Example-2-MPI_Scatterv,MPI_Send,MPI_Recv

此例将例1扩展一点，根进程分散数据后，进程间再进行数据交换，具体来说，第i个进程发送一定量的数据到第i+1和第i-1个进程。注意每个进程要更新当前数据，数据交换过程如下图所示：

代码如下：

#include "mpi.h"
#include <cstdio>
#include <math.h>
#include <iostream>
#include <vector>
#include <stdlib.h>

#define np 203  //number of elements need to be scattered at root
const int MASTER = 0; // Rank of of the master process

int main(int argc, char* argv[])
{
    std::vector<double> PosX(np);
    std::vector<double> RecPosX;
    int nProcs , Rank ;//number of processes, and id of rank
    int i,j;
    int reminder; //reminder if it is not divisible
    double StartTime,EndTime; //timing
    MPI_Status status;

    MPI_Init ( & argc , & argv );
    MPI_Comm_size ( MPI_COMM_WORLD , & nProcs );
    MPI_Comm_rank ( MPI_COMM_WORLD , & Rank );

    StartTime = MPI_Wtime(); //start timing

    //initialize array
    int* sendcounts = NULL ;
    int* displs = NULL ;
    int* TotLocalNum;

    int* numsendright;
    int* numsendleft;

    double *sendbufright,*recvbufright;
    double *sendbufleft,*recvbufleft;
    int rightrank,leftrank;

    //Initialize the data
    if ( Rank == MASTER )
    {
        for(i=0;i<np;i++)
        {
            //PosX.push_back(i);
            //PosX.resize(np,-1);
            PosX[i] = i * 1.0;

        }
    }

    sendcounts = new int [ nProcs ]; //allocate memory for array storing number of elements
    numsendright = new int[ nProcs ];
    numsendleft = new int[ nProcs ];
    TotLocalNum = new int[ nProcs ]; //store the total number in each process

    reminder = np%nProcs; //remineder

    //calculate number of elements need to be scattered in each process
    for(i=0;i<nProcs;i++)
    {
        sendcounts [i] = int(1.0*np/nProcs); 
    }
    //number of elements in the last process
        sendcounts [nProcs-1] = sendcounts [nProcs-1] + reminder;
    //calculate corresponding displacement  
    //std::cout<<sendcounts[Rank]<<std::endl;   
    displs = new int [ nProcs ];
    for(i=0;i<nProcs;i++)
        {
            displs[i] = 0;
            for(j=0;j<i;j++)
            {
                displs[i] = displs[i] + sendcounts[j];
            }
        }
        //allocate the receive buffer   
    for(i=0;i<sendcounts[Rank];i++)
    {
        RecPosX.push_back(i);
        RecPosX.resize(sendcounts[Rank],-100);
    }       
        MPI_Scatterv(&PosX[0], sendcounts, displs, MPI_DOUBLE, 
                     &RecPosX[0], sendcounts[Rank], MPI_DOUBLE, 0, MPI_COMM_WORLD);

    //output results after MPI_Scatterv operation
    std::cout<<"My Rank = "<<Rank<<" And data I received are: "<<sendcounts[Rank]<<std::endl;
    for(i=0;i<sendcounts[Rank];i++)
    {
        RecPosX[i] = RecPosX[i] + Rank * 10000;
        std::cout<<RecPosX[i]<<"  ";
        if((i+1) %10 == 0)std::cout<<std::endl;
    }
        std::cout<<std::endl;
    std::cout<<"Now Sending Data to the RIGHT neighboring process......"<<std::endl;
    //specify how many elements will be sent or received in each process
        for(i=0;i<nProcs;i++)
        {
            numsendright[i] = i + 5;
            numsendleft[i] = i + 4;
        }
        //Allocate the memory
        sendbufright = (double *)malloc(numsendright[Rank] * sizeof(double));
        recvbufright = (double *)malloc(numsendright[Rank] * sizeof(double));
        sendbufleft = (double *)malloc(numsendleft[Rank] * sizeof(double));
        recvbufleft = (double *)malloc(numsendleft[Rank] * sizeof(double));
   //randomly specify the data needs to be sent to the right neighbor
        int id;
        for(i=0;i<numsendright[Rank];i++)
        {
            id = rand() % sendcounts[Rank];
            sendbufright[i] = RecPosX[id];
            RecPosX.erase(RecPosX.begin()+id);
            sendcounts[Rank]=sendcounts[Rank]-1;
        }

        TotLocalNum[Rank] = sendcounts[Rank];

        rightrank = (Rank + 1) % nProcs;
        leftrank = (Rank + nProcs-1)%nProcs;

        //Each process sends data to its right process
        MPI_Send(sendbufright,numsendright[Rank],MPI_DOUBLE,rightrank,99,MPI_COMM_WORLD);

        std::cout<<"Rank= "<<Rank<<" SEND "<<numsendright[Rank]<<" Data To Rank= "<<rightrank<<std::endl;
        for(i=0;i<numsendright[Rank];i++)
        {
            std::cout<<i<<'\t'<<sendbufright[i]<<std::endl;
        }
        //Each process receives data from its left process
        MPI_Recv(recvbufright,numsendright[leftrank],MPI_DOUBLE,leftrank,99,MPI_COMM_WORLD,&status);

        sendcounts[Rank]  = sendcounts[Rank] + numsendright[leftrank];
        TotLocalNum[Rank] = TotLocalNum[Rank] + numsendright[leftrank];

        std::cout<<std::endl;
        std::cout<<"Rank= "<<Rank<<" RECEIVE "<<numsendright[leftrank]<<" Data From Rank= "<<leftrank<<std::endl;
        for(i=0;i<numsendright[leftrank];i++)
        {
            std::cout<<i<<'\t'<<recvbufright[i]<<std::endl;
        }

        //insert the received data here
        for(i=0;i<numsendright[leftrank];i++)
        {
            RecPosX.insert(RecPosX.end(),recvbufright[i]);
        }

        //MPI_Barrier(MPI_COMM_WORLD);
        std::cout<<std::endl;
        std::cout<<"Total number in Rank= "<<Rank<<" is "<<TotLocalNum[Rank]<<std::endl;
        for(i=0;i<TotLocalNum[Rank];i++)
        {
            std::cout<<RecPosX[i]<<"  ";
            if((i+1) %10 == 0)std::cout<<std::endl;
        }

        ///////////////////////////Start to send data to the left neighbor//////////////////////
        //Step 1: Prepare data need to be sent to the left neighboring process
        //randomly specify the data 
        std::cout<<std::endl;
        std::cout<<"Now Sending Data to the LEFT neighboring process......"<<std::endl;
        for(i=0;i<numsendleft[Rank];i++)
        {
            id = rand() % sendcounts[Rank];
            sendbufleft[i] = RecPosX[id];
            RecPosX.erase(RecPosX.begin()+id);
            sendcounts[Rank]=sendcounts[Rank]-1;
        }
        TotLocalNum[Rank] = sendcounts[Rank]; //Updating local number of data
        //Step 2
        //Each process sends data to its left process
        MPI_Send(sendbufleft,numsendleft[Rank],MPI_DOUBLE,leftrank,199,MPI_COMM_WORLD);

        std::cout<<"Rank= "<<Rank<<" SEND "<<numsendleft[Rank]<<" Data To Rank= "<<leftrank<<std::endl;
        for(i=0;i<numsendleft[Rank];i++)
        {
            std::cout<<i<<'\t'<<sendbufleft[i]<<std::endl;
        }
        //Each process receives data from its left process
        MPI_Recv(recvbufleft,numsendleft[rightrank],MPI_DOUBLE,rightrank,199,MPI_COMM_WORLD,&status);
        sendcounts[Rank]  = sendcounts[Rank] + numsendleft[rightrank];
        TotLocalNum[Rank] = TotLocalNum[Rank] + numsendleft[rightrank];
        std::cout<<std::endl;
        std::cout<<"Rank= "<<Rank<<" RECEIVE "<<numsendleft[rightrank]<<" Data From Rank= "<<rightrank<<std::endl;
        for(i=0;i<numsendleft[rightrank];i++)
        {
            std::cout<<i<<'\t'<<recvbufleft[i]<<std::endl;
        }

        //insert the received data here
        for(i=0;i<numsendleft[rightrank];i++)
        {
            RecPosX.insert(RecPosX.end(),recvbufleft[i]);
        }

        std::cout<<std::endl;

        std::cout<<"Total number in Rank= "<<Rank<<" is "<<TotLocalNum[Rank]<<std::endl;
        for(i=0;i<TotLocalNum[Rank];i++)
        {
            std::cout<<RecPosX[i]<<"  ";
            if((i+1)%10 == 0)std::cout<<std::endl;
        }

    EndTime = MPI_Wtime();
    std::cout<<std::endl;
    if(Rank==MASTER)
    {
            std::cout<<std::endl;
            std::cout<<"Total Time Spending For: "<<nProcs<<" Is "<<EndTime-StartTime<<" :Second"<<std::endl;
    }
    free(sendbufright);
    free(recvbufright);
    free(sendbufleft);
    free(recvbufleft);
    MPI_Finalize();
    return 0; 

}

设初始有203个数据，总共4个进程，以进程3为例，开始进程3得到53个数据：
30150————————————–30159
30160————————————–30169
30170————————————–30179
30180————————————–30189
30190————————————–30199
30200 30201 30202

然后进程3发送8个数据给进程0，随机抽取8个数据为：
第0个————30191
第1个————30157
第2个————30161
第3个————30150
第4个————30163
第5个————30182
第6个————30164
第7个————30165
进程3同时从进程2收到7个数据：
第0个————20141
第1个————20144
第2个————20148
第3个————20139
第4个————20133
第5个————20119
第6个————20142

此后，进程3共有52个数据：
30151…………………………………………………30162
30166…………………………………………………30175
30176…………………………………………………30186
30187…………………………………………………30197
30198…………………………………………………20133
20119 20142

然后开始向i-1进程发送数据，3号进程给2号发送7个数据：
第0个————30183
第1个————30194
……………………………………..
第6个————30187

然后3号进程从0号收到4个数据：
第0个————37
第1个————42
第0个————34
第1个————12

最后进程3上共有49个数据：

30151————————————-30168
30169————————————-30179
30180————————————-30193
30195————————————-20144
20148 20139——–37 42 34 12

执行完毕。
输出如图：