Please refer to this discussion on the Hadoop Mailing list:
I was running YARN(Hadoop2.0.0 Alpha) on a 8 datanode, 4 admin node Hadoop/HBase cluster. My datanodes were only having 3.2GB of memory. So, i configured the yarn.nodemanager.resource.memory-mb property in yarn-site.xml to 1200. After setting the property if i run any Yarn Job then the NodemManager wont be able to start any Map task since by default the yarn.app.mapreduce.am.resource.mb property is set to 1500 MB in mapred-site.xml.
Expected Behavior: NodeManager should give an error if yarn.app.mapreduce.am.resource.mb >= yarn.nodemanager.resource.memory-mb.
Please let me know if more information is required.