architecture diagram

[Terrence A. Pietrondi]

I am trying to plan out my map-reduce implementation and I have some questions 
of where computation should be split in order to take advantage of the 
distributed nodes. 

Looking at the architecture diagram 
(http://hadoop.apache.org/core/images/architecture.gif), are the map boxes the 
major computation areas or is the reduce the major computation area?

Thanks.

Terrence A. Pietrondi