how to shuffle sort mapreduce

Shuffling is the process by which intermediate data from mappers are transferred to 0,1 or more reducers. Each reducer receives 1 or more keys and its associated values depending on the number of reducers (for a balanced load). Further the values associated with each key are locally sorted.

http://stackoverflow.com/questions/22141631/what-is-the-purpose-of-shuffling-and-sorting-phase-in-the-reducer-in-map-reduce