Github user holdenk commented on the issue:
https://github.com/apache/spark/pull/14579
Sure - so at a bit of a high level and not like exactly on point - copying
data out of the Python back to the JVM is kind of slow so if we have multiple
python operations we can put together in the same task then we try and do this.
Since caching is handled by storing the data inside of the JVM a cached RDD
can't be pipelined since we need to copy the result to the JVM. You can see the
details in the PipelinedRDD in rdd.py.
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