[jira] [Created] (SPARK-17951) BlockFetch with multiple threads slows down after spark 1.6

["ding (JIRA)" <ji...@apache.org>]

ding created SPARK-17951:
----------------------------

             Summary: BlockFetch with multiple threads slows down after spark 1.6
                 Key: SPARK-17951
                 URL: https://issues.apache.org/jira/browse/SPARK-17951
             Project: Spark
          Issue Type: Bug
          Components: Block Manager, Spark Core
    Affects Versions: 1.6.2
         Environment: cluster with 8 node, each node has 28 cores. 10Gb network
            Reporter: ding


The following code demonstrates the issue:
 def main(args: Array[String]): Unit = {
    val conf = new SparkConf().setAppName(s"BMTest")
    val size = 3344570
    val sc = new SparkContext(conf)

    val data = sc.parallelize(1 to 100, 8)
    var accum = sc.accumulator(0.0, "get remote bytes")
    var i = 0
    while(i < 91) {
      accum = sc.accumulator(0.0, "get remote bytes")
      val test = data.mapPartitionsWithIndex { (pid, iter) =>
        val N = size
        val bm = SparkEnv.get.blockManager
        val blockId = TaskResultBlockId(10*i + pid)
        val test = new Array[Byte](N)
        Random.nextBytes(test)
        val buffer = ByteBuffer.allocate(N)
        buffer.limit(N)
        buffer.put(test)
        bm.putBytes(blockId, buffer, StorageLevel.MEMORY_ONLY_SER)        
        Iterator(1)
      }.count()
      
      data.mapPartitionsWithIndex { (pid, iter) =>
        val before = System.nanoTime()        
        val bm = SparkEnv.get.blockManager
        (1 to 8).map(s => {
          Future {
            val result = bm.getRemoteBytes(TaskResultBlockId(10*i + s))
          }
        }).map(Await.result(_, Duration.Inf))
        
        accum.add((System.nanoTime() - before) / 1e9)
        Iterator(1)
      }.count()
      println("get remote bytes take: " + accum.value/8)
      i += 1
    }        
  }

In spark1.6.2, average of "getting remote bytes" time is: 0.16s while
in spark 1.5.1 average of "getting remote bytes" time is: 0.07s

However if fetch block in single thread, the gap is much smaller.
spark1.6.2  get remote bytes: 0.191421s
spark1.5.1  get remote bytes: 0.181312s





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