I'll answer your questions in reverse.
> 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?
You'll find the Combiner runner somewhere buried inside MapTask.java,
hunt for the combinerRunner in there.
The Combiner only combines the output of a single map-task (after
sorting). This kicks in only if there are spills in that 1 map-task >
It does not do any cross-task actions and the MR framework (as it is
today) doesn't leave enough room for scheduling a cross-task activity
(i.e MR is strictly bi-partite).
> 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.
Your general idea is sort of chalked out in Apache Tez
(per-host/per-rack multi-level combiner trees, which is designed to be
more flexible with its plumbing) -
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