c - 为什么这个 MPI 程序停滞

Question

//#define SIZE 3
void master(int n,int size)
{
for(int j=1;j<size;j++){
    MPI_Send(&n,1,MPI_INT,j,1,MPI_COMM_WORLD);
    printf("\n Sent %d to process %d",n,j);
    fflush(stdout);
    }
}

void slave(int size)
{
for(int j=1;j<size;j++){
    int k=0;
    MPI_Status status;
    MPI_Recv(&k,1,MPI_INT,0,1,MPI_COMM_WORLD,&status);
    printf("\n Process %d has received %d",j,k);
    fflush(stdout);
    }
}

int main(int argc,char** argv)
{

MPI_Init(&argc,&argv);
int la_size;
int rank;
MPI_Comm_size(MPI_COMM_WORLD,&la_size);

MPI_Comm_rank(MPI_COMM_WORLD,&rank);

for(int i=0;i<3;i++){
    if(rank==0)
        master(i,la_size);
    else
        slave(la_size);

}
MPI_Finalize();
printf("\nprogram finished...");
fflush(stdout);
return 0;

}  

上面的程序看起来很简单但是它停滞了。这是一个僵局吗？ 输出是:

     Sent 0 to process 1  
 Sent 0 to process 2  
 Sent 1 to process 1  
 Sent 1 to process 2  
 Process 1 has received 0  
 Process 2 has received 1  
 Process 1 has received 2  
 Sent 2 to process 1  
 Sent 2 to process 2  
 Process 1 has received 0  
 Process 2 has received 1  
 Process 1 has received 2

Answer 1

在 main 循环的每次迭代中，等级 0 对许多从等级中的每一个进行一次发送，但是每个从等级都发送与从等级中的从等级一样多的接收全部的。由于没有发布的发送来匹配后来的接收，接收者无限期地阻塞，程序挂起。