the "mapred.child.java.opts"  are the Java opts for the task tracker child processes. So say the conf file can contains "mapred.child.java.opts" : " -Xmx1024m -verbose:gc -Xloggc:/tmp/@taskid@.gc" . Now notice the last character for Xmx value is 'm', which implies 1024 megabytes. We need to remove this m to get the actual number value which is to be used as long in further calculations. The space split is necessary since there could be more things being passed in the mapred.child.java.opts