Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/6352#discussion_r31084101
--- Diff:
core/src/test/scala/org/apache/spark/scheduler/DAGSchedulerSuite.scala ---
@@ -342,6 +342,35 @@ class DAGSchedulerSuite
assert(locs === Seq(Seq("hostA", "hostB"), Seq("hostB", "hostC"),
Seq("hostC", "hostD")))
}
+ /**
+ * +---+ shuffle +---+ +---+
+ * | A |<--------| B |<---| C |<--+
+ * +---+ +---+ +---+ | +---+
+ * +--| E |
+ * +---+ | +---+
+ * | D |<--+
+ * +---+
+ * Here, E has one-to-one dependencies on C and D. C is derived from A
by performing a shuffle
+ * and then a map. If we're trying to determine which ancestor stages
need to be computed in
+ * order to compute E, we need to figure out whether the shuffle A -> B
should be performed.
+ * If the RDD C, which has only one ancestor via a narrow dependency, is
cached, then we won't
+ * need to compute A, even if it has some unavailable output partitions.
The same goes for B:
+ * if B is 100% cached, then we can avoid the shuffle on A.
+ */
+ test("SPARK-7826: regression test for getMissingParentStages") {
--- End diff --
I was considering something like "getMissingParentStages should consider
all ancestor RDDs' cache statuses", since not doing that was the cause of the
bug in an earlier version of this patch. That's still a bit cryptic, though.
I guess that we don't technically need `D` to trigger the bug, so I suppose
we could remove it as a simplification.
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