[Boost-users] Boost.Asio thread pool - add/remove threads at runtime

Subject: [Boost-users] Boost.Asio thread pool - add/remove threads at runtime
From: Vance Grkov (vanceg_at_[hidden])
Date: 2010-04-14 17:34:19

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.

                                               
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