[SPARK-17951] BlockFetch with multiple threads slows down after spark 1.6

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Not A Problem
• Affects Version/s: 1.6.2
• Component/s: Block Manager, Spark Core

Environment

cluster with 8 node, each node has 28 cores. 10Gb network

• Assignee: Unassigned
• Reporter: ding
• Created: 15/Oct/16 00:50
• Updated: 17/Oct/16 09:55
• Resolved: 17/Oct/16 09:55

Description

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
        (0 to 7).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: 0.19 s while
in spark 1.5.1 average of "getting remote bytes" time is: 0.09 s

However if fetch block in single thread, the gap is much smaller.
spark1.6.2 get remote bytes: 0.21 s
spark1.5.1 get remote bytes: 0.20 s