Add unpersist in Graph and GraphImpl

Author: maropu

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

@@ -96,6 +96,13 @@ abstract class Graph[VD: ClassTag, ED: ClassTag] protected () extends Serializab
    */
   def cache(): Graph[VD, ED]
 
+  /**
+   * Uncaches both vertices and edges of this graph. It is useful that iterative algorithms build
+   * a new graph in each iteration, and the vertices and edges of previous iterations are no
+   * longer needed for following iterations.
+   */
+  def unpersist(blocking: Boolean = true): Graph[VD, ED]
+
   /**
    * Uncaches only the vertices of this graph, leaving the edges alone. This is useful in iterative
    * algorithms that modify the vertex attributes but reuse the edges. This method can be used to

graphx/src/main/scala/org/apache/spark/graphx/impl/GraphImpl.scala

@@ -65,6 +65,12 @@ class GraphImpl[VD: ClassTag, ED: ClassTag] protected (
     this
   }
 
+  override def unpersist(blocking: Boolean = true): Graph[VD, ED] = {
+    unpersistVertices(blocking)
+    replicatedVertexView.edges.unpersist(blocking)
+    this
+  }
+
   override def unpersistVertices(blocking: Boolean = true): Graph[VD, ED] = {
     vertices.unpersist(blocking)
     // TODO: unpersist the replicated vertices in `replicatedVertexView` but leave the edges alone