Made DFS shuffle's "reduce tasks" fetch inputs in a random order so they

don't all hit the same nodes at the same time.

src/scala/spark/DfsShuffle.scala

@@ -80,8 +80,8 @@ extends Logging
       }
       val fs = DfsShuffle.getFileSystem()
       val outputStreams = (0 until numOutputSplits).map(i => {
-        val path = new Path(dir, "intermediate-%d-%d".format(myIndex, i))
-        new ObjectOutputStream(fs.create(path, 1.toShort))
+        val path = new Path(dir, "%d-to-%d".format(myIndex, i))
+        new ObjectOutputStream(fs.create(path, true))
       }).toArray
       for ((k, c) <- combiners) {
         val bucket = k.hashCode % numOutputSplits
@@ -96,8 +96,8 @@ extends Logging
         override def default(key: K) = createCombiner()
       }
       val fs = DfsShuffle.getFileSystem()
-      for (i <- 0 until numInputSplits) {
-        val path = new Path(dir, "intermediate-%d-%d".format(i, myIndex))
+      for (i <- Utils.shuffle(0 until numInputSplits)) {
+        val path = new Path(dir, "%d-to-%d".format(i, myIndex))
         val inputStream = new ObjectInputStream(fs.open(path))
         try {
           while (true) {

src/scala/spark/Utils.scala

@@ -4,13 +4,14 @@ import java.io._
 import java.util.UUID
 
 import scala.collection.mutable.ArrayBuffer
+import scala.util.Random
 
 /**
  * Various utility methods used by Spark.
  */
 object Utils {
   def serialize[T](o: T): Array[Byte] = {
-    val bos = new ByteArrayOutputStream
+    val bos = new ByteArrayOutputStream()
     val oos = new ObjectOutputStream(bos)
     oos.writeObject(o)
     oos.close
@@ -95,4 +96,19 @@ object Utils {
       out.close()
     }
   }
+
+  // Shuffle the elements of a collection into a random order, returning the
+  // result in a new collection. Unlike scala.util.Random.shuffle, this method
+  // uses a local random number generator, avoiding inter-thread contention.
+  def shuffle[T](seq: Seq[T]): Seq[T] = {
+    val buf = ArrayBuffer(seq: _*)
+    val rand = new Random()
+    for (i <- (buf.size - 1) to 1 by -1) {
+      val j = rand.nextInt(i)
+      val tmp = buf(j)
+      buf(j) = buf(i)
+      buf(i) = tmp
+    }
+    buf
+  }
 }