用于并行计算的C ++ + openmp：如何在visual studio中进行设置？

Question

I have a c++ program that creates an object and then calls 2 functions of this object that are independent from one another. So it looks like this:

Object myobject(arg1, arg2);
double answer1 = myobject.function1();
double answer2 = myobject.function2();

I would like to have those 2 computations run in parallel to save computation time. I've seen that this could be done using openmp, but couldn't figure out how to set it up. The only examples I found were sending the same calculation (ie "hello world!" for example) to the different cores and the output was 2 times "hello world!". How can I do it in this situation?

I use Windows XP with Visual Studio 2005.

Answer 1

You should look into the sections construct of OpenMP. It works like this:

#pragma omp parallel sections
{
   #pragma omp section
   {
      ... section 1 block ...
   }
   #pragma omp section
   {
      ... section 2 block ...
   }
}

Both blocks might execute in parallel given that there are at least two threads in the team but it is up to the implementation to decide how and where to execute each section.

There is a cleaner solution using OpenMP tasks, but it requires that your compiler supports OpenMP 3.0. MSVC only supports OpenMP 2.0 (even in VS 11!).

You should explicitly enable OpenMP support in your project's settings. If you are doing compilation from the command line, the option is /openmp .

Answer 2

If the memory that is is required for your code is not a lot you can use MPI library too. For this purpose first of all install MPI on your visual studio from this tutorial Compiling MPI Programs in Visual Studio or from here: MS-MPI with Visual Studio 2008 use this mpi hello world code :

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

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

int mynode, totalnodes;

MPI_Init(&argc, &argv); 
MPI_Comm_size(MPI_COMM_WORLD, &totalnodes);
MPI_Comm_rank(MPI_COMM_WORLD, &mynode);

cout << "Hello world from process " << mynode;
cout << " of " << totalnodes << endl;

MPI_Finalize();
return 0;
}

for your base code, add your functions to it and declare job of each process with this example if statement:

if(mynode== 0 ){function1}
if(mynode== 1 ){function2}

function1 and function2 can be any thing that you like executes at the same time; but be careful that these two functions independent of each others. thats it!

Answer 3

The first part of this is getting OpenMP up and running with Visual Studio 2005, which is quite old; it takes some doing, but it's described in the answer to this question .

Once that's done, it's fairly easy to do this simple form of task parallelism if you have two methods which are genuinely completely independant. Note that qualifier; if the methods are reading the same data, that's ok, but if they're updating any state that the other method uses, or calling any other routines that do so, then things will break.

As long as the methods are completly independant, you can use sections for these ( tasks are actually the more modern, OpenMP 3.0 way of doing this, but you probably won't be able to get OpenMP 3.0 support for such an old compiler); you will also see people misusing parallel for loops to achieve this, which at least has the advantage of letting you control the thread assignments, so I include that here for completeness even though I can't really recommend it:

#include <omp.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int f1() {
    int tid = omp_get_thread_num();
    printf("Thread %d in function f1.\n", tid);
    sleep(rand()%10); 
    return 1;
}

int f2() {
    int tid = omp_get_thread_num();
    printf("Thread %d in function f2.\n", tid);
    sleep(rand()%10); 
    return 2;
}


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

    int answer;
    int ans1, ans2;

    /* using sections */
#pragma omp parallel num_threads(2) shared(ans1, ans2, answer) default(none)
    {
#pragma omp sections 
        {
#pragma omp section
            ans1 =  f1();

#pragma omp section
            ans2 =  f2();
        }  

#pragma omp single
        answer = ans1+ans2;

    }  

    printf("Answer = %d\n", answer);

    /* hacky appraoch, mis-using for loop */
    answer = 0;
#pragma omp parallel for schedule(static,1) num_threads(2) reduction(+:answer) default(none)
    for (int i=0; i<2; i++)  {    
        if (i==0)
            answer += f1();
        if (i==1)
            answer += f2();
    }

    printf("Answer = %d\n", answer);

    return 0;
}

Running this gives

$  ./sections 
Thread 0 in function f1.
Thread 1 in function f2.
Answer = 3
Thread 0 in function f1.
Thread 1 in function f2.
Answer = 3