hadoop - On what basis mapreduce framework decides whether to launch a combiner or not

Question

As per definition "The Combiner may be called 0, 1, or many times on each key between the mapper and reducer."

I want to know that on what basis mapreduce framework decides how many times cobiner will be launched.

Answer 1

Simply the number of spills to disk. Sorting happens after the MapOutputBuffer filled up, at the same time the combining will take place.

You can tune the number of spills to disk with the parameters io.sort.mb, io.sort.spill.percent, io.sort.record.percent - those are also explained in the documentation (books and online resources).

Example for specific numbers of combiner runs:

0 -> no combiner was defined

1 -> a combiner was defined and the MapOutputBuffer filled up once

>1 -> a combiner was defined and the MapOutputBuffer filled up more than once

Note that even if the MapOutputBuffer never fills up completely, this buffer must be flushed at the end of the map stage and thus triggers the combiner to run at least once (if defined).