When do Reduce tasks start in Hadoop

Question

When do the Reducers start in any certain Hadoop Job?

Does it follow any sort of prerequisites that a certain amount of Mapper task should be finished? If so, then please elaborate it.

Answer 1

As much I understand Reduce phase start with the map phase and keep consuming the record from maps. However, since there are sort and shuffle phase after the map phase all the outputs have to be sorted and sent to the reducer. So logically you can imagine that reduce phase starts only after map phase but actually for performance reason reducers are also initialized with the mappers.