diff --git a/core/src/main/scala/org/apache/spark/rdd/util/PeriodicRDDCheckpointer.scala b/core/src/main/scala/org/apache/spark/rdd/util/PeriodicRDDCheckpointer.scala
index ab72addb2466..facbb830a60d 100644
--- a/core/src/main/scala/org/apache/spark/rdd/util/PeriodicRDDCheckpointer.scala
+++ b/core/src/main/scala/org/apache/spark/rdd/util/PeriodicRDDCheckpointer.scala
@@ -50,6 +50,7 @@ import org.apache.spark.util.PeriodicCheckpointer
  * {{{
  *  val (rdd1, rdd2, rdd3, ...) = ...
  *  val cp = new PeriodicRDDCheckpointer(2, sc)
+ *  cp.update(rdd1)
  *  rdd1.count();
  *  // persisted: rdd1
  *  cp.update(rdd2)
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/util/PeriodicGraphCheckpointer.scala b/graphx/src/main/scala/org/apache/spark/graphx/util/PeriodicGraphCheckpointer.scala
index fda501aa757d..539b66f747cc 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/util/PeriodicGraphCheckpointer.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/util/PeriodicGraphCheckpointer.scala
@@ -50,6 +50,7 @@ import org.apache.spark.util.PeriodicCheckpointer
  * {{{
  *  val (graph1, graph2, graph3, ...) = ...
  *  val cp = new PeriodicGraphCheckpointer(2, sc)
+ *  cp.updateGraph(graph1)
  *  graph1.vertices.count(); graph1.edges.count()
  *  // persisted: graph1
  *  cp.updateGraph(graph2)