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Hadoop >> mail # dev >> Number of reducers

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Number of reducers
Hi all,

I just want to know that, based on what factor map reduce framework decides number of reducers to launch for a job

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.

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.

Sorry for this question, am little confused on this number of reducers.

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