Correct me if I'm wrong, but my understanding is that Hadoop does not use MPI for communication between different nodes.

What are the technical reasons for this?

I could hazard a few guesses, but I do not know enough of how MPI is implemented "under the hood" to know whether or not I'm right.

Come to think of it, I'm not entirely familiar with Hadoop's internals either. I understand the framework at a conceptual level (map/combine/shuffle/reduce and how that works at a high level) but I don't know the nitty gritty implementation details. I've always assumed Hadoop was transmitting serialized data structures (perhaps GPBs) over a TCP connection, eg during the shuffle phase. Let me know if that's not true.