Github user dbtsai commented on a diff in the pull request: https://github.com/apache/spark/pull/11242#discussion_r56432618 --- Diff: core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala --- @@ -62,7 +64,21 @@ class UnionRDD[T: ClassTag]( var rdds: Seq[RDD[T]]) extends RDD[T](sc, Nil) { // Nil since we implement getDependencies + // Evaluate partitions in parallel. Partitions of each rdd will be cached by the `partitions` + // val in `RDD`. + private lazy val evaluatePartitions: Unit = { + val threshold = conf.getInt("spark.rdd.parallelListingThreshold", 10) + if (threshold > 0 && rdds.length > threshold) { + val parArray = rdds.toParArray + parArray.tasksupport = new ForkJoinTaskSupport(new ForkJoinPool(threshold)) + parArray.foreach(_.partitions) + } else { --- End diff -- `false` and remove `rdds.foreach(_.partitions)` to make it evaluate in the original path. Thus, you can check if we go through this path in unit test. Thanks.
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