Tariq probably meant distribution of keys from <key, value> pair emitted by mapper. Partitioner distributes these pairs to different reducers based on key. If data is such that keys are skewed then most of the records may go to same reducer.
Regards, Ajay Srivastava On 17-Apr-2013, at 11:08 AM, <bejoy.had...@gmail.com<mailto:bejoy.had...@gmail.com>> <bejoy.had...@gmail.com<mailto:bejoy.had...@gmail.com>> wrote: Uniform Data distribution across HDFS is one of the factor that ensures map tasks are uniformly distributed across nodes. But reduce tasks doesn't depend on data distribution it is purely based on slot availability. Regards Bejoy KS Sent from remote device, Please excuse typos ________________________________ From: Mohammad Tariq <donta...@gmail.com<mailto:donta...@gmail.com>> Date: Wed, 17 Apr 2013 10:46:27 +0530 To: user@hadoop.apache.org<mailto:user@hadoop.apache.org><user@hadoop.apache.org<mailto:user@hadoop.apache.org>>; Bejoy Ks<bejoy.had...@gmail.com<mailto:bejoy.had...@gmail.com>> Subject: Re: How to balance reduce job Just to add to Bejoy's comments, it also depends on the data distribution. Is your data properly distributed across the HDFS? Warm Regards, Tariq https://mtariq.jux.com/ cloudfront.blogspot.com<http://cloudfront.blogspot.com/> On Wed, Apr 17, 2013 at 10:39 AM, <bejoy.had...@gmail.com<mailto:bejoy.had...@gmail.com>> wrote: Hi Rauljin Few things to check here. What is the number of reduce slots in each Task Tracker? What is the number of reduce tasks for your job? Based on the availability of slots the reduce tasks are scheduled on TTs. You can do the following Set the number of reduce tasks to 8 or more. Play with the number of slots (not very advisable for tweaking this on a job level ) The reducers are scheduled purely based on the slot availability so it won't be that easy to ensure that all TT are evenly loaded with same number of reducers. Regards Bejoy KS Sent from remote device, Please excuse typos ________________________________ From: rauljin <liujin666...@sina.com<mailto:liujin666...@sina.com>> Date: Wed, 17 Apr 2013 12:53:37 +0800 To: user@hadoop.apache.org<mailto:user@hadoop.apache.org><user@hadoop.apache.org<mailto:user@hadoop.apache.org>> ReplyTo: user@hadoop.apache.org<mailto:user@hadoop.apache.org> Subject: How to balance reduce job 8 datanode in my hadoop cluseter ,when running reduce job,there is only 2 datanode running the job . I want to use the 8 datanode to run the reduce job,so I can balance the I/O press. Any ideas? Thanks. ________________________________ rauljin
