Add comments about object re-use to the affected functions.

Author: darabos

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

@@ -56,6 +56,9 @@ class EdgePartition[@specialized(Char, Int, Boolean, Byte, Long, Float, Double)
    * Construct a new edge partition by applying the function f to all
    * edges in this partition.
    *
+   * Be careful not to keep references to the objects passed to `f`.
+   * To improve GC performance the same object is re-used for each call.
+   *
    * @param f a function from an edge to a new attribute
    * @tparam ED2 the type of the new attribute
    * @return a new edge partition with the result of the function `f`
@@ -182,6 +185,9 @@ class EdgePartition[@specialized(Char, Int, Boolean, Byte, Long, Float, Double)
   /**
    * Get an iterator over the edges in this partition.
    *
+   * Be careful not to keep references to the objects from this iterator.
+   * To improve GC performance the same object is re-used in `next()`.
+   *
    * @return an iterator over edges in the partition
    */
   def iterator = new Iterator[Edge[ED]] {
@@ -210,6 +216,9 @@ class EdgePartition[@specialized(Char, Int, Boolean, Byte, Long, Float, Double)
   /**
    * Get an iterator over the cluster of edges in this partition with source vertex id `srcId`. The
    * cluster must start at position `index`.
+   *
+   * Be careful not to keep references to the objects from this iterator. To improve GC performance
+   * the same object is re-used in `next()`.
    */
   private def clusterIterator(srcId: VertexId, index: Int) = new Iterator[Edge[ED]] {
     private[this] val edge = new Edge[ED]