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Hadoop, mail # dev - Number of reducers


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Abhishek 2012-08-28, 03:02
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Harsh J 2012-08-28, 03:29
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abhiTowson cal 2012-08-28, 15:13
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Re: Number of reducers
Harsh J 2012-08-28, 15:28
Ah, well, my bad. See instead the description for mapred.reduce.tasks
in mapred-default.xml, which states this: "Typically set to 99% of the
cluster's reduce capacity, so that if a node fails the reduces can
still be executed in a single wave."

FWIW, I set it manually to the level of parallelism I require (given
my partitioned data, etc.).

On Tue, Aug 28, 2012 at 8:43 PM, abhiTowson cal
<[EMAIL PROTECTED]> wrote:
> 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 <[EMAIL PROTECTED]> wrote:
>> Hi,
>>
>> 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

--
Harsh J