CoGroupPartition, part of CoGroupedRDD, includes references to each RDD that the CoGroupedRDD narrowly depends on, and a reference to the ShuffleHandle. The partition is serialized separately from the RDD, so when the RDD and partition arrive on the worker, the references in the partition and in the RDD no longer point to the same object.
This is a relatively minor performance issue (the closure can be 2x larger than it needs to be because the rdds and partitions are serialized twice; see numbers below) but is more annoying as a developer issue (this is where I ran into): if any state is stored in the RDD or ShuffleHandle on the worker side, subtle bugs can appear due to the fact that the references to the RDD / ShuffleHandle in the RDD and in the partition point to separate objects. I'm not sure if this is enough of a potential future problem to fix this old and central part of the code, so hoping to get input from others here.
I did some simple experiments to see how much this effects closure size. For this example:
$ val a = sc.parallelize(1 to 10).map((_, 1))
$ val b = sc.parallelize(1 to 2).map(x => (x, 2*x))
$ a.cogroup(b).collect()
the closure was 1902 bytes with current Spark, and 1129 bytes after my change. The difference comes from eliminating duplicate serialization of the shuffle handle.
For this example:
$ val sortedA = a.sortByKey()
$ val sortedB = b.sortByKey()
$ sortedA.cogroup(sortedB).collect()
the closure was 3491 bytes with current Spark, and 1333 bytes after my change. Here, the difference comes from eliminating duplicate serialization of the two RDDs for the narrow dependencies.
The ShuffleHandle includes the ShuffleDependency, so this difference will get larger if a ShuffleDependency includes a serializer, a key ordering, or an aggregator (all set to None by default). It would also get bigger for a big RDD -- although I can't think of any examples where the RDD object gets large. The difference is not affected by the size of the function the user specifies, which (based on my understanding) is typically the source of large task closures.
Author: Kay Ousterhout <kayousterhout@gmail.com>
Closes#4145 from kayousterhout/SPARK-5360 and squashes the following commits:
85156c3 [Kay Ousterhout] Better comment the narrowDeps parameter
cff0209 [Kay Ousterhout] Fixed spelling issue
658e1af [Kay Ousterhout] [SPARK-5360] Eliminate duplicate objects in serialized CoGroupedRDD
c035c0f2d7