C ++ 2011：std :: thread：简单的例子来并行化一个循环？ [英] C++ 2011 : std::thread : simple example to parallelize a loop?

问题描述

C ++ 2011包含了非常酷的新功能，但是我找不到很多并行化for循环的例子。
所以我非常幼稚的问题是：你如何并行化一个简单的for循环（如使用omp parallel for）与std :: thread？ （我搜索一个例子）。

C++ 2011 includes very cool new features, but I can't find a lot of example to parallelize a for-loop. So my very naive question is : how do you parallelize a simple for loop (like using "omp parallel for") with std::thread ? (I search for an example).

非常感谢。

推荐答案

std :: thread 不一定意味着将环路解析。它的意思是低层抽象构建像一个parallel_for算法的结构。如果你想将你的循环并置，你应该自己编写一个parallel_for算法或使用现有的库提供基于任务的parallism。

std::thread is not necessarily meant to parallize loops. It is meant to be the lowlevel abstraction to build constructs like a parallel_for algorithm. If you want to parallize your loops, you should either wirte a parallel_for algorithm yourself or use existing libraires which offer task based parallism.

以下示例显示了如何将一个简单循环并行化，但另一方面也显示了缺点，如缺少负载平衡和简单循环的复杂性。

The following example shows how you could parallize a simple loop but on the other side also shows the disadvantages, like the missing load-balancing and the complexity for a simple loop.

  typedef std::vector<int> container;
  typedef container::iterator iter;

  container v(100, 1);

  auto worker = [] (iter begin, iter end) {
    for(auto it = begin; it != end; ++it) {
      *it *= 2;
    }
  };


  // serial
  worker(std::begin(v), std::end(v));

  std::cout << std::accumulate(std::begin(v), std::end(v), 0) << std::endl; // 200

  // parallel
  std::vector<std::thread> threads(8);
  const int grainsize = v.size() / 8;

  auto work_iter = std::begin(v);
  for(auto it = std::begin(threads); it != std::end(threads) - 1; ++it) {
    *it = std::thread(worker, work_iter, work_iter + grainsize);
    work_iter += grainsize;
  }
  threads.back() = std::thread(worker, work_iter, std::end(v));

  for(auto&& i : threads) {
    i.join();
  }

  std::cout << std::accumulate(std::begin(v), std::end(v), 0) << std::endl; // 400

使用库提供 parallel_for template，可以简化为

Using a library which offers a parallel_for template, it can be simplified to

parallel_for(std::begin(v), std::end(v), worker);

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