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.
Sent from remote device, Please excuse typos
From: rauljin <[EMAIL PROTECTED]>
Date: Wed, 17 Apr 2013 12:53:37
To: [EMAIL PROTECTED]<[EMAIL PROTECTED]>
Reply-To: [EMAIL PROTECTED]
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.