http://wiki.apache.org/hadoop/HowManyMapsAndReduces
It is possible to have a single mapper if the input is not splittable BUT it is rarely seen as a feature. One could ask why you want to use a platform for distributed computing for a job that shouldn't be distributed. Regards Bertrand On Tue, Feb 26, 2013 at 12:09 PM, Arindam Choudhury < arindamchoudhu...@gmail.com> wrote: > Hi all, > > I am trying to run terasort using one map and one reduce. so, I generated > the input data using: > > hadoop jar hadoop-examples-1.0.4.jar teragen -Dmapred.map.tasks=1 > -Dmapred.reduce.tasks=1 32000000 /user/hadoop/input32mb1map > > Then I launched the hadoop terasort job using: > > hadoop jar hadoop-examples-1.0.4.jar terasort -Dmapred.map.tasks=1 > -Dmapred.reduce.tasks=1 /user/hadoop/input32mb1map /user/hadoop/output1 > > I thought it will run the job using 1 map and 1 reduce, but when inspect > the job statistics I found: > > hadoop job -history /user/hadoop/output1 > > Task Summary > ============================ > Kind Total Successful Failed Killed StartTime FinishTime > > Setup 1 1 0 0 26-Feb-2013 10:57:47 26-Feb-2013 > 10:57:55 (8sec) > Map 24 24 0 0 26-Feb-2013 10:57:57 26-Feb-2013 > 11:05:37 (7mins, 40sec) > Reduce 1 1 0 0 26-Feb-2013 10:58:21 26-Feb-2013 > 11:08:31 (10mins, 10sec) > Cleanup 1 1 0 0 26-Feb-2013 11:08:32 26-Feb-2013 > 11:08:36 (4sec) > ============================ > > so, though I mentioned to launch one map tasks, there are 24 of them. > > How to solve this problem. How to tell hadoop to launch only one map. > > Thanks, >
