spark sortShuffleWriter源码学习

查看的源码为spark2.3

调用ShuffleMapTask的runTask方法

org.apache.spark.scheduler.ShuffleMapTask#runTask

ShuffleMapTask继承了org.apache.spark.scheduler.Task，重写了Task的runTask方法，在该方法中关于shuffle部分主要是获取shuffleManager，然后得到sortShuffleManager，然后再通过manager获取writer，得到sortShuffleWriter，然后调用writer方法

  override def runTask(context: TaskContext): MapStatus = {
    // Deserialize the RDD using the broadcast variable.
    val threadMXBean = ManagementFactory.getThreadMXBean
    val deserializeStartTime = System.currentTimeMillis()
    val deserializeStartCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime
    } else 0L
    val ser = SparkEnv.get.closureSerializer.newInstance()
    val (rdd, dep) = ser.deserialize[(RDD[_], ShuffleDependency[_, _, _])](
      ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
    _executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime
    _executorDeserializeCpuTime = if (threadMXBean.isCurrentThreadCpuTimeSupported) {
      threadMXBean.getCurrentThreadCpuTime - deserializeStartCpuTime
    } else 0L

//定义writer对象
    var writer: ShuffleWriter[Any, Any] = null
    try {
//获取shuffleManager
      val manager = SparkEnv.get.shuffleManager
//通过shuffleManager获取Writer对象,这里的partitionId传入的其实是mapId,每个map有个mapId
      writer = manager.getWriter[Any, Any](dep.shuffleHandle, partitionId, context)
//调用write方法。write方法如下
      writer.write(rdd.iterator(partition, context).asInstanceOf[Iterator[_ <: Product2[Any, Any]]])
      writer.stop(success = true).get
    } catch {
      case e: Exception =>
        try {
          if (writer != null) {
            writer.stop(success = false)
          }
        } catch {
          case e: Exception =>
            log.debug("Could not stop writer", e)
        }
        throw e
    }
  }

调用SortShuffleWriter的write方法

org.apache.spark.shuffle.sort.SortShuffleWriter#write

SortShuffleWriter继承了org.apache.spark.shuffle.ShuffleWriter并重写了其write方法

  /** Write a bunch of records to this task's output */
  override def write(records: Iterator[Product2[K, V]]): Unit = {
//根据是否存在map端聚合获取ExternalSorter对象（sorter）
    sorter = if (dep.mapSideCombine) {
      require(dep.aggregator.isDefined, "Map-side combine without Aggregator specified!")
      new ExternalSorter[K, V, C](
        context, dep.aggregator, Some(dep.partitioner), dep.keyOrdering, dep.serializer)
    } else {
      // In this case we pass neither an aggregator nor an ordering to the sorter, because we don't
      // care whether the keys get sorted in each partition; that will be done on the reduce side
      // if the operation being run is sortByKey.如果没有map-side聚合，那么创建sorter对象时候，aggregator和ordering将不传入对应的值
      new ExternalSorter[K, V, V](
        context, aggregator = None, Some(dep.partitioner), ordering = None, dep.serializer)
    }
//通过insertAll方法先写数据到buffer
    sorter.insertAll(records)

    // Don't bother including the time to open the merged output file in the shuffle write time,
    // because it just opens a single file, so is typically too fast to measure accurately
    // (see SPARK-3570).

//通过blockManager获取对应mapId.shuffleId的文件输出路径
    val output = shuffleBlockResolver.getDataFile(dep.shuffleId, mapId)
//返回与“path”位于同一目录中的临时文件的路径。
    val tmp = Utils.tempFileWith(output)
    try {
      val blockId = ShuffleBlockId(dep.shuffleId, mapId, IndexShuffleBlockResolver.NOOP_REDUCE_ID)
//将所有的数据合并到一个文件中
      val partitionLengths = sorter.writePartitionedFile(blockId, tmp)
//生成index文件，也就是每个reduce通过该index文件得知它哪些是属于它的数据
      shuffleBlockResolver.writeIndexFileAndCommit(dep.shuffleId, mapId, partitionLengths, tmp)
      mapStatus = MapStatus(blockManager.shuffleServerId, partitionLengths)
    } finally {
      if (tmp.exists() && !tmp.delete()) {
        logError(s"Error while deleting temp file ${tmp.getAbsolutePath}")
      }
    }
  }

ExternalSorter类

创建ExternalSorter对象时，各参数对应的意思。

class ExternalSorter[K, V, C](
    context: TaskContext,
    aggregator: Option[Aggregator[K, V, C]] = None,
    partitioner: Option[Partitioner] = None,
    ordering: Option[Ordering[K]] = None,
    serializer: Serializer = SparkEnv.get.serializer)

aggregator：在RDD shuffle时，map/reduce-side使用的aggregator
partitioner：对shuffle的输出，使用哪种partitioner对数据做分区，比如hashPartitioner或者rangePartitioner
ordering：根据哪个key做排序
serializer：使用哪种序列化，如果没有显示指定，默认使用spark.serializer参数值

从一个high level的角度看ExternalSorter到底做了什么？
第一：反复的将数据填充到内存buffer中（如果需要通过key做map-side聚合，则使用PartitionedAppendOnlyMap；如果不需要，则使用PartitionedPairBuffer），如下

// Data structures to store in-memory objects before we spill. Depending on whether we have an
  // Aggregator set, we either put objects into an AppendOnlyMap where we combine them, or we
  // store them in an array buffer.
  @volatile private var map = new PartitionedAppendOnlyMap[K, C]
  @volatile private var buffer = new PartitionedPairBuffer[K, C]

第二：在buffer中，通过key计算partition ID，通过partition ID对数据进行排序（partition ID可以理解为reduce ID,意思就是数据被分给了哪个reduce），为了避免对key调用多次partitioner，spark会将partition ID跟每一条数据一起存储。

第三：当buffer达到内存限制时（buffer默认大小32k，由spark.shuffle.file.buffer参数决定），会将buffer中的数据spill到文件中（每次spill都会生成一个文件），如果我们需要做map-side聚合，该文件生成时会通过partition ID先做排序，然后通过key或者key的hashcode值做二次排序。
第四：将spill形成的多个文件合并包括还在内存中的数据，文件合并时候将会排序，排序方式跟上面一样，生成数据文件dataFile以及索引文件indexFile
第五：最后调用stop方法，删除所有中间文件

结合下图更好理解

mapTask通过externalSorter生成多个文件，也就是fileSegment，最后每个map任务的所有filesegment将会合并成一个file

上图数据插入的是appendOnlyMap，也就是使用了map-side聚合，所以有merger value，appendOnlyMap在满了以后（默认32k）将spill成文件，多次spill生成多个文件，最后merge所有文件包括还在内存buffer中的数据。

调用ExternalSorter的insertAll方法

这一步主要是往buffer写数据，对数据分partition ID，buffer满了spill数据到磁盘且对数据排序

def insertAll(records: Iterator[Product2[K, V]]): Unit = {
    // TODO: stop combining if we find that the reduction factor isn't high如果合并比例不高的话，就不会继续合并了
    // 通过创建ExternalSorter对象时传入的aggregator获取是否存在合并
    val shouldCombine = aggregator.isDefined

    if (shouldCombine) {
      // Combine values in-memory first using our AppendOnlyMap
      val mergeValue = aggregator.get.mergeValue
      val createCombiner = aggregator.get.createCombiner
      var kv: Product2[K, V] = null
      val update = (hadValue: Boolean, oldValue: C) => {
          //合并值方式
        if (hadValue) mergeValue(oldValue, kv._2) else createCombiner(kv._2)
      }
      while (records.hasNext) {
        addElementsRead()
        kv = records.next()
        //这个map就是该类中定义的PartitionedAppendOnlyMap，getPartition方法通过key获取所属Partition ID（hashPartitioner）
        map.changeValue((getPartition(kv._1), kv._1), update)
        // buffer满的话将内存中的数据spill成文件
        maybeSpillCollection(usingMap = true)
      }
    } else {
      // Stick values into our buffer
      while (records.hasNext) {
        addElementsRead()
        val kv = records.next()
        //这个buffer就是该类中定义的PartitionedPairBuffer
        buffer.insert(getPartition(kv._1), kv._1, kv._2.asInstanceOf[C])
        maybeSpillCollection(usingMap = false)
      }
    }
  }

insertAll方法中调用maybeSpillCollection方法

  /**
   * Spill the current in-memory collection to disk if needed.
   *
   * @param usingMap whether we're using a map or buffer as our current in-memory collection
   * 不同的数据结构（也就是buffer）调用不同的方法
   */
  private def maybeSpillCollection(usingMap: Boolean): Unit = {
    var estimatedSize = 0L
    if (usingMap) {
      estimatedSize = map.estimateSize()
//maybeSpill方法会尝试申请buffer内存，如果申请到内存，则spill且返回false。否则true
      if (maybeSpill(map, estimatedSize)) {
//appendOnlyMap的数据spill以后，创建一个新的appendOnlyMap
        map = new PartitionedAppendOnlyMap[K, C]
      }
    } else {
      estimatedSize = buffer.estimateSize()
      if (maybeSpill(buffer, estimatedSize)) {
        buffer = new PartitionedPairBuffer[K, C]
      }
    }

    if (estimatedSize > _peakMemoryUsedBytes) {
      _peakMemoryUsedBytes = estimatedSize
    }
  }

maybeSpillCollection方法中调用maybeSpill方法，判断是否应该执行spill

  /**
   * Spills the current in-memory collection to disk if needed. Attempts to acquire more
   * memory before spilling.
   *    在spill之前会尝试申请内存，最后才判断是否真正执行spill
   * @param collection collection to spill to disk
   * @param currentMemory estimated size of the collection in bytes
   * @return true if `collection` was spilled to disk; false otherwise
   */
  protected def maybeSpill(collection: C, currentMemory: Long): Boolean = {
    var shouldSpill = false
    if (elementsRead % 32 == 0 && currentMemory >= myMemoryThreshold) {
      // Claim up to double our current memory from the shuffle memory pool；从上次spill以后，每读取32个元素判断一次，声明申请额外内存
      val amountToRequest = 2 * currentMemory - myMemoryThreshold
      val granted = acquireMemory(amountToRequest)
      myMemoryThreshold += granted
      // If we were granted too little memory to grow further (either tryToAcquire returned 0,
      // or we already had more memory than myMemoryThreshold), spill the current collection
      shouldSpill = currentMemory >= myMemoryThreshold
    }
    shouldSpill = shouldSpill || _elementsRead > numElementsForceSpillThreshold
    // Actually spill
    if (shouldSpill) {
      _spillCount += 1
      logSpillage(currentMemory)
      spill(collection)
      _elementsRead = 0
      _memoryBytesSpilled += currentMemory
      releaseMemory()
    }
    shouldSpill
  }

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