c++ - 使用 openMPI 发送对象

Question

我正在使用 magick++ 库来管理图像。我想使用 openMPI 分发我的算法，是否可以发送对象？

例如在我的代码中

  Image image(imgName);
  int w = image.columns();
  int h = image.rows();
  PixelPacket *pixels = image.getPixels(0, 0, w, h);

我可以pixels用 MPI_Send 或 Scatter 发送吗？如果是，使用哪种数据类型？

Answer 1

一般来说，除非你有一个特殊的库来为你打包，否则永远不可能在 MPI 中发送一个专门的对象。内置数据类型在 MPI 标准中列出（MPI 3.0 第 665 页，定义的值和句柄有一个列表），但在高层次上，它们是：

• MPI_CHAR
• MPI_INT
• MPI_FLOAT
• MPI_DOUBLE
• MPI_BYTE

还有很多，但其中大多数最终都是这样的。

您可以采用这些类型并将它们组合在一起以创建您自己的自定义数据类型。例如，如果您知道要发送一堆包含以下内容的结构：

{
    int index;
    char[100] name;
    double value;
}

您可以构造一个类型来保存它，它将是名称的连续类型，然后是整个数据类型的结构类型（它将包含 int、为名称构造的类型和双精度）。

您所要做的就是创建一个数据类型来描述您的PixelPacket.

Answer 2

我不是 C++ 程序员，但以下内容可以满足您的要求。基本上，我启动了 8 个 MPI 进程（我碰巧使用mpich了），主人使用 Magick++ 读取图像（Lena 当然是从PNG文件中），然后将她发送给每个奴隶。JPEGslave 接收 Lena 并根据他们收到的数据重建她，每个 slave以不同的名称写出自己的本地副本。

我在尺寸上作弊，因为计算尺寸并通过它们很容易，但与我所展示的内容无关。

#include <cstdlib>
#include <iostream>
#include <Magick++.h>
#include "mpi.h"

using namespace std;

int main ( int argc, char *argv[] )
{
   int id,p;
   int number;

   // Initialise MPI
   MPI::Init (argc,argv);

   // Initialize ImageMagick and image processing stuff
   Magick::InitializeMagick(*argv);
   int row,col;
   Magick::Image image;
   int bytes=512*512*3;           // I happen to know Lena is 512x512 and RGB - i.e. 3 bytes/pixel
   unsigned char buffer[bytes];

   //  Get the number of processes
   p = MPI::COMM_WORLD.Get_size();

   //  Get the individual process ID
   id = MPI::COMM_WORLD.Get_rank();

   // Master will read in Lena and send her to all slaves
   if(id==0) 
   {
      cout << "MASTER: The number of processes is " << p << endl;
      // Read in Lena and put her in a buffer to send via MPI
      image.read("lena.png");

      // Convert Lena to a bunch of bytes
      image.write(0,0,512,512,"RGB",Magick::CharPixel,buffer);

      // Send the luscious Lena to all slaves
      for(int z=1;z<p;z++){
         cout << "MASTER: Sending Lena to slave " << z << endl;
         MPI_Send(buffer,bytes,MPI_BYTE,z,0,MPI_COMM_WORLD);
      }
   }else{
      // All slaves will receive Lena and write her out as a JPEG
      cout << "Process:" << id << " Started and waiting for Lena..." << endl;
      MPI_Recv(buffer,bytes,MPI_BYTE,0,0,MPI_COMM_WORLD,MPI_STATUS_IGNORE);
      cout << "Process:" << id << " Received Lena" << endl;

      // Rebuild Lena from the bunch of bytes and write to "Lena-Rebuilt-<id>.jpg"
      Magick::Image rebuilt(512,512,"RGB",Magick::CharPixel,buffer);
      char filename[100];
      sprintf(filename,"Lena-Rebuilt-%d.jpg",id);
      rebuilt.write(filename);
   }

   //  Terminate MPI
   MPI::Finalize();
   return 0;
}

我的Makefile样子是这样的：

all:    main

run:    main
        mpirun -n 8 ./main

main:   main.cpp
        mpic++ main.cpp -o main $$(Magick++-config --cxxflags --libs)

它像这样运行：

make run
mpirun -n 8 ./main
MASTER: The number of processes is 8
Process:1 Started and waiting for Lena...
Process:3 Started and waiting for Lena...
Process:4 Started and waiting for Lena...
Process:5 Started and waiting for Lena...
Process:7 Started and waiting for Lena...
Process:6 Started and waiting for Lena...
Process:2 Started and waiting for Lena...
MASTER: Sending Lena to slave 1
MASTER: Sending Lena to slave 2
Process:1 Received Lena
MASTER: Sending Lena to slave 3
Process:2 Received Lena
MASTER: Sending Lena to slave 4
Process:3 Received Lena
MASTER: Sending Lena to slave 5
Process:4 Received Lena
MASTER: Sending Lena to slave 6
Process:5 Received Lena
MASTER: Sending Lena to slave 7
Process:6 Received Lena
Process:7 Received Lena

输出如下所示：

-rw-r--r--@ 1 mark  staff  150467 23 Oct 15:34 lena.png
-rw-r--r--  1 mark  staff    1786 23 Oct 17:04 main.cpp
-rwxr-xr-x  1 mark  staff   51076 23 Oct 17:14 main
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-7.jpg
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-6.jpg
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-5.jpg
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-4.jpg
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-3.jpg
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-2.jpg
-rw-r--r--@ 1 mark  staff   64755 23 Oct 17:14 Lena-Rebuilt-1.jpg