Just finished reading the following paper on MapReduce. One question - does reduce wait until all map operations are finished or does can it start once some results are available?
In a MapReduce job reducers do not start executing the reduce method until the all Map jobs have completed. Reducers start copying intermediate key-value pairs from the mappers as soon as they are available.That's why we are able to see in the Job-tracker that reduce show few % when some maps are still running ..