What does the last reducer do?

[MemphisMeng]

I think the first two map/reduce process has completed all you need to do. So the last "AggregateReducer" looks like a redundance because it just repeats what the second reducer does. Am I right? Or is there anything I missed?

[swalkaus]

Not quite. Typically there would be multiple instances of CountTrianglesReducer, on different cores / machines. The last job sums across all of them.