CODEWITHSUNDEEP
multithreadingthreadpool
BAD_SEED
BAD_SEED

Reputation: 5056

Limit number of concurrent thread in a thread pool

In my code I have a loop, inside this loop I send several requests to a remote webservice. WS providers said: "The webservice can host at most n threads", so i need to cap my code since I can't send n+1 threads.

If I've to send m threads I would that first n threads will be executed immediately and as soon one of these is completed a new thread (one of the remaining m-n threads) will be executed and so on, until all m threads are executed.

I have thinked of a Thread Pool and explicit setting of the max thread number to n. Is this enough?

Upvotes: 1

Views: 1206

Answers (1)

MoonKnight
MoonKnight

Reputation: 23833

For this I would avoid the use of multiple threads. Instead, wrapping the entire loop up which can be run on a single thread. However, if you do want to launch multiple threads using the/a thread pool then I would use the Semaphore class to facilitate the required thread limit; here's how...

A semaphore is like a mean night club bouncer, it has been provide a club capacity and is not allowed to exceed this limit. Once the club is full, no one else can enter... A queue builds up outside. Then as one person leaves another can enter (analogy thanks to J. Albahari).

A Semaphore with a value of one is equivalent to a Mutex or Lock except that the Semaphore has no owner so that it is thread ignorant. Any thread can call Release on a Semaphore whereas with a Mutex/Lock only the thread that obtained the Mutex/Lock can release it.

Now, for your case we are able to use Semaphores to limit concurrency and prevent too many threads from executing a particular piece of code at once. In the following example five threads try to enter a night club that only allows entry to three...

class BadAssClub
{
    static SemaphoreSlim sem = new SemaphoreSlim(3);
    static void Main()
    {
        for (int i = 1; i <= 5; i++) 
            new Thread(Enter).Start(i);
    }

    // Enfore only three threads running this method at once.
    static void Enter(int i)
    {
        try
        {
            Console.WriteLine(i + " wants to enter.");
            sem.Wait();
            Console.WriteLine(i + " is in!");
            Thread.Sleep(1000 * (int)i);
            Console.WriteLine(i + " is leaving...");
        }
        finally
        {
            sem.Release();
        }
    }
}

I hope this helps.

Edit. You can also use the ThreadPool.SetMaxThreads Method. This method restricts the number of threads allowed to run in the thread pool. But it does this 'globally' for the thread pool itself. This means that if you are running SQL queries or other methods in libraries that you application uses then new threads will not be spun-up due to this blocking. This may not be relevant to you, in which case use the SetMaxThreads method. If you want to block for a particular method however, it is safer to use Semphores.

Upvotes: 2

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