Github user srowen commented on the issue: https://github.com/apache/spark/pull/18343 OK I get it. Hm, I wonder why some classes in the code extend `Externalizable` instead of `Serializable`? I see a comment about controlling serialization, but `Serializable` also lets you do that. I imagine Kryo understands what to do with `Serializable` but not `Externalizable`, because i see most classes the implement the latter also implement `KryoSerializable`. That seems like the right fix? Because this seems to just cause default serialization to take over, and that's not desirable, apparently. If so then we might have a similar problem with `CompressedMapStatus`, `DirectTaskResult`, `LongHashedRelation`, `StorageLevel`, `UpdateBlockInfo`, `BlockManagerId`. It may not make a difference there, or I might misunderstand.
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