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From Harsh J <ha...@cloudera.com>
Subject Re: Number of reducers
Date Tue, 28 Aug 2012 03:29:54 GMT

On Tue, Aug 28, 2012 at 8:32 AM, Abhishek <abhishek.dodda1@gmail.com> wrote:
> Hi all,
> I just want to know that, based on what factor map reduce framework decides number of
reducers to launch for a job

The framework does not auto-determine the number of reducers for a
job. That is purely user-or-client-program-supplied presently.

> By default only one reducer will be launched for a given job is this right? If we explicitly
does not mention number to launch via command line or driver class.

Yes, by default the number of reduce tasks is configured to be one.

> If i choose to decide number of reducers to mention explicitly, what should I consider.Because
choosing in appropriate number of reducer hampers the performance.

See http://wiki.apache.org/hadoop/HowManyMapsAndReduces

Harsh J

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