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Hadoop >> mail # dev >> Number of reducers

Abhishek 2012-08-28, 03:02
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Re: Number of reducers

On Tue, Aug 28, 2012 at 8:32 AM, Abhishek <[EMAIL PROTECTED]> 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
abhiTowson cal 2012-08-28, 15:13
Harsh J 2012-08-28, 15:28