SPARK-774: cogroup should also disable map side combine by default

core/src/main/scala/spark/rdd/CoGroupedRDD.scala

@@ -6,7 +6,7 @@ import java.util.{HashMap => JHashMap}
 import scala.collection.JavaConversions
 import scala.collection.mutable.ArrayBuffer
 
-import spark.{Aggregator, Logging, Partition, Partitioner, RDD, SparkEnv, TaskContext}
+import spark.{Aggregator, Partition, Partitioner, RDD, SparkEnv, TaskContext}
 import spark.{Dependency, OneToOneDependency, ShuffleDependency}
 
 
@@ -49,12 +49,16 @@ private[spark] class CoGroupAggregator
  *
  * @param rdds parent RDDs.
  * @param part partitioner used to partition the shuffle output.
- * @param mapSideCombine flag indicating whether to merge values before shuffle step.
+ * @param mapSideCombine flag indicating whether to merge values before shuffle step. If the flag
+ *                       is on, Spark does an extra pass over the data on the map side to merge
+ *                       all values belonging to the same key together. This can reduce the amount
+ *                       of data shuffled if and only if the number of distinct keys is very small,
+ *                       and the ratio of key size to value size is also very small.
  */
 class CoGroupedRDD[K](
   @transient var rdds: Seq[RDD[(K, _)]],
   part: Partitioner,
-  val mapSideCombine: Boolean = true,
+  val mapSideCombine: Boolean = false,
   val serializerClass: String = null)
   extends RDD[(K, Seq[Seq[_]])](rdds.head.context, Nil) {