What determines the number of threads for a TaskFactory spawned jobs?

Question

I have the following code:

var factory = new TaskFactory();
for (int i = 0; i < 100; i++)
{
    var i1 = i;
    factory.StartNew(() => foo(i1));
}

sta         


        

Answer 1

If you increase the number of tasks to for example 1000000 you will see a lot more threads spawned over time. The TPL tends to inject one every 500ms.

The TPL threadpool does not understand IO-bound workloads (sleep is IO). It's not a good idea to rely on the TPL for picking the right degree of parallelism in these cases. The TPL is completely clueless and injects more threads based on vague guesses about throughput. Also to avoid deadlocks.

Here, the TPL policy clearly is not useful because the more threads you add the more throughput you get. Each thread can process one item per second in this contrived case. The TPL has no idea about that. It makes no sense to limit the thread count to the number of cores.

What determines the number of threads used at a time?

Barely documented TPL heuristics. They frequently go wrong. In particular they will spawn an unlimited number of threads over time in this case. Use task manager to see for yourself. Let this run for an hour and you'll have 1000s of threads.

How can I retrieve this number? How can I change this number?

You can retrieve some of these numbers but that's not the right way to go. If you need a guaranteed DOP you can use AsParallel().WithDegreeOfParallelism(...) or a custom task scheduler. You also can manually start LongRunning tasks. Do not mess with process global settings.