How to make loop with multi-core computation in VC++?

Question

No idea about multi-core parallelization. But for simple loops, it might modify little. For the following example, how to make simple loops with multi-core computation in VC++?

#include <iostream>
#include <vector>
#include <ctime>
using namespace std;

void foo(int n, double* a, double* b, double *c, double*d, double* e, double* f, double* g)
{
    for (int i = 0; i < n; ++i)
    {
        a[i] = b[i] * a[i] + c[i] * (d[i] + e[i] + f[i] + g[i]);
    }
}

int main()
{
    int m = 1001001;
    vector<double> a(m), b(m), c(m), d(m), f(m);

    std::clock_t startcputime = std::clock();
    for (int i = 0; i < 1000; ++i)
        foo(1000000, &a[0], &b[0], &c[0], &d[0], &d[1], &f[0], &f[1000]);
    double cpu_duration = (std::clock() - startcputime) / (double)CLOCKS_PER_SEC;
    std::cout << "Finished in " << cpu_duration << " seconds [CPU Clock] " << std::endl;
}

I believe that your first step should be to study basics of concurrency and parallelism, so that you have "some idea". — eerorika
– eerorika, Commented Jun 29, 2016 at 13:35

Andreas H. · Accepted Answer · 2016-06-29 13:46:40Z

5

The concurrency namespace in the Parallel Patterns Library (part of VC++) contains parallel_for which is exactly what you want.

void parallel_foo(int n, double* a, double* b, double *c, double*d, double* e, double* f, double* g)
{
    concurrency::parallel_for(static_cast<size_t>(0), static_cast<size_t>(n), [&](size_t i) {
        a[i] = b[i] * a[i] + c[i] * (d[i] + e[i] + f[i] + g[i]);
    });
}

I'd recommend using size_t for 'n'. This way it gets a little cleaner:

void parallel_foo(size_t n, double* a, double* b, double *c, double*d, double* e, double* f, double* g)
{
    concurrency::parallel_for(0, n, [&](size_t i) {
        a[i] = b[i] * a[i] + c[i] * (d[i] + e[i] + f[i] + g[i]);
    });
}

You should also take a look at the example at msdn

You could use 'amp' as an alternative. Its more powerfull and more complex.

edited Jun 29, 2016 at 13:46

answered Jun 29, 2016 at 13:36

Andreas H.

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Inline · Accepted Answer · 2017-11-16 13:32:13Z

3

You can use OpenMP, just add #pragma omp parallel for before for loop.

void foo(int n, double* a, double* b, double *c, double*d, double* e, double* f, double* g)
{
    #pragma omp parallel for
    for (int i = 0; i < n; ++i)
    {
        a[i] = b[i] * a[i] + c[i] * (d[i] + e[i] + f[i] + g[i]);
    }
}

Also, if you using Microsoft Visual studio, you need to enable OpenMP Support in settings

Open the project's Property Pages dialog box.
Expand the Configuration Properties node.
Expand the C/C++ node.
Select the Language property page.
Modify the OpenMP Support property.

edited Nov 16, 2017 at 13:32

answered Jun 29, 2016 at 13:37

Inline

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