c - C语言中使用MPI添加数组

Question

我是 MPI 编程的新手。在下面的代码中，我尝试使用流程 1 添加前 3 个元素，使用流程 2 添加最后 3 个元素。结果应显示前三个元素的 Sum is 11 和 Sum is 18 最后三个元素。我看到我的问题的原因是在第 2 级(进程 2)它读取 arr[3] 作为数组的第一个元素。我真的很迷惑为什么它不能在第 2 位正确读取数组的第 3 个元素

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

int main (int argc, char *argv[])
{
        MPI_Init(&argc, &argv);
        int world_rank;
        MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
        int world_size;
        MPI_Comm_size(MPI_COMM_WORLD, &world_size);
        int tag2 = 1;
        int tag1 = 2;       
        int arr[7] = { 6,2,3,9,4,5};

        printf ("\n--Current Rank: %d\n", world_rank);
        int index;
        int source = 0;
        int dest;
        if (world_rank == 0)
        {
            printf("* Rank 0 excecuting");
            index = 0;
            dest = 1;
            MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
            MPI_Send(&arr, 2, MPI_INT, dest, tag2, MPI_COMM_WORLD); 

            index = 3;
            dest = 2;
            MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
            MPI_Send(&arr, 1, MPI_INT, dest, tag2, MPI_COMM_WORLD);
        }
        else 
        {
            int sum = 0;
            int i;
            MPI_Recv(&index, 1, MPI_INT, source, tag1, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            MPI_Recv(&arr[index], 20, MPI_INT, source, tag2, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("At Rank: %d index is: %d\n", world_rank, index);
            for(i = index; i<=index+2; i++)
            {   
                printf("i: %d and arr[i]: %d\n", i, arr[i]);
                sum = arr[i]+sum;
            }
            printf("\n Sum is: %d and arr[3]: %d\n", sum, arr[3]);
        }       

        MPI_Finalize();
}

使用命令后得到的结果:mpirun -np 3 test1

--Current Rank: 0

--Current Rank: 1
At Rank: 1 index is: 0
i: 0 and arr[i]: 6
i: 1 and arr[i]: 2
i: 2 and arr[i]: 3

 Sum is: 11 and arr[3]: 9 //here it shows the correct value of 3rd array element

--Current Rank: 2
At Rank: 2 index is: 3
i: 3 and arr[i]: 6 //Error happens here
i: 4 and arr[i]: 4
i: 5 and arr[i]: 5

 Sum is: 15 and arr[3]: 6 //Show the wrong array value for 3rd element
* Rank 0 excecuting

Answer 1

嗯，代码按照您的要求执行...

        index = 3;
        dest = 2;
        MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
        MPI_Send(&arr, 1, MPI_INT, dest, tag2, MPI_COMM_WORLD);

也许你更想要:

        index = 3;
        dest = 2;
        MPI_Send(&index, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD);
        MPI_Send(&arr[index], 3, MPI_INT, dest, tag2, MPI_COMM_WORLD);

而其他索引“有效”的事实是因为初始分配...

这段代码还有一些问题:

1. int arr[7] = { 6,2,3,9,4,5};:也许更确切地说 int arr[] = {6,2,3 ,9,4,5};

2. MPI_Send(&arr, 2, MPI_INT, dest, tag2, MPI_COMM_WORLD); 应该是 MPI_Send(&arr[index], 3, MPI_INT, dest, tag2, MPI_COMM_WORLD ); 虽然我不太确定这是你打算做的。

3. MPI_Recv(&arr[index], 20, MPI_INT, source, tag2, MPI_COMM_WORLD, MPI_STATUS_IGNORE);:20 从哪里来？不应该是 3 吗？

希望这对您有意义。