Well ,I m not sure But I think it might be the pull.. because physically the mappers and the reducers are the same nodes ,So if the Mappers had to push , it might be the case that all nodes are mapping and there are no reducers to accept it. May be for this reason ,unless all of the Mapper tasks are finished, the reducers might not want to start reducing anything @all..
There is also this sort shuffle layer between maping and reducing , it clearly demarcates the phases.. whihc seem to suggest that its the pull rather than the push .. You might think of this as a performance bottle neck, but in reality it seems it isnt . btw, Wait for some expert to answer, I m a beginner too ! On Mon, Oct 26, 2009 at 9:05 PM, Jeff Zhang <[email protected]> wrote: > Hi all, > > I'd like to know does the map task push map output to reduce task or reduce > task pull it from map task ? Which way is real in hadoop ? > > Thank you very much. > > > Jeff zhang > -- Hari
