1->n MPI_Scatter MPI_Bcast

1. 函数 MPI_Scatter

MPI_Scatter (&sendbuf,sendcnt,sendtype,&recvbuf,recvcnt,recvtype,root,comm)

Distributes distinct messages from a single source task to each task in the group.

2. 举例

#include<stdio.h>#include"mpi.h"#define SIZE 4int main(int argc, char *argv[]){        int totalNumTasks, rankID;        float sendBuf[SIZE][SIZE] = {                {1.0,   2.0,    3.0,    4.0},                {5.0,   6.0,    7.0,    8.0},                {9.0,   10.0,   11.0,   12.0},                {13.0,  14.0,   15.0,   16.0}        };        MPI_Init(&argc, &argv);        MPI_Comm_rank(MPI_COMM_WORLD, &rankID);        MPI_Comm_size(MPI_COMM_WORLD, &totalNumTasks);        if(totalNumTasks == SIZE){                int source = 0;                int sendCount = SIZE;                int recvCount = SIZE;                float recvBuf[SIZE];                //scatter data from source process to all processes in MPI_COMM_WORLD                MPI_Scatter(sendBuf, sendCount, MPI_FLOAT,                             recvBuf, recvCount, MPI_FLOAT, source, MPI_COMM_WORLD);                printf("my rankID = %d, receive Results: %f %f %f %f, total = %f\n",                         rankID, recvBuf[0], recvBuf[1], recvBuf[2], recvBuf[3],                        recvBuf[0] + recvBuf[1] + recvBuf[2] + recvBuf[3]);        }else if(totalNumTasks == 8){                int source = 0;                int sendCount = 2;                int recvCount = 2;                float recvBuf[2];                MPI_Scatter(sendBuf, sendCount, MPI_FLOAT,                             recvBuf, recvCount, MPI_FLOAT, source, MPI_COMM_WORLD);                printf("my rankID = %d, receive result: %f %f, total = %f\n",                        rankID, recvBuf[0], recvBuf[1], recvBuf[0] + recvBuf[1]);        }else{                printf("Please specify -n %d or -n %d\n", SIZE, 2*SIZE);        }        MPI_Finalize();        return 0;}

3. 编译执行

[amao@amao991 mpi-study]$ mpicc collectiveScatter.c [amao@amao991 mpi-study]$ mpiexec -n 4 -f machinefile ./a.out my rankID = 0, receive Results: 1.000000 2.000000 3.000000 4.000000, total = 10.000000my rankID = 1, receive Results: 5.000000 6.000000 7.000000 8.000000, total = 26.000000my rankID = 3, receive Results: 13.000000 14.000000 15.000000 16.000000, total = 58.000000my rankID = 2, receive Results: 9.000000 10.000000 11.000000 12.000000, total = 42.000000[amao@amao991 mpi-study]$ mpiexec -n 8 -f machinefile ./a.out my rankID = 6, receive result: 13.000000 14.000000, total = 27.000000my rankID = 7, receive result: 15.000000 16.000000, total = 31.000000my rankID = 4, receive result: 9.000000 10.000000, total = 19.000000my rankID = 0, receive result: 1.000000 2.000000, total = 3.000000my rankID = 1, receive result: 3.000000 4.000000, total = 7.000000my rankID = 5, receive result: 11.000000 12.000000, total = 23.000000my rankID = 3, receive result: 7.000000 8.000000, total = 15.000000my rankID = 2, receive result: 5.000000 6.000000, total = 11.000000[amao@amao991 mpi-study]$ mpiexec -n 2 -f machinefile ./a.out Please specify -n 4 or -n 8Please specify -n 4 or -n 8

4. 总结

(1) 由于集合通信是阻塞的，因此，每个进程自动等待，完成接收后，执行printf语句

(2) 为了能够平均划分数据，本例执行时最好-n 4 或者-n 8

(3) 在每个进程接受到数据后，可以对其求和。若要把每个进程的求和结果汇总到某个进程，要用n->1即就是MPI_Gather或者MPI_Reduce函数。

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1. 函数

MPI_Bcast (&buffer,count,datatype,root,comm)

Broadcasts (sends) a message from the process with rank "root" to all other processes in the group. 

2. 图示

3. 举例

#include<stdio.h>#include"mpi.h"int main(int argc, char *argv[]){        int rankID, totalNumTasks;        MPI_Init(&argc, &argv);        MPI_Barrier(MPI_COMM_WORLD);        double elapsed_time = -MPI_Wtime();        MPI_Comm_rank(MPI_COMM_WORLD, &rankID);        MPI_Comm_size(MPI_COMM_WORLD, &totalNumTasks);        int sendRecvBuf[3] = {0, 0, 0};        if(!rankID){                sendRecvBuf[0] = 3;                sendRecvBuf[1] = 6;                sendRecvBuf[2] = 9;        }        int count = 3;        int root = 0;        MPI_Bcast(sendRecvBuf, count, MPI_INT, root, MPI_COMM_WORLD); //MPI_Bcast can be seen from all processes        printf("my rankID = %d, sendRecvBuf = {%d, %d, %d}\n", rankID, sendRecvBuf[0], sendRecvBuf[1], sendRecvBuf[2]);        elapsed_time += MPI_Wtime();        if(!rankID){                printf("total elapsed time = %10.6f\n", elapsed_time);        }        MPI_Finalize();        return 0;}

4. 编译执行

[amao@amao991 mpi-study]$ mpicc broadcast.c

[amao@amao991 mpi-study]$ mpiexec -f machinefile -n 5 ./a.outmy rankID = 0, sendRecvBuf = {3, 6, 9}total elapsed time =   0.000045my rankID = 2, sendRecvBuf = {3, 6, 9}my rankID = 4, sendRecvBuf = {3, 6, 9}my rankID = 1, sendRecvBuf = {3, 6, 9}my rankID = 3, sendRecvBuf = {3, 6, 9}

5. 总结

(1) 在记录起始时间之前，用MPI_Barrier(MPI_COMM_WORLD);对所有进程做同步

(2) MPI_Bcast(sendRecvBuf, count, MPI_INT, root, MPI_COMM_WORLD); 

要对所有进程可见，若只对process 0可见，是错误的。其他进程也需要知道它而接收啊！