Question

If Reducers do not start before all Mappers finish then why does the progress on Mapreduce Job shows something like Map 50 percents Reduce 10 percents and why Reducers progress percentage is displayed when Mapper is not Finished yet?

Answer 1

Reducers start copying intermediate key-value pairs from the mappers as soon as they are available. The progress calculation also takes in account the processing of data transfer which is done by reduce process, therefore the reduce progress starts showing up as soon as any intermediate key-value pair for a mapper is available to be transferred to reducer. Though the reducer progress is updated still the programmer defined reduce method is called only after all the mappers have finished.