Does reduce start only after the map is completed?

[Mithila Nagendra]

Hey all
Im using the hadoop version 0.18.3, and was wondering if the reduce phase
starts only after the mapping is completed? Is it required that the Map
phase is a 100% done, or can it be programmed in such a way that the reduce
starts earlier?

Thanks!
Mithila Nagendra
Arizona State University