Full utilization of all cores in Hadoop pseudo-distributed mode
mapreduce.tasktracker.map.tasks.maximum and mapreduce.tasktracker.reduce.tasks.maximum properties control the number of map and reduce tasks per node. For a 4 core processor, start with 2/2 and from there change the values if required. A slot is a map or a reduce slot, setting the values to 4/4 will make the Hadoop framework launch 4 map and 4 reduce tasks simultaneously. A total of 8 map and reduce tasks run at a time on a node.
mapred.map.tasks and mapred.reduce.tasks properties control the total number of map/reduce tasks for the job and not the # of tasks per node. Also, mapred.map.tasks is a hint to the Hadoop framework and the total # of map tasks for the job equals the # of InputSplits.
mapred.map.tasks and mapred.reduce.tasks will control this, and (I believe) would be set in mapred-site.xml. However this establishes these as cluster-wide defaults; more usually you would configure these on a per-job basis. You can set the same params on the java command line with -D