Github user aljoscha commented on the pull request:
https://github.com/apache/flink/pull/1196#issuecomment-144396998
They are called `nonParallel` there because non-keyed windows cannot be
computed in parallel in the general case. (The general case is when you want to
evaluate the window over all elements in a map-style function). Only in special
cases can the window be pre-computed in parallel and then merged.
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