Github user andrewor14 commented on a diff in the pull request:
https://github.com/apache/spark/pull/11242#discussion_r62237486
--- Diff: core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala ---
@@ -62,8 +64,22 @@ 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 isPartitionListingParallel: Boolean =
+ rdds.length > conf.getInt("spark.rdd.parallelListingThreshold", 10)
+
+ @transient private lazy val partitionEvalTaskSupport =
+ new ForkJoinTaskSupport(new ForkJoinPool(8))
+
override def getPartitions: Array[Partition] = {
- val array = new Array[Partition](rdds.map(_.partitions.length).sum)
+ val parRDDs = if (isPartitionListingParallel) {
+ val parArray = rdds.par
+ parArray.tasksupport = partitionEvalTaskSupport
+ parArray
+ } else {
+ rdds
+ }
+ val array = new Array[Partition](parRDDs.map(_.partitions.length).sum)
--- End diff --
I think the problem is this line: after you call a map Scala will use the
original global execution context for the sum. But here it's just a list of
numbers so we can sum that ourselves synchronously, so it'll work if you do
```
parRDDs.map(_.partitions.length).seq.sum
```
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