[SPARK-5701] Only set ShuffleReadMetrics when task has shuffle deps

The updateShuffleReadMetrics method in TaskMetrics (called by the executor heartbeater) will currently always add a ShuffleReadMetrics to TaskMetrics (with values set to 0), even when the task didn't read any shuffle data. ShuffleReadMetrics should only be added if the task reads shuffle data.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #4488 from kayousterhout/SPARK-5701 and squashes the following commits:

673ed58 [Kay Ousterhout] SPARK-5701: Only set ShuffleReadMetrics when task has shuffle deps

core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala

@@ -177,8 +177,8 @@ class TaskMetrics extends Serializable {
    * Once https://issues.apache.org/jira/browse/SPARK-5225 is addressed,
    * we can store all the different inputMetrics (one per readMethod).
    */
-  private[spark] def getInputMetricsForReadMethod(readMethod: DataReadMethod):
-    InputMetrics =synchronized {
+  private[spark] def getInputMetricsForReadMethod(
+      readMethod: DataReadMethod): InputMetrics = synchronized {
     _inputMetrics match {
       case None =>
         val metrics = new InputMetrics(readMethod)
@@ -195,15 +195,17 @@ class TaskMetrics extends Serializable {
    * Aggregates shuffle read metrics for all registered dependencies into shuffleReadMetrics.
    */
   private[spark] def updateShuffleReadMetrics(): Unit = synchronized {
-    val merged = new ShuffleReadMetrics()
-    for (depMetrics <- depsShuffleReadMetrics) {
-      merged.incFetchWaitTime(depMetrics.fetchWaitTime)
-      merged.incLocalBlocksFetched(depMetrics.localBlocksFetched)
-      merged.incRemoteBlocksFetched(depMetrics.remoteBlocksFetched)
-      merged.incRemoteBytesRead(depMetrics.remoteBytesRead)
-      merged.incRecordsRead(depMetrics.recordsRead)
+    if (!depsShuffleReadMetrics.isEmpty) {
+      val merged = new ShuffleReadMetrics()
+      for (depMetrics <- depsShuffleReadMetrics) {
+        merged.incFetchWaitTime(depMetrics.fetchWaitTime)
+        merged.incLocalBlocksFetched(depMetrics.localBlocksFetched)
+        merged.incRemoteBlocksFetched(depMetrics.remoteBlocksFetched)
+        merged.incRemoteBytesRead(depMetrics.remoteBytesRead)
+        merged.incRecordsRead(depMetrics.recordsRead)
+      }
+      _shuffleReadMetrics = Some(merged)
     }
-    _shuffleReadMetrics = Some(merged)
   }
 
   private[spark] def updateInputMetrics(): Unit = synchronized {

core/src/test/scala/org/apache/spark/executor/TaskMetricsSuite.scala

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.executor
+
+import org.scalatest.FunSuite
+
+class TaskMetricsSuite extends FunSuite {
+  test("[SPARK-5701] updateShuffleReadMetrics: ShuffleReadMetrics not added when no shuffle deps") {
+    val taskMetrics = new TaskMetrics()
+    taskMetrics.updateShuffleReadMetrics()
+    assert(taskMetrics.shuffleReadMetrics.isEmpty)
+  }
+}