c++ - 提高C++的性能

我正在尝试在C++中创建线程。我当然觉得在for循环内创建线程并不意味着并行。但是我想并行化下面的代码逻辑。

for(int i = 0; i < 100000; i++) // for each instance in the dataset
{
    for(int j = 0; j < 100000; j++) // target each other instance
    {
        if(i == j) continue;

        float distance = 0;

        for(int k = 0; k < 2000; k++)
        {
            float a = dataset->get_instance(i)->get(k)->operator float();
            float b = dataset->get_instance(j)->get(k)->operator float();
            float diff = a - b
            distance += diff * diff;
        }

        distance = distance + 10;

    }

}

上面的代码段中是否存在并行性的可能性？或者任何人都可以给我提供一些代码示例，以了解类似的线程并行化。

最佳答案

如果显示的功能均没有副作用，则可以在i循环的每个迭代中仅运行一个线程，可以创建N个线程并将外部i循环的迭代数除以每个线程，或者可以使用std::async:

struct ShortestDistance {
    float distance;
    int distClass;
};

ShortestDistance inner_loop(const Dataset* dataset, int i)
{
    ShortestDistance dist { MAX_FLT, 0 };

    for(int j = 0; j < dataset->num_instances(); j++) // target each other instance
    {
        if(i == j) continue;

        float distance = 0;

        for(int k = 0; k < dataset->num_attributes() - 1; k++) // compute the distance between the two instances
        {
            float a = dataset->get_instance(i)->get(k)->operator float();
            float b = dataset->get_instance(j)->get(k)->operator float();
            float diff = a - b
            distance += diff * diff;
        }

        distance = sqrt(distance);
        if (distance < dist.distance) {
            dist.distance = distance;
            dist.distClass = dataset->get_instance(j)->get(dataset->num_attributes() - 1)->operator int32();
        }
    }

    return dist;
}

void outer_loop(const Dataset* dataset)
{
    std::vector<std::future<ShortestDistance>> vec;
    for(int i = 0; i < dataset->num_instances(); i++) // for each instance in the dataset
    {
        vec[i] = std::async(inner_loop, dataset, i);
    }

    DistanceResult overallResult { FLT_MAX, 0 };
    for (auto&& fut : vec)
    {
        DistanceResult threadResult = fut.get();
        if (threadResult.distance < overallResult.distance)
            overallResult = threadResult);
    }
}

关于c++ - 提高C++的性能，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/39444011/