This is a good discussion on setting reducers: https://community.hortonworks.com/questions/28073/how-do-you-force-the-number-of-reducers-in-a-map-r.html As with all performance tuning, best to isolate a bottleneck and tune that vs. simply trying a lot of things at the same time. So yes, among other tuning ... set this and see if it works. If not, move to the next suspected bottleneck.
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