c - MPI vector 乘法

Question

#include<stdio.h>
#include<mpi.h>

int main()
{
        int a_r = 0, a_c = 0, v_s = 0, i = 0, rank = 0, size = 0;
        int local_row = 0, partial_sum = 0, sum = 0, j = 0;
        int my_first_ele = 0, my_last_ele = 0;
        int a[10][10], v[10], partial_mul[10] = {0}, mul[10] = {0};

        MPI_Init(NULL, NULL);
        MPI_Comm_rank(MPI_COMM_WORLD, &rank);
        MPI_Comm_size(MPI_COMM_WORLD, &size);

        if(rank == 0)
        {
                printf("Enter the row of array A: ");
                scanf("%d", &a_r);
                printf("Enter the column of array A: ");
                scanf("%d", &a_c);
                printf("Enter the array A: ");

                for(i = 0; i < a_r; i++)
                {
                        for(j = 0; j < a_c; j++)
                                scanf("%d", &a[i][j]);
                }

                printf("Enter the size of vector array: ");
                scanf("%d", &v_s);
                printf("Enter the vector array: ");
                for(i = 0; i < v_s; i++)
                {
                        scanf("%d", &v[i]);
                }

                MPI_Bcast(&a_r, 1, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(&a_c, 1, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(&v_s, 1, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(a, a_r*a_c, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(v, v_s, MPI_INT, 0, MPI_COMM_WORLD);

                local_row = a_r / size;
                my_first_ele = rank * local_row;
                my_last_ele = (rank+1) * local_row;

                if(a_c == v_s)
                {      
                        for(i = my_first_ele; i < my_last_ele; i++)
                        {
                                for(j = 0; j < a_c; j++)
                                {
                                        partial_mul[i] = partial_mul[i] + (a[i][j]*v[j]);
                                }
                        }
                        printf("\nPartial multiplication in Rank 0: \n");
                        for(i = my_first_ele; i < my_last_ele; i++)
                                printf("%d \n", partial_mul[i]);

                        MPI_Gather(partial_mul, local_row, MPI_INT, mul, local_row, MPI_INT, 0, MPI_COMM_WORLD);

                        printf("\n \nGlobal Multiplication: \n");
                        for(i = 0; i < a_r; i++)
                        {
                                printf("%d \n", mul[i]);
                        }
                }
                else
                        printf("\nCan't multiply. \n");
        }

        else
        {
                MPI_Bcast(&a_r, 1, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(&a_c, 1, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(&v_s, 1, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(a, a_r*a_c, MPI_INT, 0, MPI_COMM_WORLD);
                MPI_Bcast(v, v_s, MPI_INT, 0, MPI_COMM_WORLD);

                local_row = a_r / size;
                my_first_ele = rank * local_row;
                my_last_ele = (rank+1) * local_row;
                if(a_c == v_s)
                {     
                        for(i = my_first_ele; i < my_last_ele; i++)
                        {
                                for(j = 0; j < a_c; j++)
                                {
                                        partial_mul[i] = partial_mul[i] + (a[i][j]*v[j]);
                                }
                        }
                        printf("\nPartial multiplication in Rank %d: \n", rank);
                        for(i = my_first_ele; i < my_last_ele; i++)
                                printf("%d \n", partial_mul[i]);

                        MPI_Gather(partial_mul, local_row, MPI_INT, mul, local_row, MPI_INT, 0, MPI_COMM_WORLD);

                }
                else
                        printf("\nCan't multiply. \n");
        }
        MPI_FINALIZE();
}

我对上面的代码有疑问。我的部分乘法值是正确的。但在我的总体乘法中，我只能收集排名 0 的元素，其余值被打印为 0。任何人都可以解释一下问题是什么。

Answer 1

看看你的数据布局，我认为你误解了 MPI 中的数据结构:所有数据在每个等级中都是分开保存的，没有隐式共享或分布。您的 vector partial_sum 在每个等级上都是独立的，每个等级都有完整的 10 个元素。因此，假设 size=2、a_r=10 和零初始化，计算后内容将如下所示:

• 排名 0:{x0,x1,x2,x3,x4,0,0,0,0,0}
• 排名 1:{0,0,0,0,0,x5,x6,x7,x8,x9}

其中 x 是正确的计算值。然后，Gather 会收集每个等级的前 local_row=5 元素，从而得到 {x0,x1,x2,x3,x4,0,0,0,0,0}.

您可以通过添加正确的偏移量来解决此问题:

MPI_Gather(&partial_mul[my_first_ele], local_row, MPI_INT, mul, local_row, MPI_INT, 0, MPI_COMM_WORLD);

但请不要这样做。相反，您应该重新考虑数据结构以真正分布数据，为 vector/数组的每个部分保留正确的大小。要将部分数据发送到每个等级，请使用 MPI_Scatter(与 MPI_Gather 相反)。最困难的是得到正确的矩阵。 this excellent answer对此进行了详细解释。 .