So it looks like the default value for mapreduce.map.memory.mb is 1024 as per the settings in MRJobConfig.java 

So it does not seem like a good idea to set defaults to random values that one particular instance of hRaven might be using.  Do you see the default setting to be 1536 in your code base? 

https://svn.apache.org/repos/asf/hadoop/common/trunk/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/MRJobConfig.java

A better way to handle this would be to set the param in your cluster config files.