The partitioner is configurable. The default partitioner, from what I remember, computes the partition as the hashcode modulo number of reducers/partitions. For random input, it is balanced, but some cases can have very skewed key distribution. Also, as you have pointed out, the number of values per key can also vary. Together, both of them determine "weight" of each partition as you call it.
Karthik On Mon, Jul 9, 2012 at 8:15 PM, Grandl Robert <rgra...@yahoo.com> wrote: > Thanks Arun. > > So just for my clarification. The map will create partitions according to > the number of reducers s.t. each reducer to get almost same number of keys > in its partition. However, each key can have different number of values so > the "weight" of each partition will depend on that. Also when a new <key, > value> is added into a partition a hash on the partition ID will be > computed to find the corresponding partition ? > > Robert > > ------------------------------ > *From:* Arun C Murthy <a...@hortonworks.com> > *To:* mapreduce-user@hadoop.apache.org > *Sent:* Monday, July 9, 2012 4:33 PM > > *Subject:* Re: Basic question on how reducer works > > > On Jul 9, 2012, at 12:55 PM, Grandl Robert wrote: > > 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). > > > Look at TaskTracker.MapOutputServlet. > > 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 ? > > > Essentially you can think of the partition-id as the 'primary key' and the > actual 'key' in the map-output of <key, value> as the 'secondary key'. > > hth, > Arun > > 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/ > > > > > > -- > Arun C. Murthy > Hortonworks Inc. > http://hortonworks.com/ > > > > >
