I am creating a simple task and timer thread pool using Boost.Asio.

I am searching for the recommended way to dynamically add and remove threads to the thread pool. I don’t want to have to queue up tasks most of the time, only when the thread count gets very high (100s) queuing is ok, also I want for the thread count to go down to example 5 threads when there is no more work presented to the thread pool.  This is because some of the tasks can take some time to complete and I don't want to hold off other tasks when computer resource are still available.

Basically this is the solution I have in mind now. Given the following configuration values:

    gInitialThreadPoolSize = 4;

    gMaximumThreadPoolSize = 1024;

    gNumberOfThreadsDoingWork = 0;

And 2 events:

    HANDLE moreWorkAvailable;

    HANDLE exit;

Create a thread pool using this worker function:

    void ThreadWorkerFunction(boost::asio::io_service *io, boost::thread &thread)

    {

        for (;;) {

            HANDLE objects[] = { moreWorkAvailable, exit };

            errorCode = WaitForMultipleObjects(objects)

            if (errorCode == WAIT_OBJECT_0) { // More work is available, lets start processing...

                gNumberOfThreadsDoingWork++;

                io->run();

                gNumberOfThreadsDoingWork--;

                // When all work has been processed, go back to the original thread pool size

                if (threads.size() < gInitialThreadPoolSize ) {

                    threads.remove(thread);

                    break;

                }

            } else if (errorCode == WAIT_OBJECT_1) {

                threads.remove(thread);

                break;

            }

        }

    }

This is now the thread pool is created:

      std::vector<boost::shared_ptr<boost::thread> > threads;

      for (std::size_t i = 0; i < gInitialThreadPoolSize; ++i)

      {    

            boost::shared_ptr<boost::thread> thread(new boost::thread(

                  boost::bind(&ThreadWorkerFunction, &io, thread)

                  ));

            threads.push_back(thread);

      }

Next every time a task is inserted into the queue we check if all worker threads are busy, and in case they are busy, create a new thread

    if (threadsDoingWork  == threads.size()) {

        boost::shared_ptr<boost::thread> thread(new boost::thread(

              boost::bind(&worker, &io)

              ));

        threads.push_back(thread);

    }

    SetEvent(moreWorkAvailable);

Also in the ThreadWorkerFunction(...) function, after io->run() returns, we can check if more than N threads are idle and keep exiting until we get at minimum.

I would appreciate if you can let me know if there is some better and more simpler way to achieve this than implementing this myself. I am new to Asio so I might have overlooked something.