How big is your cluster? Assuming you are running a single node cluster,
Hadoop-default.xml has a parameter 'mapred.map.tasks' that is set to 2.
By default, no matter how many map tasks are calculated by framework,
only 2 map task will execute on a single node cluster.
From: Gopal Gandhi
Sent: Thursday, July 31, 2008 4:38 AM
Subject: How can I control Number of Mappers of a job?
The motivation is to control the max # of mappers of a job. For example,
the input data is 246MB, divided by 64M is 4. If by default there will
be 4 mappers launched on the 4 blocks.
What I want is to set its max # of mappers as 2, so that 2 mappers are
launched first and when they completes on the first 2 blocks, another 2
mappers start on the rest 2 blocks. Does Hadoop provide a way?