When using Task what happens if the ThreadPool is full/busy?

Question

When I am using the .Net 4 Task class which uses the ThreadPool, what happens if all Threads are busy?

Does the TaskScheduler create a new thread and extend the ThreadPool maximum number of threads or does it sit and wait until a thread is available?

Answer 1

The maximum number of threads in the ThreadPool is set to around 1000 threads on a .NET4.0 32-bit system. It's less for older versions of .NET. If you have 1000 threads going, say they're blocking for some reason, when you queue the 1001st task, it won't ever execute.

You will never hit the max thread count in a 32 bit process. Keep in mind that each thread takes at least 1MB of memory (that's the size of the user-mode stack), plus any other overhead. You already lose a lot of memory from the CLR and native DLLs loaded, so you'll hit an OutOfMemoryException before you use that many threads.

You can change the number of threads the ThreadPool can use, by calling the ThreadPool.SetMaxThreads method. However, if you're expecting to use that many threads, you have a far larger problem with your code. I do NOT recommend you mess around with ThreadPool configurations like that. You'll most likely just get worse performance.

Keep in mind with Task and ThreadPool.QueueUserWorkItem, the threads are re-used when they're done. If you create a task or queue a threadpool thread, it may or may not create a new thread to execute your code. If there are available threads already in the pool, it will re-use one of those instead of creating (an expensive) new thread. Only if the methods you're executing in the tasks never return, should you worry about running out of threads, but like I said, that's an entirely different problem with your code.

Answer 2

By default, the MaxThreads of the ThreadPool is very high. Usually you'll never get there, your app will crash first.

So when all threads are busy the new tasks are queued and slowly, at most 1 per 500 ms, the TP will allocate new threads.