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MapReduce >> mail # dev >> How to modify the Map-Reduce execution order?

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How to modify the Map-Reduce execution order?
(Please correct me if I am wrong) So the original chain is:
InputSplits-->Mapper--> [Sorting/Shuffling, etc]-->Reducer-->...

Now I don't want the input splits to get to the Mappers first, but to go to
some other new stage instead (we can call it Pre-Mapper for example, this
class will be created by myself).

So the new order will be: InputSplits -> Pre-Mapper->Mapper ->...

I'm currently reading the source code. However, I still cannot find any
clue (what classes I should touch). Any suggestion is welcome. Thank you
very much :)