venkata91 commented on a change in pull request #30164:
URL: https://github.com/apache/spark/pull/30164#discussion_r517736090



##########
File path: 
resource-managers/yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackend.scala
##########
@@ -161,6 +170,27 @@ private[spark] abstract class YarnSchedulerBackend(
     totalRegisteredExecutors.get() >= totalExpectedExecutors * 
minRegisteredRatio
   }
 
+  override def getMergerLocations(
+      numPartitions: Int,
+      resourceProfileId: Int): Seq[BlockManagerId] = {
+    // Currently this is naive way of calculating numMergersNeeded for a 
stage. In future,
+    // we can use better heuristics to calculate numMergersNeeded for a stage.
+    val tasksPerExecutor = sc.resourceProfileManager
+        .resourceProfileFromId(resourceProfileId).maxTasksPerExecutor(sc.conf)
+    val numMergersNeeded = math.min(
+      math.max(1, math.ceil(numPartitions / tasksPerExecutor).toInt), 
maxNumExecutors)
+    val minMergersThreshold = math.max(minMergersStaticThreshold,
+      math.floor(numMergersNeeded * minMergersThresholdRatio).toInt)

Review comment:
       `minMergersThreshold` calculates mergers required both from 
`minMergersThresholdRatio` as well as the `minMergersStaticThreshold` - this 
doesn't consider `numMergersNeeded`. If `numMergersNeeded` < 
`minMergersThreshold` we would want to return the mergers obtained from 
`BlockManagerMaster`. Am I missing something here?




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