Boost MultiArray 中访问元素的最快方法 [英] Fastest method of accessing elements in Boost MultiArray

问题描述

什么更快 - 使用元素选择运算符访问多数组的元素，还是使用迭代器遍历多数组?

What is faster - accessing elements of a multiarray using element selection operator, or traversing over the multiarray using iterators?

就我而言，我每次都需要对多数组的所有元素进行完整传递.

In my case, I need to do a full pass over all elements of multiarray each time.

推荐答案

访问 boost::multi_array 的每个元素的最快方法是通过 data() 和num_elements().

The fastest way to access every element of a boost::multi_array is via data() and num_elements().

使用 data() 您可以访问底层原始存储(包含数组数据的连续块)，因此不需要多个索引计算(还要考虑 multi_array 可以从不同于 0 的基数索引数组，这是一个进一步的复杂性).

With data() you gain access to the underlying raw storage (a contiguous block that contains the array‘s data) so there isn't need for multiple index calculations (consider also that multi_array can index arrays from bases different from 0 and this is a further complication).

一个简单的测试给出:

g++ -O3 -fomit-frame-pointer -march=native   (GCC v4.8.2)
Writing (index): 9.70651
Writing (data):  2.22353
Reading (index): 4.5973 (found 1)
Reading (data):  3.53811 (found 1)

clang++ -O3 -fomit-frame-pointer -march=native   (CLANG v3.3)
Writing (index): 5.49858
Writing (data):  2.13678
Reading (index): 5.07324 (found 1)
Reading (data):  2.55109 (found 1)

默认情况下，boost 访问方法执行范围检查.如果提供的索引超出为数组定义的范围，则断言将中止程序.要禁用范围检查，您可以在应用程序中包含 multi_array.hpp 之前定义 BOOST_DISABLE_ASSERTS 预处理器宏.

By default boost access methods perform range checking. If a supplied index is out of the range defined for an array, an assertion will abort the program. To disable range checking, you can define the BOOST_DISABLE_ASSERTS preprocessor macro prior to including multi_array.hpp in your application.

这将大大减少性能差异:

This will reduce a lot the performance difference:

g++ -O3 -fomit-frame-pointer -march=native   (GCC v4.8.2)
Writing (index): 3.15244
Writing (data):  2.23002
Reading (index): 1.89553 (found 1)
Reading (data):  1.54427 (found 1)

clang++ -O3 -fomit-frame-pointer -march=native   (CLANG v3.3)
Writing (index): 2.24831
Writing (data):  2.12853
Reading (index): 2.59164 (found 1)
Reading (data):  2.52141 (found 1)

性能差异增加(即 data() 更快):

Performance difference increases (i.e. data() is faster):

• 具有更多维度；
• 元素较少(对于大量元素，对元素的访问不会像将这些元素加载到 CPU 缓存的缓存压力那么重要.预取器将坐在那里尝试加载这些元素，这将占用大部分时间).

无论如何，这种优化不太可能在实际程序中产生可测量的差异.除非您通过大量测试最终确定这是某种瓶颈的根源，否则您不必担心这一点.

Anyway this kind of optimization is unlikely to make a measurable difference in a real program. You shouldn't worry about this unless you've conclusively determined, through extensive testing, that it is the source of some sort of bottleneck.

来源:

#include <chrono>
#include <iostream>

// #define BOOST_DISABLE_ASSERTS
#include <boost/multi_array.hpp>

int main()
{
  using array3 = boost::multi_array<unsigned, 3>;
  using index = array3::index;

  using clock = std::chrono::high_resolution_clock;
  using duration = std::chrono::duration<double>;

  constexpr unsigned d1(300), d2(400), d3(200), sup(100);

  array3 A(boost::extents[d1][d2][d3]);

  // Writing via index
  const auto t_begin1(clock::now());
  unsigned values1(0);
  for (unsigned n(0); n < sup; ++n)
    for (index i(0); i != d1; ++i)
      for (index j(0); j != d2; ++j)
        for (index k(0); k != d3; ++k)
          A[i][j][k] = ++values1;
  const auto t_end1(clock::now());

  // Writing directly
  const auto t_begin2(clock::now());
  unsigned values2(0);
  for (unsigned n(0); n < sup; ++n)
  {
    const auto sup(A.data() + A.num_elements());

    for (auto i(A.data()); i != sup; ++i)
      *i = ++values2;
  }
  const auto t_end2(clock::now());

  // Reading via index
  const auto t_begin3(clock::now());
  bool found1(false);
  for (unsigned n(0); n < sup; ++n)
    for (index i(0); i != d1; ++i)
      for (index j(0); j != d2; ++j)
        for (index k(0); k != d3; ++k)
          if (A[i][j][k] == values1)
            found1 = true;
  const auto t_end3(clock::now());

  // Reading directly
  const auto t_begin4(clock::now());
  bool found2(false);
  for (unsigned n(0); n < sup; ++n)
  {
    const auto sup(A.data() + A.num_elements());

    for (auto i(A.data()); i != sup; ++i)
      if (*i == values2)
        found2 = true;
  }
  const auto t_end4(clock::now());

  std::cout << "Writing (index): "
            << std::chrono::duration_cast<duration>(t_end1 - t_begin1).count()
            << std::endl
            << "Writing (data):  "
            << std::chrono::duration_cast<duration>(t_end2 - t_begin2).count()
            << std::endl
            << "Reading (index): "
            << std::chrono::duration_cast<duration>(t_end3 - t_begin3).count()
            << " (found " << found1 << ")" << std::endl
            << "Reading (data):  "
            << std::chrono::duration_cast<duration>(t_end4 - t_begin4).count()
            << " (found " << found2 << ")" << std::endl;

  return 0;
}

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