MPI 异步发送和接收未按预期工作

Question

我有一个关于使用 C 语言中的 MPI 库进行通信的问题。这是我的示例代码：

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

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

    // Initialize the MPI Environment
    MPI_Init (&argc, &argv);

    int world_size;
    MPI_Comm_size (MPI_COMM_WORLD, &world_size);

    int world_rank;
    MPI_Comm_rank (MPI_COMM_WORLD, &world_rank);

    int workers = world_size - 1;

    if (world_rank == 0) {
    }
    else if (world_rank == 1) {
        int to_recv;
        int to_send = world_rank;
        MPI_Request request1, request2;
        MPI_Irecv (&to_recv, sizeof (int), MPI_INT, world_rank + 1, 0, MPI_COMM_WORLD, &request1);
        MPI_Isend (&to_send, sizeof (int), MPI_INT, world_rank + 1, 0, MPI_COMM_WORLD, &request2);

        MPI_Wait (&request1, MPI_STATUS_IGNORE);
        MPI_Wait (&request2, MPI_STATUS_IGNORE);

        printf("I am %d\n", world_rank);
        printf("I received rank %d\n", to_recv);
    }
    else if (world_rank == workers) {
        int to_recv;
        int to_send = world_rank;
        MPI_Request request1, request2;
        MPI_Irecv (&to_recv, sizeof (int), MPI_INT, world_rank - 1, 0, MPI_COMM_WORLD, &request1);
        MPI_Isend (&to_send, sizeof (int), MPI_INT, world_rank - 1, 0, MPI_COMM_WORLD, &request2);

        MPI_Wait (&request1, MPI_STATUS_IGNORE);
        MPI_Wait (&request2, MPI_STATUS_IGNORE);

        printf("I am %d\n", world_rank);
        printf("I received rank %d\n", to_recv);
    }
    else {
        int to_recvl, to_recvr;
        int to_send1 = world_rank, to_send2 = world_rank;

        MPI_Request request1, request2, request3, request4;

        MPI_Irecv (&to_recvl, sizeof (int), MPI_INT, world_rank - 1, 0, MPI_COMM_WORLD, &request1);
        MPI_Isend (&to_send1, sizeof (int), MPI_INT, world_rank - 1, 0, MPI_COMM_WORLD, &request2);

        MPI_Irecv (&to_recvr, sizeof (int), MPI_INT, world_rank + 1, 0, MPI_COMM_WORLD, &request3);
        MPI_Isend (&to_send2, sizeof (int), MPI_INT, world_rank + 1, 0, MPI_COMM_WORLD, &request4);

        MPI_Wait (&request1, MPI_STATUS_IGNORE);
        MPI_Wait (&request2, MPI_STATUS_IGNORE);

        MPI_Wait (&request3, MPI_STATUS_IGNORE);
        MPI_Wait (&request4, MPI_STATUS_IGNORE);

        printf("I am %d\n", world_rank);
        printf("I received ranks %d and %d\n", to_recvl, to_recvr);
    }

    MPI_Finalize ();

    return 0;
}

所以这就是我基本上想要完成的：

假设我产生了 11 个进程（0 是主进程，进程 1-10 工人）。那么，

• 进程 1 将其排名发送给进程 2
• 进程 1 从进程 2 接收排名
• 进程 10 将其排名发送给进程 9
• 进程 10 从进程 9 接收排名
• 进程 i 将其排名发送给进程 i+1 和 i-1 (1
• 进程 i 从进程 i+1 和 i-1 (1

在运行程序时，在某些情况下我会得到垃圾值，如下面添加的：

I am 2
I received ranks 1 and 3
I am 3
I received ranks 2 and 4
I am 4
I received ranks 1499161032 and 5
I am 6
I received ranks 1488867784 and 7
I am 9
I received rank 8
I am 32767
I received rank 2
I am 5
I received ranks 1507000776 and 6
I am 7
I received ranks 6 and 8
I am 8
I received ranks 7 and 9

我不知道是什么导致了问题。任何帮助将不胜感激。

谢谢！

Answer 1

一个非常简单的问题，MPI_Isend() 和MPI_Irecv() 中的第二个参数应该是您希望发送或接收的元素的count。您在大多数系统中使用sizeof(int)，即4，而您只想发送/接收 1 个元素，因此计数应为 1。