mridulm commented on a change in pull request #34743:
URL: https://github.com/apache/spark/pull/34743#discussion_r762406709



##########
File path: core/src/main/scala/org/apache/spark/scheduler/TaskSetManager.scala
##########
@@ -259,21 +259,29 @@ private[spark] class TaskSetManager(
       loc match {
         case e: ExecutorCacheTaskLocation =>
           pendingTaskSetToAddTo.forExecutor.getOrElseUpdate(e.executorId, new 
ArrayBuffer) += index
+          pendingTaskSetToAddTo.forHost.getOrElseUpdate(loc.host, new 
ArrayBuffer) += index
         case e: HDFSCacheTaskLocation =>
           val exe = sched.getExecutorsAliveOnHost(loc.host)
           exe match {
             case Some(set) =>
               for (e <- set) {
                 pendingTaskSetToAddTo.forExecutor.getOrElseUpdate(e, new 
ArrayBuffer) += index
               }
+              pendingTaskSetToAddTo.forHost.getOrElseUpdate(loc.host, new 
ArrayBuffer) += index
               logInfo(s"Pending task $index has a cached location at ${e.host} 
" +
                 ", where there are executors " + set.mkString(","))
             case None => logDebug(s"Pending task $index has a cached location 
at ${e.host} " +
               ", but there are no executors alive there.")
           }
-        case _ =>
+        case _: HostTaskLocation =>
+          val exe = sched.getExecutorsAliveOnHost(loc.host)
+          exe match {
+            case Some(_) =>
+              pendingTaskSetToAddTo.forHost.getOrElseUpdate(loc.host, new 
ArrayBuffer) += index
+            case _ => logDebug(s"Pending task $index has a location at 
${loc.host} " +
+              ", but there are no executors alive there.")
+          }
       }
-      pendingTaskSetToAddTo.forHost.getOrElseUpdate(loc.host, new ArrayBuffer) 
+= index

Review comment:
       Couple of things:
   
   a) Tasks expressing preferred locality does not necessarily mean spark will 
wait at that preferred level. Take a look at `computeValidLocalityLevels`.
   In your case, if scheduler is waiting for NODE_LOCAL schedules, then it 
means there is some executor which is alive on some host which is preferred.
   
   b) If a task is not run at one level, it would at the next level - after a 
modest delay which is configurable.
   It does not result in task never getting executed.




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