Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/20091#discussion_r162549412
--- Diff: core/src/main/scala/org/apache/spark/Partitioner.scala ---
@@ -67,31 +69,32 @@ object Partitioner {
None
}
- if (isEligiblePartitioner(hasMaxPartitioner, rdds)) {
+ val defaultNumPartitions = if
(rdd.context.conf.contains("spark.default.parallelism")) {
+ rdd.context.defaultParallelism
+ } else {
+ rdds.map(_.partitions.length).max
+ }
+
+ // If the existing max partitioner is an eligible one, or its
partitions number is larger
+ // than the default number of partitions, use the existing partitioner.
+ if (hasMaxPartitioner.nonEmpty &&
(isEligiblePartitioner(hasMaxPartitioner.get, rdds) ||
+ defaultNumPartitions < hasMaxPartitioner.get.getNumPartitions)) {
--- End diff --
It depends on how you define "default". In this case, if we can benefit
from reusing an existing partitioner, we should pick that partitioner. If we
want to respect `spark.default.parallelism` strictly, we should not reuse
partitioner at all.
For this particular case, picking the existing partitioner is obviously a
better choice and it was the behavior before #20002 , so I'm +1 on this change.
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