how to make a threadpool with boost::thread

Question

boost::thread is not-a-thread, a new thread is created when the ftor passed to it is called and thread exits when ftor returns.

We use threadpool to minimize thread creation and destruction cost. but each thread in threadpool is also destroyed when the supplied ftor returns.

So whats the basic concept behind building a threadpool ? is there any permanent thread where I can assign ftors to that thread ?

look at http://stackoverflow.com/questions/4084777/creating-a-thread-pool-using-boost — ForEveR, Jul 27 '12 at 06:06
What I am asking is a different question. I know Icn use bost::asio as threadpool and there is an unofficial threadpool class — Dipro Sen, Jul 27 '12 at 06:18
See also: http://stackoverflow.com/questions/19500404/how-to-create-a-thread-pool-using-boost-in-c — Jeroen, Mar 11 '14 at 21:31

score 1 · Accepted Answer · answered Jul 27 '12 at 07:24

1

A thread pool is just a bunch of threads that already running, and that are all running the same function. This functions basically just waits on a queue, and when there is a "function" in the queue it extracts and executes it.

Pseudo-code:

void thread_pool_function()
{
    while (true)
    {
        wait_for_signal_that_queue_is_not_empty();

        function_to_call = queue.remove_top();

        unklock_queue_semaphore();

        function_to_call();
    }
}

create_thread(thread_pool_function);
create_thread(thread_pool_function);
create_thread(thread_pool_function);
create_thread(thread_pool_function);

In the "code" above there are now four threads, all initially waiting for something to be put in a "queue". When there is something in the queue, it extracts it, and calls it as a function.

This is probably the simplest way to implement a thread pool.

answered Jul 27 '12 at 07:24

Some programmer dude

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actually what I need is only `threadpool.tryStart(f)` instead of `start` e.g. I don't need a queue. as the queue can be infinite long because my deposit rate is much much higher than execution rate. and on the other hand as deposit rate is too high it will eventually work as polling. So is it the right decision not to maintain the queue ? (the `f` that I'll run in a different thread is an Image comparator. I can always make a proxy callback that can give me a control on whether I'll put the same callback multiple times in queue or not but still, is not having a queue a right decision ?) – Dipro Sen Jul 27 '12 at 07:38
+1 I use explicit thread pools like this a lot, in multiple languages. It gives better control than some 'built-in' threadpool implementations in more modern languages, some of which seem to make inappropriate decisions about, say, when to add extra threads. – Martin James Jul 27 '12 at 10:37

Martin James · Answer 2 · 2012-07-27T10:38:49.737

In addtion to what @Joachim posted:

One way to flow-control such a system (and one I use a lot), is to use a 'pool queue', (blocking producer-consumer queue), of tasks, created and filled at startup with a fixed number of task objects. Any thread that wants to issue a task has to get one from the pool first and tasks are returned to the pool after completion handling. This limits the number of tasks in the system and, if the pool empties, requesting threads just have to wait, blocked on the empty pool, until some 'used' tasks come back in.

This works well, provides flow-control, prevents memory-runaway and eliminates continual task create/destroy. It's also easy to periodically display/write the pool queue depth on a timer, so you can see how 'busy' your app is, (and detect any leaks:).

Edit: Also, it removes the need for any bounded queues in the system. Unbounded queues are simpler and tend to need fewer system calls.

how to make a threadpool with boost::thread

2 Answers2