Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/11505#discussion_r55261972
--- Diff: core/src/main/scala/org/apache/spark/MapOutputTracker.scala ---
@@ -386,28 +384,36 @@ private[spark] class MapOutputTrackerMaster(conf:
SparkConf)
fractionThreshold: Double)
: Option[Array[BlockManagerId]] = {
- if (mapStatuses.contains(shuffleId)) {
- val statuses = mapStatuses(shuffleId)
- if (statuses.nonEmpty) {
- // HashMap to add up sizes of all blocks at the same location
- val locs = new HashMap[BlockManagerId, Long]
- var totalOutputSize = 0L
- var mapIdx = 0
- while (mapIdx < statuses.length) {
- val status = statuses(mapIdx)
- val blockSize = status.getSizeForBlock(reducerId)
- if (blockSize > 0) {
- locs(status.location) = locs.getOrElse(status.location, 0L) +
blockSize
- totalOutputSize += blockSize
+ val statuses = mapStatuses.get(shuffleId).orNull
+ if (statuses != null) {
+ statuses.synchronized {
+ if (statuses.nonEmpty) {
+ // HashMap to add up sizes of all blocks at the same location
+ val locs = new HashMap[BlockManagerId, Long]
+ var totalOutputSize = 0L
+ var mapIdx = 0
+ while (mapIdx < statuses.length) {
+ val status = statuses(mapIdx)
+ // status may be null here if we are called between
registerShuffle, which creates an
+ // array with null entries for each output, and
registerMapOutputs, which populates it
+ // with valid status entries. This is possible if one thread
schedules a job which
+ // depends on an RDD which is currently being computed by
another thread.
+ if (status != null) {
--- End diff --
Is that true? It's conceptually immutable, so that's a little surprising.
Is it worth making this change anyway as a very cheap defensive step?
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