MPI_Comm_spawn和MPI_Reduce

Question

我有两个程序.生成"工人"的"主人"执行一些计算,我希望主人从工人那里得到结果并存储金额.我正在尝试使用MPI_Reduce来收集工作者的结果,并且工作人员使用MPI_Reduce发送给主人MPI_Comm.我不确定这是否正确.这是我的节目:

主:

#include <mpi.h>
#include <iostream>
using namespace std;

int main(int argc, char *argv[]) { 
    int world_size, universe_size, *universe_sizep, flag; 

    int rc, send, recv;

    // intercommunicator
    MPI_Comm everyone;

    MPI_Init(&argc, &argv); 
    MPI_Comm_size(MPI_COMM_WORLD, &world_size); 

    if (world_size != 1) {
        cout << "Top heavy with management" << endl;
    } 

    MPI_Attr_get(MPI_COMM_WORLD, MPI_UNIVERSE_SIZE, &universe_sizep, &flag);  
    if (!flag) { 
        cout << "This MPI does not support UNIVERSE_SIZE. How many processes total?";
        cout << "Enter the universe size: ";
        cin >> universe_size; 
    } else {
        universe_size = *universe_sizep;
    }
    if (universe_size == 1) {
        cout << "No room to start workers" << endl;
    }

    MPI_Comm_spawn("so_worker", MPI_ARGV_NULL, universe_size-1,  
             MPI_INFO_NULL, 0, MPI_COMM_SELF, &everyone,  
             MPI_ERRCODES_IGNORE);

    send = 0;

    rc = MPI_Reduce(&send, &recv, 1, MPI_INT, MPI_SUM, 0, everyone);

    // store result of recv ...
    // other calculations here
    cout << "From spawned workers recv: " << recv << endl;

    MPI_Finalize(); 
    return 0; 
}

工人:

#include <mpi.h>
#include <iostream>
using namespace std;

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

    int rc, send,recv;


    int parent_size, parent_id, my_id, numprocs; 
    // parent intercomm
    MPI_Comm parent; 
    MPI_Init(&argc, &argv); 

    MPI_Comm_get_parent(&parent); 
    if (parent == MPI_COMM_NULL) {
        cout << "No parent!" << endl;
    }
    MPI_Comm_remote_size(parent, &parent_size); 
    MPI_Comm_rank(parent, &parent_id) ; 
    //cout << "Parent is of size: " << size << endl;
    if (parent_size != 1) {
        cout << "Something's wrong with the parent" << endl;
    }

    MPI_Comm_rank(MPI_COMM_WORLD, &my_id) ;     
    MPI_Comm_size(MPI_COMM_WORLD, &numprocs) ;  

    cout << "I'm child process rank "<< my_id << " and we are " << numprocs << endl;
    cout << "The parent process rank "<< parent_id << " and we are " << parent_size << endl;

    // get value of send
    send = 7; // just an example
    recv = 0;

    rc = MPI_Reduce(&send, &recv, 1, MPI_INT, MPI_SUM, parent_id, parent);
    if (rc != MPI_SUCCESS)
        cout << my_id << " failure on mpi_reduce in WORKER" << endl;

    MPI_Finalize(); 
    return 0; 
} 

我编译了两个并执行这样的(mpic ++ for osx):

mpic++ so_worker.cpp -o so_worker
mpic++ so_master.cpp -o so_master
mpirun -n 1 so_master

这是运行产生工人的主人的正确方法吗？

在Master中我总是从MPI_Reduce返回0.我可以使用来自intercommunicators的MPI_reduce,还是应该使用来自worker的MPI_Send和来自master的MPI_Recv？我真的不确定为什么它不起作用.

任何帮助,将不胜感激.谢谢!

Answer 1

MPI_Comm_get_parent返回包含原始进程和所有生成进程的父对等通道.在这种情况下,调用MPI_Comm_rank(parent, &parent_id)不返回父级别,而是返回互通者本地组中当前进程的级别:

I'm child process rank 0 and we are 3
The parent process **rank 0** and we are 1
I'm child process rank 1 and we are 3
The parent process **rank 1** and we are 1
I'm child process rank 2 and we are 3
The parent process **rank 2** and we are 1

(观察突出显示的值如何不同 - 人们会期望父进程的等级应该相同,不应该吗？)

这就是为什么MPI_Reduce()调用不会成功的原因,因为所有工作进程都为根级别指定了不同的值.由于最初有一个主进程,它在远程组中的等级parent将是0,因此所有工作者应指定0为根目录MPI_Reduce:

//
// Worker code
//
rc = MPI_Reduce(&send, &recv, 1, MPI_INT, MPI_SUM, 0, parent);

这只是问题的一半.另一半是根深蒂固的集体行动(例如MPI_REDUCE)与互通者的运作有点不同.首先必须决定两个组中的哪一个将托管根.标识根组后,根进程必须MPI_ROOT作为rootin 的值传递,MPI_REDUCE并且根组中的所有其他进程必须通过MPI_PROC_NULL.也就是说,接收组中的进程根本不参与有根集合的操作.由于写入主代码使得主组中只有一个进程,因此将MPI_Reduce主代码中的调用更改为:

//
// Master code
//
rc = MPI_Reduce(&send, &recv, 1, MPI_INT, MPI_SUM, MPI_ROOT, everyone);

请注意,主服务器本身也不参与还原操作,例如sendbuf(&send在这种情况下)的值是无关紧要的,因为根不会发送要减少的数据 - 它只是收集对来自以下值的减少的结果远程组中的进程.