### What changes were proposed in this pull request?
This PR allows Python toLocalIterator to prefetch the next partition while the first partition is being collected. The PR also adds a demo micro bench mark in the examples directory, we may wish to keep this or not.
### Why are the changes needed?
In https://issues.apache.org/jira/browse/SPARK-23961 / 5e79ae3b40 we changed PySpark to only pull one partition at a time. This is memory efficient, but if partitions take time to compute this can mean we're spending more time blocking.
### Does this PR introduce any user-facing change?
A new param is added to toLocalIterator
### How was this patch tested?
New unit test inside of `test_rdd.py` checks the time that the elements are evaluated at. Another test that the results remain the same are added to `test_dataframe.py`.
I also ran a micro benchmark in the examples directory `prefetch.py` which shows an improvement of ~40% in this specific use case.
>
> 19/08/16 17:11:36 WARN NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
> Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties
> Setting default log level to "WARN".
> To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).
> Running timers:
>
> [Stage 32:> (0 + 1) / 1]
> Results:
>
> Prefetch time:
>
> 100.228110831
>
>
> Regular time:
>
> 188.341721614
>
>
>
Closes#25515 from holdenk/SPARK-27659-allow-pyspark-tolocalitr-to-prefetch.
Authored-by: Holden Karau <hkarau@apple.com>
Signed-off-by: Holden Karau <hkarau@apple.com>
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