如何在 MPI_Send 中发送一个集合对象

How to send a set object in MPI_Send(如何在 MPI_Send 中发送一个集合对象)

问题描述

我搜索发送一个集合对象，发现最接近的是向量(它不同，不适用于集合).

I searched to send a set object and the closest I found was with vector (it's different and don't work with set).

如何在 MPI_Send 中发送一个集合对象?(不使用boost库)任何人都可以举一个简单的例子吗?

How can I send a set object in MPI_Send? (without using boost library) Anyone can put a simple example?

推荐答案

在 MPI 中，无论你必须将复杂的数据结构写入文件还是通过网络写入，问题都是一样的；您必须将数据提取到Plain Old Data"(POD)中，保存，然后输出，同样能够将保存的数据解包到相同的结构中.通常，这称为序列化.

Whether you have to write a complex data structure to a file or over the network in MPI, the issues are the same; you have to extract the data into "Plain Old Data" (POD), save it, and then output it, and likewise be able to unpack the saved data into the same sort of structure. In general, this is called serialization.

对于任何给定的结构，您始终可以编写自己的例程来执行此操作，但在 C++ 中，Boost 中有一个称为 Boost 序列化库 用于执行此操作；对于这个例子来说它有点重量级，但它适用于大多数(所有?)STL 容器，并且有一些钩子可以添加对您自己的类的支持.

For any given structure you can always write your own routines for doing this, but in C++, there's a framework in Boost called the Boost Serialization Library for doing this; it's a bit heavyweight for this example but it will work for most (all?) STL containers and there are hooks for adding support for your own classes.

为此使用 Boost 的主要技巧是 Boost 库(以及所有示例)使将数据写入文件变得非常容易，但在这里您希望将其保存在内存中并通过网络;这意味着要跳过几个圈以确保序列化到您可以访问的数组中.这个SO答案在这方面很有帮助.

The main trick with using Boost for this is that the Boost libraries (and all the examples) make it very easy to write the data to a file, but here you want to keep it in memory and send/receive it over the network; that means jumping through a couple more hoops to make sure the serialization is into an array you can access. This SO answer is very helpful in this regard.

所以一个完整的工作示例如下所示:

So a complete working example looks like this:

#include <mpi.h>
#include <set>
#include <string>
#include <boost/archive/binary_iarchive.hpp>
#include <boost/archive/binary_oarchive.hpp>
#include <boost/serialization/set.hpp>
#include <boost/iostreams/stream_buffer.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/device/back_inserter.hpp>

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

    int size, rank;

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

    if (size < 2) {
        if (rank == 0)
            std::cerr << "Require at least 2 tasks" << std::endl;
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

   const int lentag=0;
   const int datatag=1;
   if (rank == 0) {
        int nums[] = {1,4,9,16};
        std::set<int> send_set(nums, nums+4);

        std::cout << "Rank " << rank << " sending set: ";
        for (std::set<int>::iterator i=send_set.begin(); i!=send_set.end(); i++)
            std::cout << *i << " ";
        std::cout << std::endl;

        // We're going to serialize into a std::string of bytes, and then send this
        std::string serial_str;
        boost::iostreams::back_insert_device<std::string> inserter(serial_str);
        boost::iostreams::stream<boost::iostreams::back_insert_device<std::string> > s(inserter);
        boost::archive::binary_oarchive send_ar(s);

        send_ar << send_set;
        s.flush();
        int len = serial_str.size();

        // Send length, then data
        MPI_Send( &len, 1, MPI_INT, 1, lentag, MPI_COMM_WORLD );
        MPI_Send( (void *)serial_str.data(), len, MPI_BYTE, 1, datatag, MPI_COMM_WORLD );
    } else if (rank == 1) {
        int len;
        MPI_Recv( &len, 1, MPI_INT, 0, lentag, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

        char data[len+1];
        MPI_Recv( data, len, MPI_BYTE, 0, datatag, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        data[len] = '';

        boost::iostreams::basic_array_source<char> device(data, len);
        boost::iostreams::stream<boost::iostreams::basic_array_source<char> > s(device);
        boost::archive::binary_iarchive recv_ar(s);

        std::set<int> recv_set;
        recv_ar >> recv_set;

        std::cout << "Rank " << rank << " got set: ";
        for (std::set<int>::iterator i=recv_set.begin(); i!=recv_set.end(); i++)
            std::cout << *i << " ";
        std::cout << std::endl;
    }

    MPI_Finalize();
    return 0;
}

跑步给出:

$ mpic++ mpi-set.cxx -o mpiset -lboost_serialization
$ mpirun -np 2 ./mpiset
Rank 0 sending set: 1 4 9 16
Rank 1 got set: 1 4 9 16

如果您真的不想使用 Boost，因为您实际上无法直接查看设置的数据结构，所以没有太多选择，只能将数据提取到数组或向量中，然后以这种方式发送数据:

If you really don't want to use Boost, since you can't actually see directly into the set data structure, there's not much alternative but to extract the data into an array or vector, and send the data that way:

#include <mpi.h>
#include <set>
#include <vector>

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

    int size, rank;

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

    if (size < 2) {
        if (rank == 0)
            std::cerr << "Require at least 2 tasks" << std::endl;
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

   const int lentag=0;
   const int datatag=1;
   if (rank == 0) {
        int nums[] = {1,4,9,16};
        std::set<int> send_set(nums, nums+4);

        std::cout << "Rank " << rank << " sending set: ";
        for (std::set<int>::iterator i=send_set.begin(); i!=send_set.end(); i++)
            std::cout << *i << " ";
        std::cout << std::endl;

        // Send length, then data
        int len = send_set.size();
        MPI_Send( &len, 1, MPI_INT, 1, lentag, MPI_COMM_WORLD );

        std::vector<int> send_vec;
        for (std::set<int>::iterator i=send_set.begin(); i!=send_set.end(); i++) 
            send_vec.push_back(*i);

        MPI_Send( send_vec.data(), len, MPI_INT, 1, datatag, MPI_COMM_WORLD );

    } else if (rank == 1) {

        int len;
        MPI_Recv( &len, 1, MPI_INT, 0, lentag, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

        int recv_data[len];
        MPI_Recv( recv_data, len, MPI_INT, 0, datatag, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

        std::set<int> recv_set;
        for (int i=0; i<len; i++) 
            recv_set.insert(recv_data[i]);

        std::cout << "Rank " << rank << " got set: ";
        for (std::set<int>::iterator i=recv_set.begin(); i!=recv_set.end(); i++)
            std::cout << *i << " ";
        std::cout << std::endl;
    }

    MPI_Finalize();
    return 0;
}

跑步给出:

$ mpicxx -o mpisetvector mpi-set-vector.cxx 
$ mpirun -np 2 mpisetvector
Rank 0 sending set: 1 4 9 16 
Rank 1 got set: 1 4 9 16 

但实际上，如果您也打算对其他类型的对象执行此操作，那么 Boost 是您的最佳选择.

But really, if you're going to be doing this with other types of object as well, Boost is the way to go.

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