RecommenderJob Hadoop execution times

[Nikolaos Romanos Katsipoulakis]

Hey everybody.
I am working on a recommender system that uses Hadoop for generating 
Item Similarities. Since mahout has the RecommenderJob example, I tried 
to run the recommender in my hadoop (pseudo-) cluster. I noticed that on 
the MapReduce job : CoocurencesMapper - SimilarityReducer, there is a 
big overhead (approximately 6 minutes). When the mapping ends, there is 
a huge time gap until the reducer initiates, but one CPU was fully 
loaded during that time. Why is this happening? Is there an I/O 
operation hidden on this time gap?

Thank you