ThreadPool max threads

Question

I've got some trouble with .NET's ThreadPool (.NET 4).

I've read that by default .NET has a limit of 25 threads per processor, but according to forum posts on SO and on other places, I can increase the limit with the below code.

void SetThreads(int threads)
{
    ThreadPool.SetMaxThreads(threads, threads);
    ThreadPool.SetMinThreads(threads, threads);
}

However, when I set the above to some arbitrarily high number, for example, 2000, and queue ~1000 items, I still only have ~33 threads running (.NET CLR takes ~5 threads), and ThreadPool.GetAvailableThreads() returns 1971 threads remaining.

Why doesn't the code above work?

Answer 1

Firstly, your "knowledge" of the defaults is incorrect. The limit of 25 threads per processor was back from .NET 1.1. It was increased in .NET 2, and now:

Beginning with the .NET Framework version 4, the default size of the thread pool for a process depends on several factors, such as the size of the virtual address space. A process can call the GetMaxThreads method to determine the number of threads.

However, there's something else at play: the thread pool doesn't immediately create new threads in all situations. In order to cope with bursts of small tasks, it limits how quickly it creates new threads. IIRC, it will create one thread every 0.5 seconds if there are outstanding tasks, up to the maximum number of threads. I can't immediately see that figure documented though, so it may well change. I strongly suspect that's what you're seeing though. Try queuing a lot of items and then monitor the number of threads over time.