Thanks a lot guys for answers.
Still I am not able to find exactly the code for the following things: 1. reducer to read from a Map output only its partition. I looked into ReduceTask#getMapOutput which do the actual read in ReduceTask#shuffleInMemory, but I don't see where it specify which partition to read(reduceID). 2. still don't understand very well in which part of the code(MapTask.java) the intermediate data is written do which partition. So MapOutputBuffer is the one who actually writes the data to buffer and spill after buffer is full. Could you please elaborate a bit on how the data is written to which partition ? Thanks, Robert ________________________________ From: Arun C Murthy <a...@hortonworks.com> To: mapreduce-user@hadoop.apache.org Sent: Monday, July 9, 2012 9:24 AM Subject: Re: Basic question on how reducer works Robert, On Jul 7, 2012, at 6:37 PM, Grandl Robert wrote: Hi, > > >I have some questions related to basic functionality in Hadoop. > > >1. When a Mapper process the intermediate output data, how it knows how many >partitions to do(how many reducers will be) and how much data to go in each >partition for each reducer ? > > >2. A JobTracker when assigns a task to a reducer, it will also specify the >locations of intermediate output data where it should retrieve it right ? But >how a reducer will know from each remote location with intermediate output >what portion it has to retrieve only ? To add to Harsh's comment. Essentially the TT *knows* where the output of a given map-id/reduce-id pair is present via an output-file/index-file combination. Arun -- Arun C. Murthy Hortonworks Inc. http://hortonworks.com/
