c - MPI : Different value received? 中的 ISend 和 Recv

Question

在我的矩阵加法代码中，我使用 ISend 和 Tag 1 将下限传输到其他进程，但是当我编译代码时，所有其他从属进程都声称具有相同的下限。我不明白为什么？

输出:

I am process 1 and I received 1120 as lower bound
I am process 1 and my lower bound is 1120 and my upper bound is 1682
I am process 2 and I received 1120 as lower bound
I am process 2 and my lower bound is 1120 and my upper bound is 1682
Process 0 here: I am sending lower bound 0 to process 1
Process 0 here: I am sending lower bound 560 to process 2
Process 0 here: I am sending lower bound 1120 to process 3
Timings : 13.300698 Sec
I am process 3 and I received 1120 as lower bound
I am process 3 and my lower bound is 1120 and my upper bound is 1682

代码:

#define N_ROWS 1682
#define N_COLS 823
#define MASTER_TO_SLAVE_TAG 1 //tag for messages sent from master to slaves
#define SLAVE_TO_MASTER_TAG 4 //tag for messages sent from slaves to master

void readMatrix();
int rank, nproc, proc;
double matrix_A[N_ROWS][N_COLS];
double matrix_B[N_ROWS][N_COLS];
double matrix_C[N_ROWS][N_COLS];
int low_bound; //low bound of the number of rows of [A] allocated to a slave
int upper_bound; //upper bound of the number of rows of [A] allocated to a slave
int portion; //portion of the number of rows of [A] allocated to a slave
MPI_Status status; // store status of a MPI_Recv
MPI_Request request; //capture request of a MPI_Isend

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

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &nproc);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    double StartTime = MPI_Wtime();

        // -------------------> Process 0 initalizes matrices and sends work portions to other processes
        if (rank==0) {
            readMatrix();
            for (proc = 1; proc < nproc; proc++) {//for each slave other than the master
                portion = (N_ROWS / (nproc - 1)); // calculate portion without master
                low_bound = (proc - 1) * portion;
                if (((proc + 1) == nproc) && ((N_ROWS % (nproc - 1)) != 0)) {//if rows of [A] cannot be equally divided among slaves
                    upper_bound = N_ROWS; //last slave gets all the remaining rows
                } else {
                    upper_bound = low_bound + portion; //rows of [A] are equally divisable among slaves
                }
                //send the low bound first without blocking, to the intended slave
                printf("Process 0 here: I am sending lower bound %i to process %i \n",low_bound,proc);
                MPI_Isend(&low_bound, 1, MPI_INT, proc, MASTER_TO_SLAVE_TAG, MPI_COMM_WORLD, &request);
                //next send the upper bound without blocking, to the intended slave
                MPI_Isend(&upper_bound, 1, MPI_INT, proc, MASTER_TO_SLAVE_TAG + 1, MPI_COMM_WORLD, &request);
                //finally send the allocated row portion of [A] without blocking, to the intended slave
                MPI_Isend(&matrix_A[low_bound][0], (upper_bound - low_bound) * N_COLS, MPI_DOUBLE, proc, MASTER_TO_SLAVE_TAG + 2, MPI_COMM_WORLD, &request);
            }
        }

        //broadcast [B] to all the slaves
        MPI_Bcast(&matrix_B, N_ROWS*N_COLS, MPI_DOUBLE, 0, MPI_COMM_WORLD);


        // -------------------> Other processes do their work
        if (rank != 0) {
            //receive low bound from the master
            MPI_Recv(&low_bound, 1, MPI_INT, 0, MASTER_TO_SLAVE_TAG, MPI_COMM_WORLD, &status);
            printf("I am process %i and I received %i as lower bound \n",rank,low_bound);
            //next receive upper bound from the master
            MPI_Recv(&upper_bound, 1, MPI_INT, 0, MASTER_TO_SLAVE_TAG + 1, MPI_COMM_WORLD, &status);
            //finally receive row portion of [A] to be processed from the master
            MPI_Recv(&matrix_A[low_bound][0], (upper_bound - low_bound) * N_COLS, MPI_DOUBLE, 0, MASTER_TO_SLAVE_TAG + 2, MPI_COMM_WORLD, &status);
            printf("I am process %i and my lower bound is %i and my upper bound is %i \n",rank,low_bound,upper_bound);
            //do your work
            for (int i = low_bound; i < upper_bound; i++) {
                for (int j = 0; j < N_COLS; j++) {
                    matrix_C[i][j] = (matrix_A[i][j] + matrix_B[i][j]);
                }
            }
            //send back the low bound first without blocking, to the master
            MPI_Isend(&low_bound, 1, MPI_INT, 0, SLAVE_TO_MASTER_TAG, MPI_COMM_WORLD, &request);
            //send the upper bound next without blocking, to the master
            MPI_Isend(&upper_bound, 1, MPI_INT, 0, SLAVE_TO_MASTER_TAG + 1, MPI_COMM_WORLD, &request);
            //finally send the processed portion of data without blocking, to the master
            MPI_Isend(&matrix_C[low_bound][0], (upper_bound - low_bound) * N_COLS, MPI_DOUBLE, 0, SLAVE_TO_MASTER_TAG + 2, MPI_COMM_WORLD, &request);
        }

        // -------------------> Process 0 gathers the work
        ...

Answer 1

MPI_Isend()开始非阻塞发送。因此，修改发送的缓冲区而不检查消息是否实际发送会导致发送错误的值。

这就是您提供的代码片段中发生的情况，在进程 for (proc = 1; proc < nproc; proc++) 的循环中

1. proc=1 : low_bound被计算出来。

2. proc=1 : low_bound被发送(非阻塞)到进程 1。

3. 过程=2:low_bound被修改。消息已损坏。

存在不同的解决方案:

• 使用阻塞发送 MPI_Send() .

• 通过创建一个包含 3 个请求的数组来检查消息是否已完成 MPI_Request requests[3]; MPI_Status statuses[3]; , 使用非阻塞发送和使用 MPI_Waitall() 检查请求的完成情况。

   MPI_Isend(&low_bound, 1, MPI_INT, proc, MASTER_TO_SLAVE_TAG, MPI_COMM_WORLD, &requests[0]);
   MPI_Isend(..., &requests[1]);
   MPI_Isend(..., &requests[2]);
   MPI_Waitall(3, requests, statuses);
  

• 看看 MPI_Scatter() 和 MPI_Scatterv() !

执行此操作的“常规”方法是 MPI_Bcast()矩阵的大小。然后每个进程计算其矩阵部分的大小。进程 0 计算 sendcounts和 displs MPI_Scatterv() 需要.