c++ - 运行方法并等待完成所有 for 循环 openmp

Question

我创建了 C++ 方法来查找数字的除数。第二步是在 C++ 中使用 openmp。 不幸的是，我无法管理为什么我的函数 doStuff 会抛出 memory error。可能问题出在线程上，我在所有线程停止之前检查数组。有人可以帮助我吗？

没有必要把我的程序全部看完，问题出在doStuff()

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include "omp.h"

using namespace std;

vector<int> dividors;
int NUMBER = 1000;

bool ifContains(vector<int> element, int dividedNumber)
{
    for(int i=0; i<dividors.size(); i++)
    {
        if(dividors[i] == dividedNumber)
            return true;
    }

    return false;
}

void doStuff()
{
    int sqr = (int) sqrt(NUMBER);
    int sqrp1 = sqr + 1;

    #pragma omp parallel
    {
    #pragma omp for nowait
        for (int i = 1; i < sqrp1; i++)
        {
            if (NUMBER % i == 0)
            {
                if (!ifContains(dividors, i))
                    dividors.push_back(i);

                int dividednumber = NUMBER / i;

                if (!ifContains(dividors,dividednumber))
                    dividors.push_back(dividednumber);
            }
        }


        sort(dividors.begin(), dividors.end());

        #pragma omp for nowait
        for (int i = 0; i < dividors.size(); i++)
        {
            cout << dividors[i] << "\r\n";
        }

    }
}

int main()
{
    doStuff();
    return 0;
}

---

这个我也试过了，但是不行

void doStuff()
{
    int sqr = (int) sqrt(NUMBER);
    int sqrp1 = sqr + 1;

    #pragma omp parallel
    {
        #pragma omp for
        for (int i = 1; i < sqrp1; i++)
        {

            if (NUMBER % i == 0)
            {
                if (!ifContains(dividors, i))
                    dividors.push_back(i);

                int dividednumber = NUMBER / i;

                if (!ifContains(dividors,dividednumber))
                    dividors.push_back(dividednumber);
            }
        }



        #pragma omp single
        sort(dividors.begin(), dividors.end());

        #pragma omp single
        for (int i = 0; i < dividors.size(); i++)
        {
            cout << dividors[i] << "\r\n";
        }

    }
}

Answer 1

有几种方法可以解决这个问题。最简单的就是使用 ordered 子句。请参阅下面的代码。但是，这消除了一些并行性。更好的方法是在并行 block 中声明 prviate dividors vector (我称之为 dividors_private)，这样每个线程都有自己的私有(private)版本，然后写入 dividors vector 关键 block 。排序是在私有(private) vector 上并行完成的。最后的排序是在单个线程中对 dividors 完成的，但由于其中大部分已经排序，所以速度很快。请参见下面的第二个代码:

带顺序的代码版本:

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include "omp.h"

using namespace std;

vector<int> dividors;
int NUMBER = 1000;

bool ifContains(vector<int> dividors, int dividedNumber)
{
    for(int i=0; i<dividors.size(); i++)
    {
        if(dividors[i] == dividedNumber)
            return true;
    }

    return false;
}

void doStuff()
{
    int sqr = (int) sqrt(NUMBER);
    int sqrp1 = sqr + 1;

    #pragma omp parallel
    {
        #pragma omp for ordered
        for (int i = 1; i < sqrp1; i++)
        {

            if (NUMBER % i == 0)
            {
                #pragma omp ordered 
                {
                    dividors.push_back(i);
                    if (!ifContains(dividors, i))
                        dividors.push_back(i);

                    int dividednumber = NUMBER / i;

                    if (!ifContains(dividors, dividednumber))
                        dividors.push_back(dividednumber);
                }
            }
        }

    }
    sort(dividors.begin(), dividors.end());  
    for (int i = 0; i < dividors.size(); i++)
    {
        cout << dividors[i] << "\r\n";
    }
}

int main()
{
    doStuff();
    return 0;
}

使用私有(private)除数 vector 的代码版本

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include "omp.h"

using namespace std;

vector<int> dividors;
int NUMBER = 1000;

bool ifContains(vector<int> dividors, int dividedNumber)
{
    for(int i=0; i<dividors.size(); i++)
    {
        if(dividors[i] == dividedNumber)
            return true;
    }

    return false;
}

void doStuff()
{
    int sqr = (int) sqrt(NUMBER);
    int sqrp1 = sqr + 1;

    #pragma omp parallel
    {
        vector<int> dividors_private;
        #pragma omp for nowait
        for (int i = 1; i < sqrp1; i++)
        {

            if (NUMBER % i == 0)
            {

                dividors_private.push_back(i);
                //printf("i %d\n", i);
                if (!ifContains(dividors_private, i))
                    dividors_private.push_back(i);

                int dividednumber = NUMBER / i;

                if (!ifContains(dividors_private, dividednumber))
                    dividors_private.push_back(dividednumber);
            }
        }
        sort(dividors_private.begin(), dividors_private.end());
        #pragma omp critical
        {
            dividors.insert(dividors.end(), dividors_private.begin(), dividors_private.end());
        }
    }
    sort(dividors.begin(), dividors.end()); 
    for (int i = 0; i < dividors.size(); i++) 
    {
        cout << dividors[i] << "\r\n";
    }
}

int main()
{
    doStuff();
    return 0;
}