Spark技术内幕: Shuffle详解（二）

本文主要关注ShuffledRDD的Shuffle Read是如何从其他的node上读取数据的。

上文讲到了获取如何获取的策略都在org.apache.spark.storage.BlockFetcherIterator.BasicBlockFetcherIterator#splitLocalRemoteBlocks中。可以见注释。

    protected def splitLocalRemoteBlocks(): ArrayBuffer[FetchRequest] = {
      // Make remote requests at most maxBytesInFlight / 5 in length; the reason to keep them
      // smaller than maxBytesInFlight is to allow multiple, parallel fetches from up to 5
      // nodes, rather than blocking on reading output from one node.
      // 为了快速的得到数据，每次都会启动5个线程去最多5个node上取数据；
      // 每次请求的数据不会超过spark.reducer.maxMbInFlight（默认值为48MB） / 5。
      // 这样做的原因有几个：
      // 1. 避免占用目标机器的过多带宽，在千兆网卡为主流的今天，带宽还是比较重要的。
      //    如果一个连接将要占用48M的带宽，这个Network IO可能会成为瓶颈。
      // 2. 请求数据可以平行化，这样请求数据的时间可以大大减少。请求数据的总时间就是那个请求最长的。
      //    如果不是并行请求，那么总时间将是所有的请求时间之和。
      // 而设置spark.reducer.maxMbInFlight，也是为了不要占用过多的内存
      val targetRequestSize = math.max(maxBytesInFlight / 5, 1L)
      logInfo("maxBytesInFlight: " + maxBytesInFlight + ", targetRequestSize: " + targetRequestSize)

      // Split local and remote blocks. Remote blocks are further split into FetchRequests of size
      // at most maxBytesInFlight in order to limit the amount of data in flight.
      val remoteRequests = new ArrayBuffer[FetchRequest]
      var totalBlocks = 0
      for ((address, blockInfos) <- blocksByAddress) { //  address实际上是executor_id
        totalBlocks += blockInfos.size
        if (address == blockManagerId) { //数据在本地，那么直接走local read
          // Filter out zero-sized blocks
          localBlocksToFetch ++= blockInfos.filter(_._2 != 0).map(_._1)
          _numBlocksToFetch += localBlocksToFetch.size
        } else {
          val iterator = blockInfos.iterator
          var curRequestSize = 0L
          var curBlocks = new ArrayBuffer[(BlockId, Long)]
          while (iterator.hasNext) {
          // blockId 是org.apache.spark.storage.ShuffleBlockId，
          // 格式："shuffle_" + shuffleId + "_" + mapId + "_" + reduceId
            val (blockId, size) = iterator.next()
            // Skip empty blocks
            if (size > 0) { //过滤掉为大小为0的文件
              curBlocks += ((blockId, size))
              remoteBlocksToFetch += blockId
              _numBlocksToFetch += 1
              curRequestSize += size
            } else if (size < 0) {
              throw new BlockException(blockId, "Negative block size " + size)
            }
            if (curRequestSize >= targetRequestSize) { // 避免一次请求的数据量过大
              // Add this FetchRequest
              remoteRequests += new FetchRequest(address, curBlocks)
              curBlocks = new ArrayBuffer[(BlockId, Long)]
              logDebug(s"Creating fetch request of $curRequestSize at $address")
              curRequestSize = 0
            }
          }
          // Add in the final request
          if (!curBlocks.isEmpty) { // 将剩余的请求放到最后一个request中。
            remoteRequests += new FetchRequest(address, curBlocks)
          }
        }
      }
      logInfo("Getting " + _numBlocksToFetch + " non-empty blocks out of " +
        totalBlocks + " blocks")
      remoteRequests
    }