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From abhiTowson cal <abhishek.dod...@gmail.com>
Subject Re: Number of reducers
Date Tue, 28 Aug 2012 15:13:15 GMT
hi harsh,

Thanks for the reply.I get your first and second points and coming to
third point how is it specific to a job?
My question was specific to job.

Regards
Abhishek



On Mon, Aug 27, 2012 at 11:29 PM, Harsh J <harsh@cloudera.com> wrote:
> Hi,
>
> 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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