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From "arkady borkovsky (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-717) When there are few reducers, sorting should be done by mappers
Date Tue, 14 Nov 2006 06:49:37 GMT
When there are few reducers, sorting should be done by mappers
--------------------------------------------------------------

                 Key: HADOOP-717
                 URL: http://issues.apache.org/jira/browse/HADOOP-717
             Project: Hadoop
          Issue Type: Improvement
          Components: mapred
            Reporter: arkady borkovsky


If I understand correctly, currently, sort happens on the reducer side.
So if few hundred mappers produce few (or many) Gig of data, and there is just ONE reduce
to consume it, copying and sorting takes forever.

It may make sense to have a special case optimization for a single reducer.  (E.g. "when there
is only reducer and many mappers, sort is done by the mappers, and reducer does only a merge")

Or to have some smarter policy that makes sure that sorting uses as many CPUs as it makes
sense.   If  the map step has produced data on all the nodes of the cluster, it makes sense
to use all the nodes for sorting.


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