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Posted to mapreduce-dev@hadoop.apache.org by "Allen Wittenauer (JIRA)" <ji...@apache.org> on 2009/11/11 01:24:28 UTC

[jira] Created: (MAPREDUCE-1206) Need a better answer for "how many reduces?"

Need a better answer for "how many reduces?"
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                 Key: MAPREDUCE-1206
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1206
             Project: Hadoop Map/Reduce
          Issue Type: Bug
            Reporter: Allen Wittenauer


http://hadoop.apache.org/common/docs/current/mapred_tutorial.html#Reducer

--snip--
How Many Reduces?

The right number of reduces seems to be 0.95 or 1.75 multiplied by (<no. of nodes> * mapred.tasktracker.reduce.tasks.maximum).
--snip--

Sure, if you only ever run one job on your grid.  There should really be a better answer here, especially explaining what the impact is of a high/low number when chaining multiple map/reduce jobs.

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