Re: [PR] [CORE][SPARK-49788] Add optional TTL based cleaning for blocks to better support notebooks [spark]

["HyukjinKwon (via GitHub)" <gi...@apache.org>]

HyukjinKwon commented on code in PR #49032:
URL: https://github.com/apache/spark/pull/49032#discussion_r3974102410


##########
core/src/main/scala/org/apache/spark/MapOutputTracker.scala:
##########
@@ -1377,18 +1461,36 @@ private[spark] class MapOutputTrackerMaster(
     Seq.empty.iterator
   }
 
-  // This method is only called in local-mode.
+  // Called in local-mode, and by BlockManagerMaster.removeShuffle to snapshot the merger locations
+  // for a shuffle it is about to remove. No atime update, because of that second caller: stamping
+  // here would resurrect the atime of a shuffle that is going away.
   override def getShufflePushMergerLocations(shuffleId: Int): Seq[BlockManagerId] = {
     shuffleStatuses.get(shuffleId).map(_.getShufflePushMergerLocations).getOrElse(Seq.empty)
   }
 
+  /**
+   * The location and map id of every available map output of a shuffle. Used by the shuffle removal
+   * path to work out which blocks an external shuffle service still holds on behalf of executors
+   * that have since gone away. Reading this takes the shuffle's `ShuffleStatus` lock, so it is
+   * snapshotted on the caller's thread -- see `BlockManagerMaster.removeShuffle`. No atime update,
+   * for the same reason as `getShufflePushMergerLocations` above.
+   */
+  private[spark] def getMapOutputLocations(shuffleId: Int): Seq[(BlockManagerId, Long)] = {
+    shuffleStatuses.get(shuffleId).map { shuffleStatus =>
+      shuffleStatus.withMapStatuses { statuses =>
+        statuses.filter(_ != null).map(status => (status.location, status.mapId)).toSeq

Review Comment:
   Non-blocking: this traverses the map-status array twice and allocates an intermediate array (`filter` then `map`). `iterator` fuses it into a single pass. Minor — it runs once per shuffle removal, not on a hot path.
   
   ```suggestion
           statuses.iterator.filter(_ != null).map(status => (status.location, status.mapId)).toSeq
   ```



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