How does Hadoop choose machines for Reducers?

[Nathan Marz]

I have a MapReduce application in which I configure 16 reducers to run  
on 15 machines. My mappers output exactly 16 keys, IntWritable's from  
0 to 15. However, only 12 out of the 15 machines are used to run the  
16 reducers (4 machines have 2 reducers running on each). Is there a  
way to get Hadoop to use all the machines for reducing?