multithreading - C++ 2011 : std::thread : simple example to parallelize a loop?

Question

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).

Thank you very much.

Answer 1

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.

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

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

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