mpi enabled design doc

doc/fluid/design/dist_train/mpi_enabled_design.md

 #MPI-enabled PaddlePaddle Design doc
-## Overview
+
+# Background
+Now, PaddlePaddle Fluid with Distribution has relatively large network bottleneck, We want to use RDMA and GPUDriect to improve and solve it, so we enabled the features to PaddlePaddle with the help of MPI.
+
 We will introduce Open MPI API to PaddlePaddle, which can bring two benefits to PaddlePaddle:
-1. Enable RDMA with PaddlePaddle, which bring high performance low latency networks.
-2. Enable GPUDriect with PaddlePaddle, which bring highest throughput and lowest latency GPU read and write.
+1. Enable RDMA with PaddlePaddle, which bring high-performance low latency networks.
+2. Enable GPUDriect with PaddlePaddle, which bring the highest throughput and lowest latency GPU read and write.
 
-## Global Config
-Launch the script using the 'mpirun' launcher, For example: ```mpirun -np 3 -hosts node1,node2,node3 python train.py```. By doing this, We can number the actors (trainer/pserver/master) whith o .. (n-1). The actor's number is the Rank of the calling process in group of comm (integer),  The MPI processes identify each other using an Rank ID. We have to create a mapping between PaddlePaddle's actors and there Rank ID, so that we can communicate with the correct destinations when using MPI operations.
+## Execute args
+Launch the script using the ```mpirun``` launcher, For example: ```mpirun -np 3 -hosts node1,node2,node3 python train.py```. By doing this, We can number the actors (trainer/pserver/master) with o .. (n-1). The node's number is the Rank of the calling process in a group of comm (integer),  The MPI processes identify each other using a Rank ID. We have to create a mapping between PaddlePaddle's actors and their Rank ID so that we can communicate with the correct destinations when using MPI operations.
     **We have to store the Rank ID and the mapping in global variables.**
 
-#Utils
-We will build mpi_send_recv_utils Class to unify package  interface about MPI Send and Receive.
-```c++
-#mpi send and receive utils
-class Mpi_ISend {
-    
-}
-class Mpi_IRecv {
-    
-}
-
-class MPIUtils {
-    public:
-        const int GetRankID(const std::string& task_id);
-        void InitMPI();
-    private:
-        std::map<std::string, int> name_to_id_;
-}
-
-```
-```c++
-class MPIServer {
-    public:
-        SetCond();
-        ShutDown();
-        WaitClientGet();
-        reset();
-        Push();
-        SetScope();
-        SetDevCtx();
-        get();
-}
-```
-
-## New OP
-We won't replace all the gRPC requests to MPI requests,  the standard gRPC library is used for all administrative operations  and the MPI API will used to transfer tensor or selectRows to Pservers. Base of this idea, we create two new operators to handle requests and receives,  the two  operators are send_mpi_op and listenandserve_mpi_op. They are a little similar with [send_op](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/fluid/operators/send_op.cc) and [listen_and_serv_op](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/fluid/operators/listen_and_serv_op.cc).
-
-### send_mpi_op
-vary similar with send_op, we will replace grpc with mpi send service.
-### listenandserve_mpi_op
-vary similar with listen_and_serv_op, we will replace grpc with mpi receive service.
+## New OP/MODULE
+We won't replace all the gRPC requests to MPI requests,  the standard gRPC library is used for all administrative operations and the MPI API will be used to transfer tensor or selectRows to Pservers. The base of this idea, we create two new operators to handle requests and receives,  the two operators are send_mpi_op and listenandserve_mpi_op. They are a little similar with [send_op](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/fluid/operators/send_op.cc) and [listen_and_serv_op](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/fluid/operators/listen_and_serv_op.cc), also, We will build  a new module to package MPI send and receive process.
+
+### mpi_module
+We will build a new module to package MPI send and receive process. MPI send and recvice are defferent to gRPC, the MPI [recvice](https://www.open-mpi.org/doc/v1.8/man3/MPI_Irecv.3.php) must know receive buffer size and receive buffer element. For this reason, We have to make conmunications twice, the first one is to send metadata about gradient through gRPC, the second one is the real conmunications through MPI which send gradient data to mpi_listenandserve_op.
+The detail flow is below:
+![](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/fluid/design/dist_train/src/mpi_module.png)
+
+### mpi_send_op
+Very similar with ```send_op```, we will replace gRPC code which used to send gradient with ```mpi_module```, at the same time , we will wrap it with ```framework::Async```.
+
+### mpi_listenandserve_op
+Very similar with ```listen_and_serv_op```, we will replace gRPC code which used to receive gradient  with ```mpi_module```, at the same time , we will wrap it with ```framework::Async```.
+
+### modify distribute_transpiler.py
+Need to add args to distinguish use MPI or not. if  confirm to use MPI, we will modify  ```send_op``` to ```mpi_send_op``` in distribute_transpiler, and modify ```listenandserve_op``` to ```mpi_listenandserve_op``` also.
+
 ## Build args
-Beause MPI or CUDA need hardware supported, so we will add some build args to control compiling.
-**The specific arguments is under design**
-## Execute args
+Because MPI or CUDA need hardware supported, so we will add some build args to control compiling.
+**The specific arguments are under design**
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-Launch the script using the 'mpirun' launcher, For example: ```mpirun -np 3 -hosts node1,node2,node3 python train.py```.
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