2

我有一个 640*480 的向量，其中包含一组数字，我希望找到向量每行的最小和最大数。

for(int i = 0; i < R; i++)
    {
        Begin = m_valBuffer.begin()  + (i*C);
        End = Begin+C;

        rMinmax= minmax_element(Begin, End);
     }

但是这非常慢，有什么办法可以加快速度吗？

  • 运行时 GPU 上的当前负载仅为 34%，所以必须有办法改善这一点？
4

1 回答 1

7

此示例说明如何使用该reduce_by_key算法计算每一行的总和。您可以轻松地调整该示例来计算每行的最小值或最大值。要同时计算每行的最小值和最大值，您需要使用此策略。具体来说，您需要transform_iterator在输入数据上使用 a 并将每个值x转换为元组(x,x)，然后再应用minmax_binary_op归约运算符。

这是一个完整的例子：

#include <thrust/host_vector.h>
#include <thrust/device_vector.h>
#include <thrust/generate.h>
#include <thrust/transform_reduce.h>
#include <thrust/functional.h>
#include <thrust/extrema.h>
#include <thrust/random.h>
#include <iostream>
#include <iomanip>

// minmax_pair stores the minimum and maximum 
// values that have been encountered so far
template <typename T>
struct minmax_pair
{
  T min_val;
  T max_val;
};

// minmax_unary_op is a functor that takes in a value x and
// returns a minmax_pair whose minimum and maximum values
// are initialized to x.
template <typename T>
struct minmax_unary_op
  : public thrust::unary_function< T, minmax_pair<T> >
{
  __host__ __device__
  minmax_pair<T> operator()(const T& x) const
  {
    minmax_pair<T> result;
    result.min_val = x;
    result.max_val = x;
    return result;
  }
};

// minmax_binary_op is a functor that accepts two minmax_pair 
// structs and returns a new minmax_pair whose minimum and 
// maximum values are the min() and max() respectively of 
// the minimums and maximums of the input pairs
template <typename T>
struct minmax_binary_op
  : public thrust::binary_function< minmax_pair<T>,
                                    minmax_pair<T>,
                                    minmax_pair<T> >
{
  __host__ __device__
  minmax_pair<T> operator()(const minmax_pair<T>& x, const minmax_pair<T>& y) const 
  {
    minmax_pair<T> result;
    result.min_val = thrust::min(x.min_val, y.min_val);
    result.max_val = thrust::max(x.max_val, y.max_val);
    return result;
  }
};

// convert a linear index to a row index
template <typename T>
struct linear_index_to_row_index : public thrust::unary_function<T,T>
{
    T C; // number of columns

    __host__ __device__
    linear_index_to_row_index(T C) : C(C) {}

    __host__ __device__
    T operator()(T i)
    {
        return i / C;
    }
};

int main(void)
{
    int R = 5;     // number of rows
    int C = 8;     // number of columns
    thrust::default_random_engine rng;
    thrust::uniform_int_distribution<int> dist(0, 99);

    // initialize data
    thrust::device_vector<int> array(R * C);
    for (size_t i = 0; i < array.size(); i++)
        array[i] = dist(rng);

    // allocate storage for per-row results and indices
    thrust::device_vector< minmax_pair<int> > row_results(R);
    thrust::device_vector< int              > row_indices(R);

    // compute row sums by summing values with equal row indices
    thrust::reduce_by_key
      (thrust::make_transform_iterator(thrust::counting_iterator<int>(0), linear_index_to_row_index<int>(C)),
       thrust::make_transform_iterator(thrust::counting_iterator<int>(0), linear_index_to_row_index<int>(C)) + (R*C),
       thrust::make_transform_iterator(array.begin(), minmax_unary_op<int>()),
       row_indices.begin(),
       row_results.begin(),
       thrust::equal_to<int>(),
       minmax_binary_op<int>());

    // print data 
    for(int i = 0; i < R; i++)
    {
      minmax_pair<int> result = row_results[i];
        std::cout << "[";
        for(int j = 0; j < C; j++)
            std::cout << std::setw(3) << array[i * C + j] << " ";
        std::cout << "] = " << "(" << result.min_val << "," << result.max_val << ")\n";
    }

    return 0;
}

样本输出：

[  0   8  60  89  96  18  51  39 ] = (0,96)
[ 26  74   8  56  58  80  59  51 ] = (8,80)
[ 87  99  72  96  29  42  89  65 ] = (29,99)
[ 90  96  16  85  90  29  93  41 ] = (16,96)
[ 30  51  39  78  68  54  59   9 ] = (9,78)
于 2011-07-27T17:38:51.313 回答