What do you understand by Uber Mode?
By default, Mappers and Reducers are run by ResourceManager, where ResourceManager launches separate JVM container for map and reduce tasks. The uber mode in YARN 2, allows running mapper and reducers in the same JVM as the ApplicationMaster master, avoiding the overhead of launching and communicating with remote containers.
If we have small data set, Uber configuration is used for MapReduce.
The Uber mode runs the map and reduce tasks within its own process and avoid overhead of launching and communicating with remote nodes.