spark AE功能控制小文件的输出数量

场景：

一般我们配置spark.sql.shuffle.partition来控制reduce的个数，但是对于不同的作业，reduce个数是不同的，对于小任务，一方面是影响运行效率，另一方面对于产生的文件数有很大的影响。

 

描述：

spark自适应执行，可以动态控制shuffle read的分区数，对reduce个数进行动态调整。

 

命令：

spark.sql.adaptive.enabled	设置自适应执行
spark.sql.adaptive.minNumPostShufflePartitions	reduce阶段最小的分区数
spark.sql.adaptive.shuffle.targetPostShuffleInputSize	reduce阶段每个task最少处理的数据量，默认64M
spark.sql.adaptive.shuffle.targetPostShuffleRowCount	reduce每个分区处理的行数（这个在spark-2.3已经没有了）

 

 

 

 

 

源码分析：

以spark-2.3为例， ExchangeCoordinator类。如果设置了最小分区数(spark.sql.adaptive.minNumPostShufflePartitions), 就用最小分区数来计算每个分区inputsize的大小。如果没有设置最小分区数，直接使用spark.sql.adaptive.shuffle.targetPostShuffleInputSize来作为每个分区inputsize的大小。

def estimatePartitionStartIndices(
      mapOutputStatistics: Array[MapOutputStatistics]): Array[Int] = {
    // If we have mapOutputStatistics.length < numExchange, it is because we do not submit
    // a stage when the number of partitions of this dependency is 0.
    assert(mapOutputStatistics.length <= numExchanges)

    // If minNumPostShufflePartitions is defined, it is possible that we need to use a
    // value less than advisoryTargetPostShuffleInputSize as the target input size of
    // a post shuffle task.
    //如果最小分区数已经设置，每个分区的inputsize的大小= min(总数据量大小/ 最小分区数, 设置的单个分区的inputsize的大小)
    val targetPostShuffleInputSize = minNumPostShufflePartitions match {
      case Some(numPartitions) =>
        val totalPostShuffleInputSize = mapOutputStatistics.map(_.bytesByPartitionId.sum).sum
        // The max at here is to make sure that when we have an empty table, we
        // only have a single post-shuffle partition.
        // There is no particular reason that we pick 16. We just need a number to
        // prevent maxPostShuffleInputSize from being set to 0.
        val maxPostShuffleInputSize =
          math.max(math.ceil(totalPostShuffleInputSize / numPartitions.toDouble).toLong, 16)
        math.min(maxPostShuffleInputSize, advisoryTargetPostShuffleInputSize)

      case None => advisoryTargetPostShuffleInputSize
    }

    logInfo(
      s"advisoryTargetPostShuffleInputSize: $advisoryTargetPostShuffleInputSize, " +
      s"targetPostShuffleInputSize $targetPostShuffleInputSize.")