Github user srowen commented on a diff in the pull request: https://github.com/apache/spark/pull/22371#discussion_r216383808 --- 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 -- The theory is many fewer threads would contend here because it's per-shuffleID. If it's an issue, then your idea of a second-level ConcurrentHashMap might help. It's more complex than a usual Map but can allow for safe concurrent access by a limited number of threads. Otherwise it might be overkill as the second-level Map.
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