Github user rdblue commented on a diff in the pull request:
https://github.com/apache/spark/pull/11242#discussion_r57203120
--- Diff: core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala ---
@@ -62,7 +64,23 @@ 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[spark] lazy val parallelPartitionEval: Boolean = {
--- End diff --
I think there's a big difference between the two scenarios. While we may
have lurking concurrency bugs in getSplits calls today, this patch kicks off a
bunch of tasks that call getSplits at the same time. That's much more likely to
hit concurrency problems if they exist and I don't think we can conclude that
it is safe from your example, where a driver could be doing split calculations
in multiple threads.
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