Yes. How many Reducers in Hadoop? With 0.95, all reducers immediately launch and start transferring map outputs as the maps finish. But still I am getting a different number of mapper & reducer tasks. How to set the number of mappers and reducers of Hadoop in command line? My command is. -D mapred.reduce.tasks=0 To be backward compatible, Hadoop Streaming also supports the "-reduce NONE" option, which is equivalent to "-D mapred.reduce.tasks=0". In our example from the diagram, the reduce tasks get the following individual results: