partitioner相等，表示是一个map,不需要进行shuffle,直接在map端合并。
if (self.partitioner == Some(partitioner)) {
self.mapPartitionsWithContext((context, iter) => {
new InterruptibleIterator(context, aggregator.combineValuesByKey(iter, context))
}, preservesPartitioning = true)
} elseif (mapSideCombine) {
如果设置有在map端先进行一次合并，类似于mapreduce中的combine,先在map端执行一次合并，
并生成MapPartitionsRDD
valcombined = self.mapPartitionsWithContext((context, iter) => {
aggregator.combineValuesByKey(iter, context)
}, preservesPartitioning = true)
生成一个ShuffledRDD实例，在reduce端执行合并操作。合并的核心函数是aggregator实例中定义的相关函数。
valpartitioned = new ShuffledRDD[K, C, (K, C)](combined, partitioner)
.setSerializer(serializerClass)
partitioned.mapPartitionsWithContext((context, iter) => {
new InterruptibleIterator(context, aggregator.combineCombinersByKey(iter, context))
}, preservesPartitioning = true)
} else {
不执行combiner操作，直接在reduce端进行shuffle操作。
// Don't apply map-side combiner.
valvalues = new ShuffledRDD[K, V, (K, V)](self, partitioner).setSerializer(serializerClass)
values.mapPartitionsWithContext((context, iter) => {
new InterruptibleIterator(context, aggregator.combineValuesByKey(iter, context))
}, preservesPartitioning = true)
}
}
在Reduce端，生成为ShuffledRDD。数据计算函数通过compute函数完成。
ShuffledRDD中计算函数的实现
override def compute(split: Partition, context: TaskContext): Iterator[P] = {
valshuffledId = dependencies.head.asInstanceOf[ShuffleDependency[K, V]].shuffleId
通过指定的shuffledid,拿到shuffle完成的数据。
SparkEnv.get.shuffleFetcher.fetch[P](shuffledId, split.index, context,
SparkEnv.get.serializerManager.get(serializerClass, SparkEnv.get.conf))
}
从SparkEnv中拿到shuffleFetcher的实例。从SparkEnv生成来看，
通过spark.shuffle.fetcher 配置，默认为BlockStoreShuffleFetcher。
Sparkenv中的定义
valshuffleFetcher = instantiateClass[ShuffleFetcher](
"spark.shuffle.fetcher", "org.apache.spark.BlockStoreShuffleFetcher")
BlockStoreShuffleFetcher.fetch的函数：
override def fetch[T](
shuffleId: Int,
reduceId: Int,
context: TaskContext,
serializer: Serializer)
: Iterator[T] =
{
logDebug("Fetching outputs for shuffle %d, reduce %d".format(shuffleId, reduceId))
valblockManager = SparkEnv.get.blockManager
valstartTime = System.currentTimeMillis
在executor中的mapoutputtracker会通过GetMapOutputStatuses事件
向mapoutputtrackermaster中的MapOutputTrackerMasterActor发起得到所有的mapStatus事件。
valstatuses = SparkEnv.get.mapOutputTracker.getServerStatuses(shuffleId, reduceId)
...........................
valsplitsByAddress = new HashMap[BlockManagerId, ArrayBuffer[(Int, Long)]]
把BlockManagerid相同的map结果进行合并，index的值就是map的partition
for (((address, size), index) <- statuses.zipWithIndex) {
splitsByAddress.getOrElseUpdate(address, ArrayBuffer()) += ((index, size))
}
得到每一个map的输出文件的结果集地址，地址由shuffleid,mappartitionnum,reduceparttion组成。
valblocksByAddress: Seq[(BlockManagerId, Seq[(BlockId, Long)])] = splitsByAddress.toSeq.map {
case (address, splits) =>
(address, splits.map(s => (ShuffleBlockId(shuffleId, s._1, reduceId), s._2)))
}
def unpackBlock(blockPair: (BlockId, Option[Iterator[Any]])) : Iterator[T] = {
valblockId = blockPair._1
valblockOption = blockPair._2
blockOptionmatch {
case Some(block) => {
block.asInstanceOf[Iterator[T]]
}
case None => {
blockIdmatch {
case ShuffleBlockId(shufId, mapId, _) =>
valaddress = statuses(mapId.toInt)._1
thrownew FetchFailedException(address, shufId.toInt, mapId.toInt, reduceId, null)
case _ =>
thrownew SparkException(
"Failed to get block " + blockId + ", which is not a shuffle block")
}
}
}
}
通过blockManager从blockid中获取Iterator,用来得到数据
这里的blockManager中reduce进行shuffle的具体有两个实现，默认为BasicBlockFetcherIterator，
如果spark.shuffle.use.netty配置为true时，实现类为NettyBlockFetcherIterator。
在BasicBlockFetcherIterator中通过nio的方式使用sparkenv中的ConnectionManager来接收数据，
而NettyBlockFetcherIterator通过netty的通信框架进行操作，使用netty时，
通过reduce端spark.shuffle.copier.threads配置的线程数来获取数据，默认的线程个数为6.
valblockFetcherItr = blockManager.getMultiple(blocksByAddress, serializer)
取出每一个blockid中的values部分的iterator.
valitr = blockFetcherItr.flatMap(unpackBlock)
valcompletionIter = CompletionIterator[T, Iterator[T]](itr, {
valshuffleMetrics = new ShuffleReadMetrics
shuffleMetrics.shuffleFinishTime = System.currentTimeMillis
shuffleMetrics.remoteFetchTime = blockFetcherItr.remoteFetchTime
shuffleMetrics.fetchWaitTime = blockFetcherItr.fetchWaitTime
shuffleMetrics.remoteBytesRead = blockFetcherItr.remoteBytesRead
shuffleMetrics.totalBlocksFetched = blockFetcherItr.totalBlocks
shuffleMetrics.localBlocksFetched = blockFetcherItr.numLocalBlocks
shuffleMetrics.remoteBlocksFetched = blockFetcherItr.numRemoteBlocks
context.taskMetrics.shuffleReadMetrics = Some(shuffleMetrics)
})
new InterruptibleIterator[T](context, completionIter)
}
通过MapOutputTracker得到shuffle的stage的map完成的mapstatus
上面得到MapStatus的容器的函数定义
def getServerStatuses(shuffleId: Int, reduceId: Int): Array[(BlockManagerId, Long)] = {
检查executor本地是否有此shuffleid的mapstatuses信息，
valstatuses = mapStatuses.get(shuffleId).orNull
如果本地还没有shuffle的状态数据(所有的shuffle完成的状态都需要从master中同步过来)，
if (statuses == null) {
logInfo("Don't have map outputs for shuffle " + shuffleId + ", fetching them")
varfetchedStatuses: Array[MapStatus] = null
出于线程安全考虑，
fetching.synchronized {
如果shuffleid已经在fetching中存在，等待shuffle从master获取MapStatus完成。
这里主要是为了多个task同时来获取数据，第一个task已经向master发起申请，
第二个就不需要在发起只需要等待第一个完成申请并得到数据存储到fetchedStatuses中。
if (fetching.contains(shuffleId)) {
// Someone else is fetching it; wait for them to be done
while (fetching.contains(shuffleId)) {
try {
fetching.wait()
} catch {
casee: InterruptedException =>
}
}
}
if (fetchedStatuses == null) {
// We won the race to fetch the output locs; do so
logInfo("Doing the fetch; tracker actor = " + trackerActor)
// This try-finally prevents hangs due to timeouts:
try {
通过askTracker函数，通过actorref向MapoutputTrackerMasterActor发起GetMapOutputStatuses事件。
得到此stage完成的所有的task的MapStatus信息
valfetchedBytes =
askTracker(GetMapOutputStatuses(shuffleId)).asInstanceOf[Array[Byte]]
解析成fetchedStatuses数据。
fetchedStatuses = MapOutputTracker.deserializeMapStatuses(fetchedBytes)
logInfo("Got the output locations")
添加到executor中的MapStatuses容器中。缓存起来，共下一个task实例。
mapStatuses.put(shuffleId, fetchedStatuses)
} finally {
从master中获取数据完成，把fetching中的shuffleid移出。
fetching.synchronized {
fetching -= shuffleId
fetching.notifyAll()
}
}
}
if (fetchedStatuses != null) {
fetchedStatuses.synchronized {
通过指定的shuffleid与reduceid的值，得到此reduce在blockid中要获取数据的大小。
return MapOutputTracker.convertMapStatuses(shuffleId, reduceId, fetchedStatuses)
}
}
else {
thrownew FetchFailedException(null, shuffleId, -1, reduceId,
new Exception("Missing all output locations for shuffle " + shuffleId))
}
} else {
通过指定的shuffleid与reduceid的值，得到此reduce在blockid中要获取数据的大小。local的cache模式
statuses.synchronized {
return MapOutputTracker.convertMapStatuses(shuffleId, reduceId, statuses)
}
}
}
MapOutputTracker.convertMapStatuses函数
private def convertMapStatuses(
shuffleId: Int,
reduceId: Int,
statuses: Array[MapStatus]): Array[(BlockManagerId, Long)] = {
assert (statuses != null)
statuses.map {
status =>
if (status == null) {
thrownew FetchFailedException(null, shuffleId, -1, reduceId,
new Exception("Missing an output location for shuffle " + shuffleId))
} else {
取出MapStatus中，针对此reduce的partition中的shuffle的内容大小。
(status.location, decompressSize(status.compressedSizes(reduceId)))
}
}
}
........