MPI_Bcast
函数将信息从一个进程(即“根”)发送到所有其他进程。相较于循环遍历所有进程,这可能会更快。例如,mpich2
实现使用二叉树来分发消息。
如果您不想广播到MPI_COMM_WORLD,但要定义子组,则可以使用以下方法:
#include <stdio.h>
#include "mpi.h"
#define NPROCS 8
int main(int argc, char **argv)
{
int rank, new_rank, sendbuf, recvbuf,
ranks1[4]={0,1,2,3}, ranks2[4]={4,5,6,7};
MPI_Group orig_group, new_group;
MPI_Comm new_comm;
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
sendbuf = rank;
MPI_Comm_group(MPI_COMM_WORLD, &orig_group);
if (rank < NPROCS/2) {
MPI_Group_incl(orig_group, NPROCS/2, ranks1, &new_group);
} else {
MPI_Group_incl(orig_group, NPROCS/2, ranks2, &new_group);
}
MPI_Comm_create(MPI_COMM_WORLD, new_group, &new_comm);
MPI_Allreduce(&sendbuf, &recvbuf, 1, MPI_INT, MPI_SUM, new_comm);
MPI_Group_rank (new_group, &new_rank);
printf("rank= %d newrank= %d recvbuf= %d\n", rank, new_rank, recvbuf);
MPI_Finalize();
}
可能会产生以下输出:
rank= 7 newrank= 3 recvbuf= 22
rank= 0 newrank= 0 recvbuf= 6
rank= 1 newrank= 1 recvbuf= 6
rank= 2 newrank= 2 recvbuf= 6
rank= 6 newrank= 2 recvbuf= 22
rank= 3 newrank= 3 recvbuf= 6
rank= 4 newrank= 0 recvbuf= 22
rank= 5 newrank= 1 recvbuf= 22