问题描述

我有两个 openmpi 程序，我是这样开始的

I have two openmpi programs which I start like this

mpirun -n 4 ./prog1 : -n 2 ./prog2

现在我如何使用 MPI_Comm_size(MPI_COMM_WORLD, &size) 以便我获得大小值

Now how do I use MPI_Comm_size(MPI_COMM_WORLD, &size) such that i get size values as

prog1 size=4
prog2 size=2.

截至目前，我在两个程序中都获得了6".

As of now I get "6" in both programs.

推荐答案

这是可行的，虽然实现起来有点麻烦.其原理是根据argv[0]的值将MPI_COMM_WORLD拆分为多个通信器，其中包含可执行文件的名称.

This is doable albeit a bit cumbersome to get that. The principle is to split MPI_COMM_WORLD into communicators based on the value of argv[0], which contains the executable's name.

可能是这样的:

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

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

    MPI_Init( &argc, &argv );

    int wRank, wSize;
    MPI_Comm_rank( MPI_COMM_WORLD, &wRank );
    MPI_Comm_size( MPI_COMM_WORLD, &wSize );

    int myLen = strlen( argv[0] ) + 1;
    int maxLen;
    // Gathering the maximum length of the executable' name
    MPI_Allreduce( &myLen, &maxLen, 1, MPI_INT, MPI_MAX, MPI_COMM_WORLD );

    // Allocating memory for all of them
    char *names = malloc( wSize * maxLen );
    // and copying my name at its place in the array
    strcpy( names + ( wRank * maxLen ), argv[0] );

    // Now collecting all executable' names
    MPI_Allgather( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL,
                   names, maxLen, MPI_CHAR, MPI_COMM_WORLD );

    // With that, I can sort-out who is executing the same binary as me
    int binIdx = 0;
    while( strcmp( argv[0], names + binIdx * maxLen ) != 0 ) {
        binIdx++;
    }
    free( names );

    // Now, all processes with the same binIdx value are running the same binary
    // I can split MPI_COMM_WORLD accordingly
    MPI_Comm binComm;
    MPI_Comm_split( MPI_COMM_WORLD, binIdx, wRank, &binComm );

    int bRank, bSize;
    MPI_Comm_rank( binComm, &bRank );
    MPI_Comm_size( binComm, &bSize );

    printf( "Hello from process WORLD %d/%d running %d/%d %s binary
",
            wRank, wSize, bRank, bSize, argv[0] );

    MPI_Comm_free( &binComm );

    MPI_Finalize();

    return 0;
}

在我的机器上，我编译并运行它如下:

On my machine, I compiled and ran it as follow:

~> mpicc mpmd.c
~> cp a.out b.out
~> mpirun -n 3 ./a.out : -n 2 ./b.out
Hello from process WORLD 0/5 running 0/3 ./a.out binary
Hello from process WORLD 1/5 running 1/3 ./a.out binary
Hello from process WORLD 4/5 running 1/2 ./b.out binary
Hello from process WORLD 2/5 running 2/3 ./a.out binary
Hello from process WORLD 3/5 running 0/2 ./b.out binary

理想情况下，如果存在用于按二进制文件进行排序的相应类型，则可以通过使用 MPI_Comm_split_type() 来大大简化这一过程.不幸的是，在 3.1 MPI 标准中没有预定义这样的 MPI_COMM_TYPE_.唯一的预定义是 MPI_COMM_TYPE_SHARED 用于在运行在相同共享内存计算节点上的进程之间进行排序......太糟糕了！也许该标准的下一个版本需要考虑什么?

Ideally, this could be greatly simplified by using MPI_Comm_split_type() if the corresponding type for sorting out by binaries existed. Unfortunately, there is no such MPI_COMM_TYPE_ pre-defined in the 3.1 MPI standard. The only pre-defined one is MPI_COMM_TYPE_SHARED to sort-out between processes running on the same shared memory compute nodes... Too bad! Maybe something to consider for the next version of the standard?

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