用多个线程填充向量

我需要std::vector<unsigned int>用随机值填充一个巨大的（7734500个元素），并且我试图与多个线程并行执行以实现更高的效率。这是我到目前为止的代码：

std::random_device rd; // seed generator

std::mt19937_64 generator{rd()}; // generator initialized with seed from rd

static const unsigned int NUM_THREADS = 4;


std::uniform_int_distribution<> initialize(unsigned long long int modulus)
{
    std::uniform_int_distribution<> unifDist{0, (int)(modulus-1)};
    return unifDist;
}


void unifRandVectorThreadRoutine
    (std::vector<unsigned int>& vector, unsigned int start,
    unsigned int end, std::uniform_int_distribution<>& dist)
{
    for(unsigned int i = start ; i < end ; ++i)
    {
        vector[i] = dist(generator);
    }
}


std::vector<unsigned int> uniformRandomVector
    (unsigned int rows, unsigned int columns, unsigned long long int modulus)
{
    std::uniform_int_distribution<> dist = initialize(modulus);

    std::thread threads[NUM_THREADS];

    std::vector<unsigned int> v;
    v.resize(rows*columns);

    // number of entries each thread will take care of
    unsigned int positionsEachThread = rows*columns/NUM_THREADS;

    // all but the last thread
    for(unsigned int i = 0 ; i < NUM_THREADS - 1 ; ++i)
    {
        threads[i] = std::thread(unifRandVectorThreadRoutine, v, i*positionsEachThread,
            (i+1)*positionsEachThread, dist);
        // threads[i].join();
    }

    // last thread
    threads[NUM_THREADS - 1] = std::thread(unifRandVectorThreadRoutine, v,
        (NUM_THREADS-1)*positionsEachThread, rows*columns, dist);
    // threads[NUM_THREADS - 1].join();

    for(unsigned int i = 0 ; i < NUM_THREADS ; ++i)
    {
        threads[i].join();
    }

    return v;
}

目前，此过程大约需要0.3秒：您认为有办法提高效率吗？

编辑：给每个线程自己的生成器

我将例程修改如下

void unifRandVectorThreadRoutine
    (std::vector<unsigned int>& vector, unsigned int start,
    unsigned int end, std::uniform_int_distribution<>& dist)
{
    std::mt19937_64 generator{rd()};
    for(unsigned int i = start ; i < end ; ++i)
    {
        vector[i] = dist(generator);
    }
}

运行时间减少了一半。所以我仍然在共享，std::random_device但是每个线程都有自己的线程std::mt19937_64。

编辑：给每个线程自己的向量，然后串联

我将代码更改如下：

void unifRandVectorThreadRoutine
    (std::vector<unsigned int>& vector, unsigned int length,
    std::uniform_int_distribution<>& dist)
{
    vector.reserve(length);
    std::mt19937_64 generator{rd()};
    for(unsigned int i = 0 ; i < length ; ++i)
    {
        vector.push_back(dist(generator));
    }
}

和

std::vector<unsigned int> uniformRandomVector
    (unsigned int rows, unsigned int columns, unsigned long long int modulus)
{
    std::uniform_int_distribution<> dist = initialize(modulus);

    std::thread threads[NUM_THREADS];

    std::vector<unsigned int> v[NUM_THREADS];

    unsigned int positionsEachThread = rows*columns/NUM_THREADS;

    // all but the last thread
    for(unsigned int i = 0 ; i < NUM_THREADS - 1 ; ++i)
    {
        threads[i] = std::thread(unifRandVectorThreadRoutine, std::ref(v[i]), positionsEachThread, dist);
    }

    // last thread
    threads[NUM_THREADS - 1] = std::thread(unifRandVectorThreadRoutine, std::ref(v[NUM_THREADS-1]),
        rows*columns - (NUM_THREADS-1)*positionsEachThread, dist);

    for(unsigned int i = 0 ; i < NUM_THREADS ; ++i)
    {
        threads[i].join();
    }

    std::vector<unsigned int> finalVector;
    finalVector.reserve(rows*columns);

    for(unsigned int i = 0 ; i < NUM_THREADS ; ++i)
    {
        finalVector.insert(finalVector.end(), v[i].begin(), v[i].end());
    }

    return finalVector;
}

当我只使用一个在所有线程之间共享的向量时，执行时间比以前差一些。我是否想念某些东西还是会发生？

编辑：使用不同的PRNG +基准

使用不同的PRNG（如一些评论/答案所建议）有很多帮助：我尝试了使用xorshift+，这是我正在使用的实现：

class xorShift128PlusGenerator
{
public:
    xorShift128PlusGenerator()
    {
        state[0] = rd();
        state[1] = rd();
    };


    unsigned long int next()
    {
        unsigned long int x = state[0];
        unsigned long int const y = state[1];
        state[0] = y;
        x ^= x << 23; // a
        state[1] = x ^ y ^ (x >> 17) ^ (y >> 26); // b, c
        return state[1] + y;
    }


private:
    std::random_device rd; // seed generator
    unsigned long int state[2];

};

然后例程如下

void unifRandVectorThreadRoutine
    (std::vector<unsigned int>& vector, unsigned int start,
    unsigned int end)
{
    xorShift128PlusGenerator prng;
    for(unsigned int i = start ; i < end ; ++i)
    {
        vector[i] = prng.next();
    }
}

由于我现在在家中，并且正在使用其他（功能更强大）的计算机，因此我重新测试以比较结果。这是我得到的：

• Mersenne Twister每线程一个生成器：0.075秒
• 所有线程之间共享的xorshift128 +：0.023秒
• xorshift128 +，每个线程一个生成器：0.023秒

注意：每次重复执行的时间都不同。这些只是典型值。

因此，是否共享xorshift生成器似乎没有什么区别，但是通过所有这些改进，执行时间大大减少了。