I've run into the same thing. My conclusion of the situation was that when you
use a combiner with hadoop it adds an extra: serialization, sorting,
deserialization to the mapper
If the combiner isn't able to reduce the data a significant amount,
this is pure overhead. Some code (like scoobi's join) stuff is
organized so that each mapper can has a huge amount of reducable data,
(IIRC x100 isn't unusual) so its a huge benefit (and without the
combiner, isn't actually even feasible to run).
Maybe a simple (and ugly) combineWithoutCombiner method could be
added, that is just simply a wrapper around map [Since I kind of feel
a configuration option is too course for anything other than
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