I understand that the maximum number of concurrent map tasks is set by mapred.tasktracker.map.tasks.maximum  - however I wish to run with a smaller number of maps (am testing disk IO). I thought that I could set that within the main program using 

conf.set("mapred.tasktracker.map.tasks.maximuma", "4");

to run with 4 maps  but that seems to have no impact. I know I could just change the mapred-site.xml and restart map reduce but that's kind of a pain. Can it be set from within the code?