SPARK-1775: Unneeded lock in ShuffleMapTask.deserializeInfo

This was used in the past to have a cache of deserialized ShuffleMapTasks, but that's been removed, so there's no need for a lock. It slows down Spark when task descriptions are large, e.g. due to large lineage graphs or local variables.

Author: Sandeep <sandeep@techaddict.me>

Closes #707 from techaddict/SPARK-1775 and squashes the following commits:

18d8ebf [Sandeep] SPARK-1775: Unneeded lock in ShuffleMapTask.deserializeInfo This was used in the past to have a cache of deserialized ShuffleMapTasks, but that's been removed, so there's no need for a lock. It slows down Spark when task descriptions are large, e.g. due to large lineage graphs or local variables.

core/src/main/scala/org/apache/spark/scheduler/ShuffleMapTask.scala

@@ -57,15 +57,13 @@ private[spark] object ShuffleMapTask {
   }
 
   def deserializeInfo(stageId: Int, bytes: Array[Byte]): (RDD[_], ShuffleDependency[_,_]) = {
-    synchronized {
-      val loader = Thread.currentThread.getContextClassLoader
-      val in = new GZIPInputStream(new ByteArrayInputStream(bytes))
-      val ser = SparkEnv.get.closureSerializer.newInstance()
-      val objIn = ser.deserializeStream(in)
-      val rdd = objIn.readObject().asInstanceOf[RDD[_]]
-      val dep = objIn.readObject().asInstanceOf[ShuffleDependency[_,_]]
-      (rdd, dep)
-    }
+    val loader = Thread.currentThread.getContextClassLoader
+    val in = new GZIPInputStream(new ByteArrayInputStream(bytes))
+    val ser = SparkEnv.get.closureSerializer.newInstance()
+    val objIn = ser.deserializeStream(in)
+    val rdd = objIn.readObject().asInstanceOf[RDD[_]]
+    val dep = objIn.readObject().asInstanceOf[ShuffleDependency[_,_]]
+    (rdd, dep)
   }
 
   // Since both the JarSet and FileSet have the same format this is used for both.