运行时决定线程数量

清单2.8实现了一个并行版的std::accumulate。代码中将整体工作拆分成小任务交给每个线程去做，其中设置最小任务数，是为了避免产生太多的线程。

清单2.8 原生并行版的std::accumulate

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template<typename Iterator,typename T>
struct accumulate_block
{
  void operator()(Iterator first,Iterator last,T& result)
  {
    result=std::accumulate(first,last,result);
  }
};

template<typename Iterator,typename T>
T parallel_accumulate(Iterator first,Iterator last,T init)
{
  unsigned long const length=std::distance(first,last);

  if(!length) // 1
    return init;

  unsigned long const min_per_thread=25;
  unsigned long const max_threads=
      (length+min_per_thread-1)/min_per_thread; // 2

  unsigned long const hardware_threads=
      std::thread::hardware_concurrency();

  unsigned long const num_threads=  // 3
      std::min(hardware_threads != 0 ? hardware_threads : 2, max_threads);

  unsigned long const block_size=length/num_threads; // 4

  std::vector<T> results(num_threads);
  std::vector<std::thread> threads(num_threads-1);  // 5

  Iterator block_start=first;
  for(unsigned long i=0; i < (num_threads-1); ++i)
  {
    Iterator block_end=block_start;
    std::advance(block_end,block_size);  // 6
    threads[i]=std::thread(     // 7
        accumulate_block<Iterator,T>(),
        block_start,block_end,std::ref(results[i]));
    block_start=block_end;  // 8
  }
  accumulate_block<Iterator,T>()(
      block_start,last,results[num_threads-1]); // 9
  std::for_each(threads.begin(),threads.end(),
       std::mem_fn(&std::thread::join));  // 10

  return std::accumulate(results.begin(),results.end(),init); // 11
}