Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/11242#discussion_r59058248
--- Diff: core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala ---
@@ -62,8 +62,14 @@ class UnionRDD[T: ClassTag](
var rdds: Seq[RDD[T]])
extends RDD[T](sc, Nil) { // Nil since we implement getDependencies
+ // visible for testing
+ private[spark] val isPartitionEvalParallel: Boolean =
+ rdds.length > conf.getInt("spark.rdd.parallelListingThreshold", 10)
+
override def getPartitions: Array[Partition] = {
- val array = new Array[Partition](rdds.map(_.partitions.length).sum)
+ val parRDDs = if (isPartitionEvalParallel) rdds.par else rdds
--- End diff --
Right, I suppose I'm saying much more than just this fails in that case. I
support your suggestion. The original version always made a brand-new pool on
each call; we should avoid that at least, and also not keep a bunch of threads
around when not used if that's possible in ForkJoinSupport.
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