Add a std::packaged_task to an existing thread?

问题:

Is there an standard way to add a std::packaged_task to an existing thread? There's a nontrivial amount of overhead that must happen before the task is run, so I want to do that once, then keep the thread running and waiting for tasks to execute. I want to be able to use futures so I can optionally get the result of the task and catch exceptions.

My pre-C++11 implementation requires my tasks to inherit from an abstract base class with a Run() method (a bit of a pain, can't use lambdas), and having a std::deque collection of those that I add to in the main thread and dequeue from in the worker thread. I have to protect that collection from simultaneous access and provide a signal to the worker thread that there's something to do so it isn't spinning or sleeping. Enqueing something returns a "result" object with a synchronization object to wait for the task to complete, and a result value. It all works well but it's time for an upgrade if there's something better.

回答1:

Here is a toy thread pool:

template<class T>
struct threaded_queue {
  using lock = std::unique_lock<std::mutex>;
  void push_back( T t ) {
    {
      lock l(m);
      data.push_back(std::move(t));
    }
    cv.notify_one();
  }
  boost::optional<T> pop_front() {
    lock l(m);
    cv.wait(l, [this]{ return abort || !data.empty(); } );
    if (abort) return {};
    auto r = std::move(data.back());
    data.pop_back();
    return std::move(r);
  }
  void terminate() {
    {
      lock l(m);
      abort = true;
      data.clear();
    }
    cv.notify_all();
  }
  ~threaded_queue()
  {
    terminate();
  }
private:
  std::mutex m;
  std::deque<T> data;
  std::condition_variable cv;
  bool abort = false;
};
struct thread_pool {
  thread_pool( std::size_t n = 1 ) { start_thread(n); }
  thread_pool( thread_pool&& ) = delete;
  thread_pool& operator=( thread_pool&& ) = delete;
  ~thread_pool() = default; // or `{ terminate(); }` if you want to abandon some tasks
  template<class F, class R=std::result_of_t<F&()>>
  std::future<R> queue_task( F task ) {
    std::packaged_task<R()> p(std::move(task));
    auto r = p.get_future();
    tasks.push_back( std::move(p) );
    return r;
  }
  template<class F, class R=std::result_of_t<F&()>>
  std::future<R> run_task( F task ) {
    if (threads_active() >= total_threads()) {
      start_thread();
    }
    return queue_task( std::move(task) );
  }
  void terminate() {
    tasks.terminate();
  }
  std::size_t threads_active() const {
    return active;
  }
  std::size_t total_threads() const {
    return threads.size();
  }
  void clear_threads() {
    terminate();
    threads.clear();
  }
  void start_thread( std::size_t n = 1 ) {
    while(n-->0) {
      threads.push_back(
        std::async( std::launch::async,
          [this]{
            while(auto task = tasks.pop_front()) {
              ++active;
              try{
                (*task)();
              } catch(...) {
                --active;
                throw;
              }
              --active;
            }
          }
        )
      );
    }
  }
private:
  std::vector<std::future<void>> threads;
  threaded_queue<std::packaged_task<void()>> tasks;
  std::atomic<std::size_t> active;
};

copied from another answer of mine.

A thread_pool with 1 thread matches your description pretty much.

The above is only a toy, a real thread pool I'd replace the std::packaged_task<void()> with a move_only_function<void()>, which is all I use it for. (A packaged_task<void()> can hold a packaged_task<R()> amusingly, if inefficiencly).

You will have to reason about shutdown and make a plan. The above code locks up if you try to shut it down without first clearing the threads.