用MPI_Isend发送多条非阻塞消息，用C语言MPI_Recv接收的问题

Question

我正在编写一个并行算法，并且我对非阻塞通信有一个问题.我通过以下代码对问题进行建模：

int main( int argc, char* argv[] ) {
    MPI_Init(&argc, &argv);
    int rank, p;
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
    MPI_Comm_size( MPI_COMM_WORLD, &p );

    int a,b, i, j;
    int maxNumber = 8192;

    int(*tab)[maxNumber] = malloc(sizeof(int[maxNumber + 1][maxNumber + 1]));
    MPI_Request* r = malloc(sizeof * r);

    if(rank == 0){
        for(i = 0; i < maxNumber + 1; i++){
            for(j = 0; j < maxNumber + 1; j++){
                tab[i][j] = 2*i+i*j;
            }
            for(a = 1; a < p; a++){
                MPI_Isend(&tab[i], maxNumber + 1, MPI_INT, a, i, MPI_COMM_WORLD, r);
                printf("Process 0 send the block %d to process %d\n", i, a);
            }
        }

    }
    else{
        for(i = 1; i < p; i++){
            if(rank == i){
                for(j = 0; j < maxNumber + 1; j++){
                    printf("Process %d wait the block %d to process 0\n", i, j);
                    MPI_Recv(&tab[j], maxNumber + 1, MPI_INT, 0, j, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
                    printf("Process %d receive the block %d to process 0\n", i, j);
                }
            }           

        }
    }

    MPI_Finalize();

    return 0;
}

经过一些计算，处理器0将大小为8192 * 8192的矩阵的每一行发送给其他处理器。问题是，处理器0在其他处理器接收数据之前完成发送8192行。

这是输出的一部分：

...
...
Process 0 send the block 8187 to process 1
Process 0 send the block 8188 to process 1
Process 0 send the block 8189 to process 1
Process 0 send the block 8190 to process 1
Process 0 send the block 8191 to process 1
Process 0 send the block 8192 to process 1
Process 1 receive the block 5 to process 0
Process 1 wait the block 6 to process 0
Process 1 receive the block 6 to process 0
Process 1 wait the block 7 to process 0
Process 1 receive the block 7 to process 0
Process 1 wait the block 8 to process 0
Process 1 receive the block 8 to process 0
Process 1 wait the block 9 to process 0
Process 1 receive the block 9 to process 0
...
...

PS:对于发送，通信必须是非阻塞的，因为在我的问题中，进程0在每次迭代中都会以O(n平方公里/p)的方式进行计算，然后再发送给其他处理器，以便他们能够尽快开始计算。

你知道我能为解决这个问题做些什么吗？

Answer 1

谢谢你的回答。它能让我解决我的问题。我需要使用MPI_Ibsend来分配所需的缓冲区空间，以便将数据复制到其中，直到数据交付为止。

int main( int argc, char* argv[] ) {   
    MPI_Init(&argc, &argv);
    int rank, p;
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
    MPI_Comm_size( MPI_COMM_WORLD, &p );

    int a, i, j;
    int maxNumber = atoi(argv[1]);

    int(*tab)[maxNumber] = malloc(sizeof(int[maxNumber + 1][maxNumber + 1]));
    MPI_Request* tabReq = malloc(maxNumber * sizeof * tabReq);

    int bufsize = maxNumber * maxNumber; 
    char *buf = malloc( bufsize ); 

    if(rank == 0){
        for(i = 0; i < maxNumber + 1; i++){
            for(j = 0; j < maxNumber + 1; j++){
                tab[i][j] = 2*i+i*j;
            }
            for(a = 1; a < p; a++){
                MPI_Buffer_attach( buf, bufsize );
                MPI_Ibsend(&tab[i], maxNumber + 1, MPI_INT, a, i, MPI_COMM_WORLD, &tabReq[i]);
                MPI_Buffer_detach( &buf, &bufsize );
                printf("Process 0 send the block %d to process %d\n", i, a);
            }
        }
    }
    else{
        for(j = 0; j < maxNumber + 1; j++){
            printf("Process %d wait the block %d to process 0\n", rank, j);
            MPI_Recv(&tab[j], maxNumber + 1, MPI_INT, 0, j, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("Process %d receive the block %d to process 0\n", rank, j);
        }
    }

    MPI_Finalize();

    return 0;
}

这是输出的一部分：

...
...
Process 1 wait the block 8186 to process 0
Process 0 send the block 8185 to process 1
Process 1 receive the block 8186 to process 0
Process 1 wait the block 8187 to process 0
Process 0 send the block 8186 to process 1
Process 1 receive the block 8187 to process 0
Process 1 wait the block 8188 to process 0
Process 0 send the block 8187 to process 1
Process 1 receive the block 8188 to process 0
Process 1 wait the block 8189 to process 0
Process 0 send the block 8188 to process 1
Process 1 receive the block 8189 to process 0
Process 1 wait the block 8190 to process 0
Process 0 send the block 8189 to process 1
Process 1 receive the block 8190 to process 0
Process 1 wait the block 8191 to process 0
Process 0 send the block 8190 to process 1
Process 1 receive the block 8191 to process 0
Process 1 wait the block 8192 to process 0
Process 0 send the block 8191 to process 1
Process 1 receive the block 8192 to process 0
Process 0 send the block 8192 to process 1