When do reduce tasks start in Hadoop?

Question

In Hadoop when do reduce tasks start? Do they start after a certain percentage (threshold) of mappers complete? If so, is this threshold fixed? What kind of threshold is typ

Answer 1

Reduce starts only after all the mapper have fished there task, Reducer have to communicate with all the mappers so it has to wait till the last mapper finished its task.however mapper starts transferring data to the moment it has completed its task.