[SPARK-4183] Enable NettyBlockTransferService by default

Author: aarondav

core/src/main/scala/org/apache/spark/SparkEnv.scala

@@ -274,7 +274,7 @@ object SparkEnv extends Logging {
     val shuffleMemoryManager = new ShuffleMemoryManager(conf)
 
     val blockTransferService =
-      conf.get("spark.shuffle.blockTransferService", "nio").toLowerCase match {
+      conf.get("spark.shuffle.blockTransferService", "netty").toLowerCase match {
         case "netty" =>
           new NettyBlockTransferService(conf)
         case "nio" =>

docs/configuration.md

@@ -359,6 +359,16 @@ Apart from these, the following properties are also available, and may be useful
     map-side aggregation and there are at most this many reduce partitions.
   </td>
 </tr>
+<tr>
+  <td><code>spark.shuffle.blockTransferService</code></td>
+  <td>netty</td>
+  <td>
+    Implementation to use for transferring shuffle and cached blocks between executors. There
+    are two implementations available: <code>netty</code> and <code>nio</code>. Netty-based
+    block transfer is intended to be simpler but equally efficient and is the default option
+    starting in 1.2.
+  </td>
+</tr>
 </table>
 
 #### Spark UI