spark shuffle 相关细节整理

1.Shuffle Write 和Shuffle Read具体发生在哪里

2.哪里用到了Partitioner

3.何为mapSideCombine

4.何时进行排序

之前已经看过spark shuffle源码了，现在总结一下一些之前没有理解的小知识点，作为一个总结。

用户自定义的Partitioner存到了哪里？

假设用户在调用reduceByKey时，传递了一个自定义的Partitioner，那么，这个Partitioner会被保存到ShuffleRDD的ShuffleDependency中。在进行Shuffle Write时，会使用这个Partitioner来对finalRDD.iterator(partition)的计算结果shuffle到不同的Bucket中。

何为mapSideCombine

reduceByKey默认是开启了mapSideCombine的，在进行shuffle write时会进行本地聚合，在shuffle read时，也会合并一下。举一个例子更好：

shuffle write阶段：

partition0：[(hello,1),(hello,1)]

partition1:[(hello,1),(word,1),(word,1)]

mapSideCombine后：

partition0：[(hello,2)]

partition1:[(hello,1),(word,2)]

hash shuffle后：

[(hello,2),(hello,1)]

[(word,2)]

hash read阶段:

[(hello,3)]

[(word,2)]

何时排序

排序操作发生在shuffle read 阶段。在shuffle read 进行完mapSideCombine之后，就开始进行排序了。

reduceByKey做了什么？

假设我们对rdd1调用了reduceByKey，那么最终的RDD依赖关系如下：rdd1->ShuffleRDD。rdd1.reduceByKey中，会做如下非常重要的事情：创建ShuffleRDD，在创建ShuffleRDD的过程中最最最重要的就是会创建ShuffleDependency,这个ShuffleDependency中有Aggregator，Partitioner，Ordering,parentRDD,mapSideCombine等重要的信息。为什么说ShuffleDependency非常重要，因为他是沟通Shuffle Writer和Shuffle Reader的一个重要桥梁。

Shuffle Write

Shuffle Write 发生在ShuffleMapTask.runTask中。首先反序列出rdd1和那个ShuffleDependency:(rdd1,dep),然后调用rdd1.iterator(partition)获取计算结果，再对计算结果进行ShuffleWriter，代码如下：

override def runTask(context: TaskContext): MapStatus = {
    // Deserialize the RDD using the broadcast variable.
    val deserializeStartTime = System.currentTimeMillis()
    val ser = SparkEnv.get.closureSerializer.newInstance()
    //统计反序列化rdd和shuffleDependency的时间
    val (rdd, dep) = ser.deserialize[(RDD[_], ShuffleDependency[_, _, _])](
      ByteBuffer.wrap(taskBinary.value), Thread.currentThread.getContextClassLoader)
    _executorDeserializeTime = System.currentTimeMillis() - deserializeStartTime

    metrics = Some(context.taskMetrics)
    var writer: ShuffleWriter[Any, Any] = null
    try {
      val manager = SparkEnv.get.shuffleManager
      writer = manager.getWriter[Any, Any](dep.shuffleHandle, partitionId, context)
      writer.write(rdd.iterator(partition, context).asInstanceOf[Iterator[_ <: Product2[Any, Any]]])
      return 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
    }
  }

我们以HashSuffleWriter为例，在其write()，他就会用到mapSideCombine和Partitioner。如下：

/** Write a bunch of records to this task's output */
  override def write(records: Iterator[Product2[K, V]]): Unit = {
    val iter = if (dep.aggregator.isDefined) {
      if (dep.mapSideCombine) {
        dep.aggregator.get.combineValuesByKey(records, context)
      } else {
        records
      }
    } else {
      require(!dep.mapSideCombine, "Map-side combine without Aggregator specified!")
      records
    }

    for (elem <- iter) {
      val bucketId = dep.partitioner.getPartition(elem._1)
      shuffle.writers(bucketId).write(elem._1, elem._2)
    }
  }

Shuffle Read

shuffle Read发生在ShuffleRDD的compute中：

  override def compute(split: Partition, context: TaskContext): Iterator[(K, C)] = {
    val dep = dependencies.head.asInstanceOf[ShuffleDependency[K, V, C]]
    SparkEnv.get.shuffleManager.getReader(dep.shuffleHandle, split.index, split.index + 1, context)
      .read()
      .asInstanceOf[Iterator[(K, C)]]
  }

下面是HashShuffleReader的read():

  /** Read the combined key-values for this reduce task */
  override def read(): Iterator[Product2[K, C]] = {
    val ser = Serializer.getSerializer(dep.serializer)
    val iter = BlockStoreShuffleFetcher.fetch(handle.shuffleId, startPartition, context, ser)

    val aggregatedIter: Iterator[Product2[K, C]] = if (dep.aggregator.isDefined) {
      if (dep.mapSideCombine) {
        new InterruptibleIterator(context, dep.aggregator.get.combineCombinersByKey(iter, context))
      } else {
        new InterruptibleIterator(context, dep.aggregator.get.combineValuesByKey(iter, context))
      }
    } else {
      require(!dep.mapSideCombine, "Map-side combine without Aggregator specified!")

      // Convert the Product2s to pairs since this is what downstream RDDs currently expect
      iter.asInstanceOf[Iterator[Product2[K, C]]].map(pair => (pair._1, pair._2))
    }

    // Sort the output if there is a sort ordering defined.
    dep.keyOrdering match {
      case Some(keyOrd: Ordering[K]) =>
        // Create an ExternalSorter to sort the data. Note that if spark.shuffle.spill is disabled,
        // the ExternalSorter won't spill to disk.
        val sorter = new ExternalSorter[K, C, C](ordering = Some(keyOrd), serializer = Some(ser))
        sorter.insertAll(aggregatedIter)
        context.taskMetrics.incMemoryBytesSpilled(sorter.memoryBytesSpilled)
        context.taskMetrics.incDiskBytesSpilled(sorter.diskBytesSpilled)
        sorter.iterator
      case None =>
        aggregatedIter
    }
  }