Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/21927#discussion_r207107889
--- Diff: core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala
---
@@ -340,6 +340,22 @@ class DAGScheduler(
}
}
+ /**
+ * Check to make sure we don't launch a barrier stage with unsupported
RDD chain pattern. The
+ * following patterns are not supported:
+ * 1. Ancestor RDDs that have different number of partitions from the
resulting RDD (eg.
+ * union()/coalesce()/first()/PartitionPruningRDD);
--- End diff --
`coalesce()` is not safe when shuffle is false because it may cause the
number of tasks doesn't match the number of partitions for the RDD that uses
barrier mode.
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