[SPARK-12655][GRAPHX] GraphX does not unpersist RDDs

Some VertexRDD and EdgeRDD are created during the intermediate step of g.connectedComponents() but unnecessarily left cached after the method is done. The fix is to unpersist these RDDs once they are no longer in use.

A test case is added to confirm the fix for the reported bug.

Author: Jason Lee <cjlee@us.ibm.com>

Closes #10713 from jasoncl/SPARK-12655.

graphx/src/main/scala/org/apache/spark/graphx/Pregel.scala

@@ -151,7 +151,7 @@ object Pregel extends Logging {
       // count the iteration
       i += 1
     }
-
+    messages.unpersist(blocking = false)
     g
   } // end of apply
 

graphx/src/main/scala/org/apache/spark/graphx/lib/ConnectedComponents.scala

@@ -47,9 +47,11 @@ object ConnectedComponents {
       }
     }
     val initialMessage = Long.MaxValue
-    Pregel(ccGraph, initialMessage, activeDirection = EdgeDirection.Either)(
+    val pregelGraph = Pregel(ccGraph, initialMessage, activeDirection = EdgeDirection.Either)(
       vprog = (id, attr, msg) => math.min(attr, msg),
       sendMsg = sendMessage,
       mergeMsg = (a, b) => math.min(a, b))
+    ccGraph.unpersist()
+    pregelGraph
   } // end of connectedComponents
 }

graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala

@@ -428,4 +428,20 @@ class GraphSuite extends SparkFunSuite with LocalSparkContext {
     }
   }
 
+  test("unpersist graph RDD") {
+    withSpark { sc =>
+      val vert = sc.parallelize(List((1L, "a"), (2L, "b"), (3L, "c")), 1)
+      val edges = sc.parallelize(List(Edge[Long](1L, 2L), Edge[Long](1L, 3L)), 1)
+      val g0 = Graph(vert, edges)
+      val g = g0.partitionBy(PartitionStrategy.EdgePartition2D, 2)
+      val cc = g.connectedComponents()
+      assert(sc.getPersistentRDDs.nonEmpty)
+      cc.unpersist()
+      g.unpersist()
+      g0.unpersist()
+      vert.unpersist()
+      edges.unpersist()
+      assert(sc.getPersistentRDDs.isEmpty)
+    }
+  }
 }