Combiner Execution

[Mike Schliep]

For a class project my group and I are looking to experiment with 
combining the output from Mappers on the same node or in the same rack. 
We found the idea at http://wiki.apache.org/hadoop/HadoopResearchProjects.

According to http://developer.yahoo.com/hadoop/tutorial/module4.html the 
output is already combined over all Mappers in a node. But we can not 
find how this is happening. Can someone point us to where this combiner 
is executed?

Thanks,
Mike Schliep