In MR (Map-Reduce) model, reducings are not balanced, because the scale of partitiones are unbalanced. How to balance? We can control the size of partition, rehash the bigger parition and combine to the specified size. If a key has many values, it's necessary to execute mapreduce twice.The following is the model digram:
[image: Map-Balance-Reduce.JPG] Scheduler can regard a task as a timeslice similarly OS scheduler.
