使用 MPI_Allgather 分发结构

Question

我必须使用MPI_Allgather()向所有进程发送一个结构。 我似乎没有得到任何明显的错误，但是代码不起作用。 当我检查我是否在recv[]中收到任何值时，它没有显示。 如果我只发送一个变量而不是使用类似代码的结构，那是可行的，所以我不确定发生了什么。 结构有 static arrays，所以 memory 应该是连续的，还是我应该使用MPI_Pack什么的？ 这是代码：

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

#define NUMEL 21
struct mystruct{
 int sendarray[10];
 int a;
 char array2[10];
};

typedef struct mystruct struct_t;

int main (int argc, char ** argv)
{
 MPI_Status status;
 int rank, size;
 char *recv;
 int i, j;
 MPI_Init(&argc, &argv);
 MPI_Comm_rank(MPI_COMM_WORLD, &rank);
 MPI_Comm_size(MPI_COMM_WORLD, &size);
 // init
 struct_t * fd = (struct_t*)malloc(sizeof(*fd));;
 for (i=0;i<10;i++){
    fd->sendarray[i] = 0;
    fd->array2[i] = 0;
 }
 recv = (char *) malloc ( size*NUMEL);

 // put some stuff in your array
 for (i=0;i<size;i++){
    if(rank == i){
      fd->sendarray[i] = i *10;
      fd->array2[i] = i *20;
      fd->a = rank;
   }
  if(fd->sendarray[i] != 0)
     printf("My rank is %d, fd->sendarray[%d]  is %d\n", rank, i, fd->sendarray[i]);
     }

   // gather data from all now..
   MPI_Allgather (fd, NUMEL, MPI_BYTE, recv, NUMEL * size, MPI_INT, MPI_COMM_WORLD);

  // check if correct data has been received
  for (i=0;i<size*NUMEL;i++){
   if(recv[i] != 0)
     printf("My rank is %d and recv[i]=%d and i is %d\n", rank, recv[i],i);
  }
MPI_Finalize();
}

Answer 1

第一次看到 Allgather 时可能会有些困惑。 这里发生了一些事情。

首先，要收集的计数——发送计数和接收计数——是每个进程发送和从每个进程接收的数据量。

其次，allgather 的工作方式是连接发送的数据。 所以如果你有

int send[3];
int recv[9];

在每个进程上发送 arrays 如下所示：

send:
      +---+---+---+
      | 0 | 0 | 0 |     rank 0
      +---+---+---+

      +---+---+---+
      | 1 | 1 | 1 |     rank 1
      +---+---+---+

      +---+---+---+
      | 2 | 2 | 2 |     rank 2
      +---+---+---+

然后调用

MPI_Allgather(send, 3, MPI_INT,  recv, 3, MPI_INT,  MPI_COMM_WORLD);

会导致：

recv:
      +---+---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 1 | 1 | 1 | 2 | 2 | 2 |     
      +---+---+---+---+---+---+---+---+---+

因此，提取正确数据的代码版本是：

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

struct mystruct{
    int sendarray[10];
    int a;
    char array2[10];
};

typedef struct mystruct struct_t;

int main (int argc, char ** argv)
{
    int rank, size;
    struct_t *recv;
    int i, j;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    // init
    struct_t * fd = (struct_t*)malloc(sizeof(*fd));
    for (i=0;i<10;i++){
        fd->sendarray[i] = 0;
        fd->array2[i] = 0;
    }
    recv = malloc ( size * sizeof(*fd) );

    // put some stuff in your array
    fd->sendarray[rank] = rank*10;
    fd->array2[rank] = rank*20;
    fd->a = rank;
    printf("My rank is %d, fd->sendarray[%d]  is %d\n", rank, i, fd->sendarray[i]);

    // gather data from all now..
    MPI_Allgather (fd, sizeof(*fd), MPI_BYTE, recv, sizeof(*fd), MPI_BYTE, MPI_COMM_WORLD);


    // check if correct data has been received
    if (rank == 0) {
        printf("Received:\n");
        for (i=0;i<size;i++){
            printf("---\n");
            printf("int array:  ");
            for (j=0; j<10; j++) printf("%3d ", recv[i].sendarray[j]);
            printf("\nint:        "); printf("%3d\n", recv[i].a);
            printf("char array: ");
            for (j=0; j<10; j++) printf("%3d ", (int)(recv[i].array2[j]));
            printf("\n");
        }
    }
    MPI_Finalize();
    return 0;
}

请注意，它将这些结构收集到等效于这些结构的数组中。 使用 4 个处理器运行会给出：

My rank is 0, fd->sendarray[10]  is 0
My rank is 1, fd->sendarray[10]  is 1
My rank is 2, fd->sendarray[10]  is 2
My rank is 3, fd->sendarray[10]  is 3

Received:
---
int array:    0   0   0   0   0   0   0   0   0   0 
int:          0
char array:   0   0   0   0   0   0   0   0   0   0 
---
int array:    0  10   0   0   0   0   0   0   0   0 
int:          1
char array:   0  20   0   0   0   0   0   0   0   0 
---
int array:    0   0  20   0   0   0   0   0   0   0 
int:          2
char array:   0   0  40   0   0   0   0   0   0   0 
---
int array:    0   0   0  30   0   0   0   0   0   0 
int:          3
char array:   0   0   0  60   0   0   0   0   0   0 

如果您真的只想要收集相应的元素，那么您只需从结构中的特定位置发送一个 int/char：

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

struct mystruct{
    int sendarray[10];
    int a;
    char array2[10];
};

typedef struct mystruct struct_t;

int main (int argc, char ** argv)
{
    int rank, size;
    struct_t fd;
    struct_t recv;
    int i, j;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    // init
    for (i=0;i<10;i++){
        fd.sendarray[i] = 0;
        fd.array2[i] = 0;

        recv.sendarray[i] =999;
        recv.array2[i]    = 99;
    }
    recv.a =999;

    // put some stuff in your array
    fd.sendarray[rank] = rank*10;
    fd.array2[rank]    = (char)(rank*20);
    fd.a = rank;
    printf("My rank is %d, fd.sendarray[%d]  is %d\n", rank, rank, fd.sendarray[rank]);

    // gather data from all now.. send the int:
    MPI_Allgather (&(fd.sendarray[rank]), 1, MPI_INT,  recv.sendarray, 1, MPI_INT,  MPI_COMM_WORLD);
    // then the char
    MPI_Allgather (&(fd.array2[rank]),    1, MPI_CHAR, recv.array2,    1, MPI_CHAR, MPI_COMM_WORLD);

    // check if correct data has been received
    if (rank == 0) {
        printf("Received:\n");
        printf("---\n");
        printf("int array:  ");
        for (j=0; j<10; j++) printf("%3d ", recv.sendarray[j]);
        printf("\nint:        "); printf("%3d\n", recv.a);
        printf("char array: ");
        for (j=0; j<10; j++) printf("%3d ", (int)(recv.array2[j]));
        printf("\n");
    }
    MPI_Finalize();

    return 0;
}

如果我们用 4 个进程运行它，我们会得到：

My rank is 0, fd.sendarray[0]  is 0
My rank is 1, fd.sendarray[1]  is 10
My rank is 2, fd.sendarray[2]  is 20
My rank is 3, fd.sendarray[3]  is 30
Received:
---
int array:    0  10  20  30 999 999 999 999 999 999 
int:        999
char array:   0  20  40  60  99  99  99  99  99  99