MPI_Reduce是一个集体操作。这意味着参与通信器中的所有任务都必须进行MPI_Reduce()
调用。在上面,等级 0 永远不会调用MPI_Reduce()
,因此该程序将挂起,因为其他一些处理器等待来自永远不会到来的等级 0 的参与。
另外,因为它是对整个通信器的集体操作,所以需要做一些工作来对归约进行分区。一种方法是减少整数数组,并让每个处理器仅对数组中的元素做出贡献:
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
int main(int argc, char** argv)
{
int size, rank;
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&size);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
int localsum[2] = {0,0};
int globalsum[2] = {0,0};
if(rank % 2 == 1)
{
localsum[0] += 5;
}
else if( rank > 0 && (rank % 2 == 0))
{
localsum[1] += 10;
}
MPI_Reduce(localsum,globalsum,2,MPI_INT,MPI_SUM,0,MPI_COMM_WORLD);
if(rank==0)
{
printf("globalsum1 = %d \n",globalsum[0]);
printf("globalsum2 = %d \n",globalsum[1]);
}
MPI_Finalize();
return (EXIT_SUCCESS);
}
现在跑步的地方
$ mpicc -o reduce reduce.c
$ mpirun -np 3 ./reduce
globalsum1 = 5
globalsum2 = 10
否则,您可以创建仅连接您希望参与每个求和的处理器的通信器,并在每个通信器内进行归约。下面是一个不太漂亮的方法来做到这一点。这通常非常强大,但比第一个解决方案更复杂:
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
int main(int argc, char** argv)
{
int size, rank;
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&size);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
int localsum = 0;
int globalsum = 0;
MPI_Comm comm_evens_plus_root, comm_odds_plus_root;
MPI_Group grp_evens_plus_root, grp_odds_plus_root, grp_world;
MPI_Comm_group(MPI_COMM_WORLD, &grp_world);
int *ranks = malloc((size/2 + 1) * sizeof(rank));
int i,j;
for (i=1, j=0; i<size; i+=2, j+=1)
ranks[j] = i;
MPI_Group_excl(grp_world, j, ranks, &grp_evens_plus_root);
MPI_Comm_create(MPI_COMM_WORLD, grp_evens_plus_root, &comm_evens_plus_root);
for (i=2, j=0; i<size; i+=2, j+=1)
ranks[j] = i;
MPI_Group_excl(grp_world, j, ranks, &grp_odds_plus_root);
MPI_Comm_create(MPI_COMM_WORLD, grp_odds_plus_root, &comm_odds_plus_root);
free(ranks);
if(rank % 2 == 1)
{
localsum += 5;
MPI_Reduce(&localsum,&globalsum,1,MPI_INT,MPI_SUM,0,comm_odds_plus_root);
}
else if( rank > 0 && (rank % 2 == 0))
{
localsum += 10;
MPI_Reduce(&localsum,&globalsum,1,MPI_INT,MPI_SUM,0,comm_evens_plus_root);
}
if(rank==0)
{
MPI_Reduce(&localsum,&globalsum,1,MPI_INT,MPI_SUM,0,comm_odds_plus_root);
printf("globalsum1 = %d \n",globalsum);
MPI_Reduce(&localsum,&globalsum,1,MPI_INT,MPI_SUM,0,comm_evens_plus_root);
printf("globalsum2 = %d \n",globalsum);
}
MPI_Comm_free(&comm_odds_plus_root);
MPI_Comm_free(&comm_evens_plus_root);
MPI_Group_free(&grp_odds_plus_root);
MPI_Group_free(&grp_evens_plus_root);
MPI_Finalize();
return (EXIT_SUCCESS);
}
运行给
$ mpicc -o reduce2 reduce2.c
$ mpirun -np 3 ./reduce
globalsum1 = 5
globalsum2 = 10