c++ - 了解 boost 执行器示例

Question

我无法理解为什么在下面这个取自 boost documentation 的示例中使用了 boost::basic_thread_pool executor 的(未记录的)接口(interface)本身:

template<typename T>
struct sorter
{
    boost::basic_thread_pool pool;
    typedef std::list<T> return_type;

    std::list<T> do_sort(std::list<T> chunk_data)
    {
        if(chunk_data.empty()) {
            return chunk_data;
        }

        std::list<T> result;
        result.splice(result.begin(),chunk_data, chunk_data.begin());
        T const& partition_val=*result.begin();

        typename std::list<T>::iterator divide_point =
            std::partition(chunk_data.begin(), chunk_data.end(),
                           [&](T const& val){return val<partition_val;});

        std::list<T> new_lower_chunk;
        new_lower_chunk.splice(new_lower_chunk.end(), chunk_data,
                               chunk_data.begin(), divide_point);
        boost::future<std::list<T> > new_lower =
             boost::async(pool, &sorter::do_sort, this, std::move(new_lower_chunk));
        std::list<T> new_higher(do_sort(chunk_data));
        result.splice(result.end(),new_higher);
        while(!new_lower.is_ready()) {
            pool.schedule_one_or_yield();
        }
        result.splice(result.begin(),new_lower.get());
        return result;
    }
};

有问题的调用是 pool.schedule_one_or_yield();。如果我错了，请纠正我，但这表明提交的任务最终将被安排执行。如果是这样，不应该让之前对 boost::async(pool, &sorter::do_sort, this, std::move(new_lower_chunk)); 的每次调用都已经安排好了隐式提交任务？

我知道 boost 执行器 API 是实验性的，但你知道为什么 schedule_one_or_yield 没有记录吗？

Answer 1

schedule_one_or_yield() 函数已从当前的 boost 源代码中删除，因为它实现了忙等待。这是在

https://github.com/boostorg/thread/issues/117

loop_executor::loop is currently:

void loop()
{
  while (!closed())
  {
    schedule_one_or_yield();
  }
  while (try_executing_one())
  {
  }
}

The first loop repeatedly calls schedule_one_or_yield() which is simply

void schedule_one_or_yield()
{
    if ( ! try_executing_one())
    {
      this_thread::yield();
    }
}

当前的loop_executor::loop实现是

/**
     * The main loop of the worker thread
     */
    void loop()
    {
      while (execute_one(/*wait:*/true))
      {
      }
      BOOST_ASSERT(closed());
      while (try_executing_one())
      {
      }
}

来源:https://github.com/boostorg/thread/blob/develop/include/boost/thread/executors/loop_executor.hpp

例子中也去掉了user_scheduler，旧版本在

https://github.com/mongodb/mongo/blob/master/src/third_party/boost-1.60.0/boost/thread/user_scheduler.hpp在第 63 行使用 schedule_one_or_yield()

没有 schedule_one_or_yield() 的新版本在 https://github.com/boostorg/thread/blob/develop/example/user_scheduler.cpp 中