Hello,
If a combiner is set, the combiner might run in parallel with the map
or the reducer.
I say might because it is not guaranteed that the combiner will run.

Q: How many combiner threads in a JVM are run in parallel? One? So,
for example, suppose
a map is processing its tasks will there be a single combiner thread
(if at all) running in parallel?
Thank you
Sapsi