为了有效地实现容错，RDD提供了一种高度受限的共享内存，即RDD是只读的，并且只能通过其他RDD上的批量操作来创建（注：还可以由外部存储系数据集创建，如HDFS）
可知，我们在第九，第十篇博文所讲的是传统Hadoop MapReduce类似的，在最初从HDFS中读取数据生成HadoopRDD的过程。而RDD可以通过其他RDD上的批量操作来创建，所以这里的HadoopRDD对于下一个生成的ShuffledRDD可以视为Map端，当然下一个生成的ShuffledRDD可以被下下个ShuffledRDD视为Map端。反过来说，下一个ShuffledRDD可以被``HadoopRDD视作Reduce`端。
这篇博文，我们就来讲下Shuffle的Reduce端。其实在RDD迭代部分和第九篇博文类似，不同的是，这里调用的是rdd.ShuffledRDD.compute:
override def compute(split: Partition, context: TaskContext): Iterator[(K, C)] = { // 得到依赖
val dep = dependencies.head.asInstanceOf[ShuffleDependency[K, V, C]] // 调用getReader，传入dep.shuffleHandle 分区 上下文
// 得到Reader，调用read()
// 得到迭代器
SparkEnv.get.shuffleManager.getReader(dep.shuffleHandle, split.index, split.index + 1, context)
.read()
.asInstanceOf[Iterator[(K, C)]]
}这里调用的是shuffle.sort.SortShuffleManager的getReader：
override def getReader[K, C](
handle: ShuffleHandle,
startPartition: Int,
endPartition: Int,
context: TaskContext): ShuffleReader[K, C] = { // 生成返回 BlockStoreShuffleReader
new BlockStoreShuffleReader(
handle.asInstanceOf[BaseShuffleHandle[K, _, C]], startPartition, endPartition, context)
}shuffle.BlockStoreShuffleReader.read:
override def read(): Iterator[Product2[K, C]] = { // 实例化ShuffleBlockFetcherIterator
val blockFetcherItr = new ShuffleBlockFetcherIterator(
context,
blockManager.shuffleClient,
blockManager, // 通过消息发送获取 ShuffleMapTask 存储数据位置的元数据
mapOutputTracker.getMapSizesByExecutorId(handle.shuffleId, startPartition, endPartition), // 设置每次传输的大小
SparkEnv.get.conf.getSizeAsMb("spark.reducer.maxSizeInFlight", "48m") * 1024 * 1024, // // 设置Int的大小
SparkEnv.get.conf.getInt("spark.reducer.maxReqsInFlight", Int.MaxValue)) // 基于配置的压缩和加密来包装流
val wrappedStreams = blockFetcherItr.map { case (blockId, inputStream) =>
serializerManager.wrapStream(blockId, inputStream)
} val serializerInstance = dep.serializer.newInstance() // 对每个流生成 k/v 迭代器
val recordIter = wrappedStreams.flatMap { wrappedStream =>
serializerInstance.deserializeStream(wrappedStream).asKeyValueIterator
} // 每条记录读取后更新任务度量
val readMetrics = context.taskMetrics.createTempShuffleReadMetrics() // 生成完整的迭代器
val metricIter = CompletionIterator[(Any, Any), Iterator[(Any, Any)]](
recordIter.map { record =>
readMetrics.incRecordsRead(1)
record
},
context.taskMetrics().mergeShuffleReadMetrics()) // 传入metricIter到可中断的迭代器
// 为了能取消迭代
val interruptibleIter = new InterruptibleIterator[(Any, Any)](context, metricIter) val aggregatedIter: Iterator[Product2[K, C]] = if (dep.aggregator.isDefined) { // 若需要对数据进行聚合
if (dep.mapSideCombine) { // 若需要进行Map端（对于下一个Shuffle来说）的合并
val combinedKeyValuesIterator = interruptibleIter.asInstanceOf[Iterator[(K, C)]]
dep.aggregator.get.combineCombinersByKey(combinedKeyValuesIterator, context) // 若只需要进行Reduce端（对于下一个Shuffle来说）的合并
} else { val keyValuesIterator = interruptibleIter.asInstanceOf[Iterator[(K, Nothing)]]
dep.aggregator.get.combineValuesByKey(keyValuesIterator, context)
}
} else {
require(!dep.mapSideCombine, "Map-side combine without Aggregator specified!")
interruptibleIter.asInstanceOf[Iterator[Product2[K, C]]]
}
dep.keyOrdering match { case Some(keyOrd: Ordering[K]) => // 若需要排序
// 若spark.shuffle.spill设置为否的话
// 将不会spill到磁盘
val sorter = new ExternalSorter[K, C, C](context, ordering = Some(keyOrd), serializer = dep.serializer)
sorter.insertAll(aggregatedIter)
context.taskMetrics().incMemoryBytesSpilled(sorter.memoryBytesSpilled)
context.taskMetrics().incDiskBytesSpilled(sorter.diskBytesSpilled)
context.taskMetrics().incPeakExecutionMemory(sorter.peakMemoryUsedBytes) CompletionIterator[Product2[K, C], Iterator[Product2[K, C]]](sorter.iterator, sorter.stop()) case None =>
aggregatedIter
}
}类调用关系图：
这里写图片描述
下面我们来深入讲解下实例化ShuffleBlockFetcherIterator的过程:
// 实例化ShuffleBlockFetcherIterator
val blockFetcherItr = new ShuffleBlockFetcherIterator(
context,
blockManager.shuffleClient,
blockManager, // 通过消息发送获取 ShuffleMapTask 存储数据位置的元数据
mapOutputTracker.getMapSizesByExecutorId(handle.shuffleId, startPartition, endPartition), // 设置每次传输的大小
SparkEnv.get.conf.getSizeAsMb("spark.reducer.maxSizeInFlight", "48m") * 1024 * 1024, // // 设置Int的大小
SparkEnv.get.conf.getInt("spark.reducer.maxReqsInFlight", Int.MaxValue))获取元数据
mapOutputTracker.getMapSizesByExecutorId
首先我们会调用mapOutputTracker.getMapSizesByExecutorId：
def getMapSizesByExecutorId(shuffleId: Int, startPartition: Int, endPartition: Int)
: Seq[(BlockManagerId, Seq[(BlockId, Long)])] = {
logDebug(s"Fetching outputs for shuffle $shuffleId, partitions $startPartition-$endPartition") // 得到元数据
val statuses = getStatuses(shuffleId) // 返回格式为：
// Seq[BlockManagerId,Seq[(shuffle block id, shuffle block size)]]
statuses.synchronized { return MapOutputTracker.convertMapStatuses(shuffleId, startPartition, endPartition, statuses)
}
}mapOutputTracker.getStatuses
private def getStatuses(shuffleId: Int): Array[MapStatus] = { // 尝试从本地获取数据
val statuses = mapStatuses.get(shuffleId).orNull if (statuses == null) { // 若本地无数据
logInfo("Don't have map outputs for shuffle " + shuffleId + ", fetching them") val startTime = System.currentTimeMillis var fetchedStatuses: Array[MapStatus] = null
fetching.synchronized { // 若以及有其他人也准备远程获取这数据的话
// 则等待
while (fetching.contains(shuffleId)) { try {
fetching.wait()
} catch { case e: InterruptedException =>
}
} // 尝试直接获取数据
fetchedStatuses = mapStatuses.get(shuffleId).orNull if (fetchedStatuses == null) { // 若还是不得不远程获取，
// 则将shuffleId加入fetching
fetching += shuffleId
}
} if (fetchedStatuses == null) {
logInfo("Doing the fetch; tracker endpoint = " + trackerEndpoint) try { // 远程获取
val fetchedBytes = askTracker[Array[Byte]](GetMapOutputStatuses(shuffleId)) // 反序列化
fetchedStatuses = MapOutputTracker.deserializeMapStatuses(fetchedBytes)
logInfo("Got the output locations") // 将数据加入mapStatuses
mapStatuses.put(shuffleId, fetchedStatuses)
} finally {
fetching.synchronized {
fetching -= shuffleId
fetching.notifyAll()
}
}
}
logDebug(s"Fetching map output statuses for shuffle $shuffleId took " + s"${System.currentTimeMillis - startTime} ms") if (fetchedStatuses != null) { // 若直接获取，则直接返回
return fetchedStatuses
} else {
logError("Missing all output locations for shuffle " + shuffleId) throw new MetadataFetchFailedException(
shuffleId, -1, "Missing all output locations for shuffle " + shuffleId)
}
} else { // 若直接获取，则直接返回
return statuses
}
}mapOutputTracker.askTracker
向trackerEndpoint发送消息GetMapOutputStatuses(shuffleId)
protected def askTracker[T: ClassTag](message: Any): T = { try {
trackerEndpoint.askWithRetry[T](message)
} catch { case e: Exception =>
logError("Error communicating with MapOutputTracker", e) throw new SparkException("Error communicating with MapOutputTracker", e)
}
}MapOutputTrackerMasterEndpoint.receiveAndReply
case GetMapOutputStatuses(shuffleId: Int) => val hostPort = context.senderAddress.hostPort
logInfo("Asked to send map output locations for shuffle " + shuffleId + " to " + hostPort) val mapOutputStatuses = tracker.post(new GetMapOutputMessage(shuffleId, context))可以看到，这里并不是直接返回消息，而是调用tracker.post:
def post(message: GetMapOutputMessage): Unit = {
mapOutputRequests.offer(message)
}向mapOutputRequests加入GetMapOutputMessage(shuffleId, context)消息。这里的mapOutputRequests是链式阻塞队列。
private val mapOutputRequests = new LinkedBlockingQueue[GetMapOutputMessage]
MapOutputTrackerMaster.MessageLoop.run
MessageLoop启一个线程不断的参数从mapOutputRequests读取数据：
private class MessageLoop extends Runnable { override def run(): Unit = { try { while (true) { try { val data = mapOutputRequests.take() if (data == PoisonPill) {
mapOutputRequests.offer(PoisonPill) return
} val context = data.context val shuffleId = data.shuffleId val hostPort = context.senderAddress.hostPort
logDebug("Handling request to send map output locations for shuffle " + shuffleId + " to " + hostPort) // 若读到数据
// 则序列化
val mapOutputStatuses = getSerializedMapOutputStatuses(shuffleId) // 返回数据
context.reply(mapOutputStatuses)
} catch { case NonFatal(e) => logError(e.getMessage, e)
}
}
} catch { case ie: InterruptedException => // exit
}
}
}MapOutputTracker.convertMapStatuses
我们回到mapOutputTracker.getMapSizesByExecutorId中返回的MapOutputTracker.convertMapStatuses：
private def convertMapStatuses(
shuffleId: Int,
startPartition: Int,
endPartition: Int,
statuses: Array[MapStatus]): Seq[(BlockManagerId, Seq[(BlockId, Long)])] = {
assert (statuses != null) val splitsByAddress = new HashMap[BlockManagerId, ArrayBuffer[(BlockId, Long)]] for ((status, mapId) <- statuses.zipWithIndex) { if (status == null) { val errorMessage = s"Missing an output location for shuffle $shuffleId"
logError(errorMessage) throw new MetadataFetchFailedException(shuffleId, startPartition, errorMessage)
} else { for (part <- startPartition until endPartition) { // 返回的Seq中的结构是status.location，Seq[ShuffleBlockId,SizeForBlock]
splitsByAddress.getOrElseUpdate(status.location, ArrayBuffer()) +=
((ShuffleBlockId(shuffleId, mapId, part), status.getSizeForBlock(part)))
}
}
} // 对Seq根据status.location进行排序
splitsByAddress.toSeq
}划分本地和远程Block
让我回到new ShuffleBlockFetcherIterator
storage.ShuffleBlockFetcherIterator.initialize
当我们实例化ShuffleBlockFetcherIterator时，会调用initialize:
private[this] def initialize(): Unit = {
context.addTaskCompletionListener(_ => cleanup()) // 划分本地和远程的blocks
val remoteRequests = splitLocalRemoteBlocks() // 把远程请求随机的添加到队列中
fetchRequests ++= Utils.randomize(remoteRequests)
assert ((0 == reqsInFlight) == (0 == bytesInFlight), "expected reqsInFlight = 0 but found reqsInFlight = " + reqsInFlight + ", expected bytesInFlight = 0 but found bytesInFlight = " + bytesInFlight) // 发送远程请求获取blocks
fetchUpToMaxBytes() val numFetches = remoteRequests.size - fetchRequests.size
logInfo("Started " + numFetches + " remote fetches in" + Utils.getUsedTimeMs(startTime)) // 获取本地的Blocks
fetchLocalBlocks()
logDebug("Got local blocks in " + Utils.getUsedTimeMs(startTime))
}storage.ShuffleBlockFetcherIterator.splitLocalRemoteBlocks
private[this] def splitLocalRemoteBlocks(): ArrayBuffer[FetchRequest] = { // 是的远程请求最大长度为 maxBytesInFlight / 5
// maxBytesInFlight： 为单次航班请求的最大字节数
// 航班： 一批请求
// 1/5 ： 是为了提高请求批发度，允许5个请求分别从5个节点获取数据
val targetRequestSize = math.max(maxBytesInFlight / 5, 1L)
logDebug("maxBytesInFlight: " + maxBytesInFlight + ", targetRequestSize: " + targetRequestSize) // 缓存需要远程请求的FetchRequest对象
val remoteRequests = new ArrayBuffer[FetchRequest] // 总共 blocks 的数量
var totalBlocks = 0
// 我们从上文可知blocksByAddress是根据status.location进行排序的
for ((address, blockInfos) <- blocksByAddress) {
totalBlocks += blockInfos.size if (address.executorId == blockManager.blockManagerId.executorId) { // 若 executorId 相同 与本 blockManagerId.executorId，
// 则从本地获取
localBlocks ++= blockInfos.filter(_._2 != 0).map(_._1)
numBlocksToFetch += localBlocks.size
} else { // 否则 远程请求
// 得到迭代器
val iterator = blockInfos.iterator // 当前累计块的大小
var curRequestSize = 0L // 当前累加块
// 累加： 若向一个节点频繁的请求字节很少的Block，
// 那么会造成网络阻塞
var curBlocks = new ArrayBuffer[(BlockId, Long)] // iterator 中的block 都是同一节点的
while (iterator.hasNext) { val (blockId, size) = iterator.next() if (size > 0) {
curBlocks += ((blockId, size))
remoteBlocks += blockId
numBlocksToFetch += 1
curRequestSize += size
} else if (size < 0) { throw new BlockException(blockId, "Negative block size " + size)
} if (curRequestSize >= targetRequestSize) { // 若累加到大于远程请求的尺寸
// 往remoteRequests加入FetchRequest
remoteRequests += new FetchRequest(address, curBlocks)
curBlocks = new ArrayBuffer[(BlockId, Long)]
logDebug(s"Creating fetch request of $curRequestSize at $address")
curRequestSize = 0
}
} // 增加最后的请求
if (curBlocks.nonEmpty) {
remoteRequests += new FetchRequest(address, curBlocks)
}
}
}
logInfo(s"Getting $numBlocksToFetch non-empty blocks out of $totalBlocks blocks")
remoteRequests
}获取Block
storage.ShuffleBlockFetcherIterator.fetchUpToMaxBytes
我们回到storage.ShuffleBlockFetcherIterator.initialize的fetchUpToMaxBytes()来深入讲解下如何获取远程的Block：
private def fetchUpToMaxBytes(): Unit = { // Send fetch requests up to maxBytesInFlight
// 单次航班请求数要小于最大航班请求数
// 单次航班字节数数要小于最大航班字节数
while (fetchRequests.nonEmpty &&
(bytesInFlight == 0 ||
(reqsInFlight + 1 <= maxReqsInFlight &&
bytesInFlight + fetchRequests.front.size <= maxBytesInFlight))) {
sendRequest(fetchRequests.dequeue())
}
}storage.ShuffleBlockFetcherIterator.sendRequest
private[this] def sendRequest(req: FetchRequest) {
logDebug("Sending request for %d blocks (%s) from %s".format(
req.blocks.size, Utils.bytesToString(req.size), req.address.hostPort))
bytesInFlight += req.size
reqsInFlight += 1
// 可根据blockID查询block大小
val sizeMap = req.blocks.map { case (blockId, size) => (blockId.toString, size) }.toMap val remainingBlocks = new HashSet[String]() ++= sizeMap.keys val blockIds = req.blocks.map(_._1.toString) val address = req.address // 关于shuffleClient.fetchBlocks我们会在之后的博文讲解
shuffleClient.fetchBlocks(address.host, address.port, address.executorId, blockIds.toArray, new BlockFetchingListener { // 请求成功
override def onBlockFetchSuccess(blockId: String, buf: ManagedBuffer): Unit = { ShuffleBlockFetcherIterator.this.synchronized { if (!isZombie) {
buf.retain()
remainingBlocks -= blockId
results.put(new SuccessFetchResult(BlockId(blockId), address, sizeMap(blockId), buf,
remainingBlocks.isEmpty))
logDebug("remainingBlocks: " + remainingBlocks)
}
}
logTrace("Got remote block " + blockId + " after " + Utils.getUsedTimeMs(startTime))
} // 请求失败
override def onBlockFetchFailure(blockId: String, e: Throwable): Unit = {
logError(s"Failed to get block(s) from ${req.address.host}:${req.address.port}", e)
results.put(new FailureFetchResult(BlockId(blockId), address, e))
}
}
)
}storage.ShuffleBlockFetcherIterator.fetchLocalBlocks
我们再回过头来看获取本地blocks：
private[this] def fetchLocalBlocks() { // 获取迭代器
val iter = localBlocks.iterator while (iter.hasNext) { val blockId = iter.next() try { // 遍历获取数据
// blockManager.getBlockData 会在后续博文讲解
val buf = blockManager.getBlockData(blockId)
shuffleMetrics.incLocalBlocksFetched(1)
shuffleMetrics.incLocalBytesRead(buf.size)
buf.retain()
results.put(new SuccessFetchResult(blockId, blockManager.blockManagerId, 0, buf, false))
} catch { case e: Exception =>
logError(s"Error occurred while fetching local blocks", e)
results.put(new FailureFetchResult(blockId, blockManager.blockManagerId, e)) return
}
}
}
作者：小爷Souljoy
链接：https://www.jianshu.com/p/2131207184be
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