Github user davies commented on a diff in the pull request:
https://github.com/apache/spark/pull/11327#discussion_r61801132
--- Diff: core/src/main/scala/org/apache/spark/rdd/CoalescedRDD.scala ---
@@ -334,8 +332,41 @@ private class DefaultPartitionCoalescer(val
balanceSlack: Double = 0.10)
}
}
} else {
+ // It is possible to have unionRDD where one rdd has preferred
locations and another rdd
+ // that doesn't. To make sure we end up with the requested number of
partitions,
+ // make sure to put a partition in every group.
+
+ if (groupArr.size > initialHash.size) {
+ // we don't have a partition assigned to every group yet so first
try to fill them
+ // with the partitions with preferred locations
+ val partIter = partitionLocs.partsWithLocs.iterator
+ while (partIter.hasNext && initialHash.size < groupArr.size) {
+ var (nxt_replica, nxt_part) = partIter.next()
+ if (!initialHash.contains(nxt_part)) {
+ groupArr.find(pg => pg.numPartitions == 0).map(firstEmpty => {
+ firstEmpty.partitions += nxt_part
+ initialHash += nxt_part
+ })
+ }
+ }
+ }
+
+ // if we didn't get one partitions per group from partitions with
preferred locations
+ // use partitions without preferred locations
+ val partNoLocIter = partitionLocs.partsWithoutLocs.iterator
+ while (partNoLocIter.hasNext && initialHash.size < groupArr.size) {
+ var nxt_part = partNoLocIter.next()
+ if (!initialHash.contains(nxt_part)) {
+ groupArr.find(pg => pg.numPartitions == 0).map(firstEmpty => {
--- End diff --
This is still O(N*N) (the worst), it could be
```
groupArr.filter(pg => pg.numPartitions == 0).foreach { pg =>
while (partNoLocIter && pg.numPartitions == 0) {
if (!initialHash.contains(nxt_part)) {
pg.partitions += nxt_part
initialHash += nxt_part
}
}
}
```
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