Good doc as below and welcome to comment and modify:

https://docs.google.com/document/d/1e4anwDiS18cWP49FAghU6tqqdtnRKUcbNJJxvhIfvIA/edit#heading=h.t762l56r1094

Problem Statement

MXNet inherent distributed training mechanism, parameter server, provides efficient communication in ASGD and fault tolerance especially in cloud environment. But we found that in small-scale number of nodes (8-64) mpi allreduce can achieved the scaling efficiency close to linear while there's no extra server node deployment. So we suggest to add mpi-allreduce as an alternative choice for customer in MXNet multi-node distributed training. 

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