[RFC] Disable local execution of Spark jobs by default

Currently, local execution of Spark jobs is only used by take(), and it can
be problematic as it can load a significant amount of data onto the driver.
The worst case scenarios occur if the RDD is cached (guaranteed to load whole
partition), has very large elements, or the partition is just large and we
apply a filter with high selectivity or computational overhead.

Additionally, jobs that run locally in this manner do not show up in the web UI,
and are thus harder to track or understand what is occurring.

This PR adds a flag to disable local execution, which is turned OFF by default, with
the intention of perhaps eventually removing this functionality altogether. Removing it
now is a tougher proposition since it is part of the public runJob API. An alternative
solution would be to limit the flag to take()/first() to avoid impacting any external
users of this API, but such usage (or at least, reliance upon the feature) is hopefully
minimal.

Author: aarondav

core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala

@@ -121,6 +121,9 @@ class DAGScheduler(
 
   private[scheduler] var eventProcessActor: ActorRef = _
 
+  /** If enabled, we may run certain actions like take() and first() locally. */
+  private val localExecutionEnabled = sc.getConf.getBoolean("spark.localExecution.enabled", false)
+
   private def initializeEventProcessActor() {
     // blocking the thread until supervisor is started, which ensures eventProcessActor is
     // not null before any job is submitted
@@ -732,7 +735,9 @@ class DAGScheduler(
       logInfo("Final stage: " + finalStage + "(" + finalStage.name + ")")
       logInfo("Parents of final stage: " + finalStage.parents)
       logInfo("Missing parents: " + getMissingParentStages(finalStage))
-      if (allowLocal && finalStage.parents.size == 0 && partitions.length == 1) {
+      val shouldRunLocally =
+        localExecutionEnabled && allowLocal && finalStage.parents.isEmpty && partitions.length == 1
+      if (shouldRunLocally) {
         // Compute very short actions like first() or take() with no parent stages locally.
         listenerBus.post(SparkListenerJobStart(job.jobId, Array[Int](), properties))
         runLocally(job)

docs/configuration.md

@@ -846,6 +846,15 @@ Apart from these, the following properties are also available, and may be useful
     (in milliseconds).  
   </td>
 </tr>
+<tr>
+  <td><code>spark.localExecution.enabled</code></td>
+  <td>false</td>
+  <td>
+    Enables Spark to run certain jobs, such as first() or take() on the driver, without sending
+    tasks to the cluster. This can make certain jobs execute very quickly, but may require
+    shipping a whole partition of data to the driver.
+  </td>
+</tr>
 </table>
 
 #### Security