It depends of workload. Could you tell us more specification about
your job? In general case which reducers are bottleneck, there are
some tuning techniques as follows:
1. Allocate more memory to reducers. It decreases disk IO of reducers
when merging and running reduce functions.
2. Use combine function, which enable mapper-side aggregation
processing, if your MR job consists of the operations that satisfy
both the commutative and the associative low.
See also about combine functions:
On Tuesday, August 28, 2012, Gaurav Dasgupta wrote:
> I have run some large and small jobs and calculated the Total Shuffle Time for the jobs. I can see that the Total Shuffle Time is almost half the Total Time which was taken by the full job to complete.
> My question, here, is that how can we decrease the Total Shuffle Time? And doing so, what will be its effect on the Job?
> Gaurav Dasgupta