Github user andrewor14 commented on the pull request:
https://github.com/apache/spark/pull/1165#issuecomment-49685219
@mateiz Yes, currently we unroll it in deserialized form even if we only
want to store it in serialized form. One issue with storing it directly in
bytes is that `CacheManager` still needs to return the original iterator,
meaning, after we serialize the values we need to deserialize it back. This may
cause a performance regression for the `MEMORY_*_SER` storage levels because
the deserialization process may be expensive. (This was attempted in #1083, but
we later decided against it for this reason).
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