Github user squito commented on a diff in the pull request:

    https://github.com/apache/spark/pull/22371#discussion_r216369925
  
    --- Diff: 
core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala ---
    @@ -138,13 +154,22 @@ private[spark] class IndexShuffleBlockResolver(
           mapId: Int,
           lengths: Array[Long],
           dataTmp: File): Unit = {
    +    val mapLocks = shuffleIdToLocks.get(shuffleId)
    +    require(mapLocks != null, "Shuffle should be registered to 
IndexShuffleBlockResolver first")
    +    val lock = mapLocks.synchronized {
    --- End diff --
    
    in most cases, you'll still have contention on this lock, right, its just a 
much smaller critical section?  As the usual case is for multiple tasks in the 
same executor to be for the same stage.


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