Setting limit on post queue size with Boost Asio?

Question

I\'m using boost::asio::io_service as a basic thread pool. Some threads get added to io_service, the main thread starts posting handlers, the worker threads start r

Answer 1

You can wrap your lambda in another lambda which would take care of counting the "in-progress" tasks, and then wait before posting if there are too many in-progress tasks.

Example:

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

class ThreadPool {
  using asio_worker = std::unique_ptr;
  boost::asio::io_service service;
  asio_worker service_worker;
  std::vector grp;
  std::atomic inProgress = 0;
  std::mutex mtx;
  std::condition_variable busy;
public:
  ThreadPool(int threads) : service(), service_worker(new asio_worker::element_type(service)) {
    for (int i = 0; i < threads; ++i) {
      grp.emplace_back([this] { service.run(); });
    }
  }

  template
  void enqueue(F && f) {
    std::unique_lock lock(mtx);
    // limit queue depth = number of threads
    while (inProgress >= grp.size()) {
      busy.wait(lock);
    }
    inProgress++;
    service.post([this, f = std::forward(f)]{
      try {
        f();
      }
      catch (...) {
        inProgress--;
        busy.notify_one();
        throw;
      }
      inProgress--;
      busy.notify_one();
    });
  }

  ~ThreadPool() {
    service_worker.reset();
    for (auto& t : grp)
      if (t.joinable())
        t.join();
    service.stop();
  }
};

int main() {
  std::unique_ptr pool(new ThreadPool(4));
  for (int i = 1; i <= 20; ++i) {
    pool->enqueue([i] {
      std::string s("Hello from task ");
      s += std::to_string(i) + "\n";
      std::cout << s;
      std::this_thread::sleep_for(std::chrono::seconds(1));
    });
  }
  std::cout << "All tasks queued.\n";
  pool.reset(); // wait for all tasks to complete
  std::cout << "Done.\n";
}

Output:

Hello from task 3
Hello from task 4
Hello from task 2
Hello from task 1
Hello from task 5
Hello from task 7
Hello from task 6
Hello from task 8
Hello from task 9
Hello from task 10
Hello from task 11
Hello from task 12
Hello from task 13
Hello from task 14
Hello from task 15
Hello from task 16
Hello from task 17
Hello from task 18
All tasks queued.
Hello from task 19
Hello from task 20
Done.