Any relationship between process priority and thread pool priority (C#)

Question

I understand that thread pool priority should/can not be changed by the running process, but is the priority of particular task running on the thread pool somewhat priced-in with the calling process priority?

in other words, do all tasks in thread pool run in the same priority regardless the calling process priority?

thank you

update 1: i should have been more specific, i refer to thread inside Parallel.ForEach

Answer 1

I understand that thread pool priority should/can not be changed by the running process,

That's not exact. You can change Thread Pool's thread priority (inside delegate itself) and it'll run with new priority but default one will be restored when its task finishes and it'll be send back to pool.

ThreadPool.QueueUserWorkItem(delegate(object state) {
    Thread.CurrentThread.Priority = ThreadPriority.Highest;

    // Code in this function will run with Highest priority
});

is the priority of particular task running on the thread pool somewhat priced-in with the calling process priority?

Yes, and it doesn't apply to Thread Pool's threads only. In Windows process' priority is given by its class (from IDLE_PRIORITY_CLASS to REALTIME_PRIORITY_CLASS). Together with thread's priority (from THREAD_PRIORITY_IDLE to THREAD_PRIORITY_TIME_CRITICAL) it'll be used to calculate thread's final priority.

From MSDN:

The process priority class and thread priority level are combined to form the base priority of each thread.

Note that it's not simply a base priority plus an offset:

NORMAL_PRIORITY_CLASS + THREAD_PRIORITY_IDLE  == 1
NORMAL_PRIORITY_CLASS + THREAD_PRIORITY_TIME_CRITICAL == 15

But:

REALTIME_PRIORITY_CLASS + THREAD_PRIORITY_IDLE == 16
REALTIME_PRIORITY_CLASS + THREAD_PRIORITY_TIME_CRITICAL == 31

Moreover threads can have a temporary boost (decided and managed by Windows Scheduler). Be aware that a process can also change its own priority class.

in other words, do all tasks in thread pool run in the same priority regardless the calling process priority?

No, thread's priority depends on process' priority (see previous paragraph) and each thread in pool can temporary have a different priority. Also note that thread priority isn't affected by caller thread's priority:

ThreadPool.QueueUserWorkItem(delegate(object s1) {
    Thread.CurrentThread.Priority = ThreadPriority.Highest;

    ThreadPool.QueueUserWorkItem(delegate(object s2) {
        // This code is executed with ThreadPriority.Normal

        Thread.CurrentThread.Priority = ThreadPriority.Lowest;

        // This code is executed with ThreadPriority.Lowest
    });

    // This code is executed with ThreadPriority.Highest
});

EDIT: .NET tasks uses Thread Pool than what wrote above still applies. If, for example, you're enumerating a collecition with Parallel.ForEach to increase thread priority you have to do it inside your loop:

Parallel.ForEach(items, item => {
    Thread.CurrentThread.Priority = ThreadPriority.Highest;

    // Your code here...
});

Just a warning: be careful when you change priorities. If, for example, two threads use a shared resource (protected by a lock), there are many races to acquire that resources and one of them has highest priority then you may end with a very high CPU usage (because of spinning behavior of Monitor.Enter). This is just one issue, please refer to MSDN for more details (increasing thread's priority may even result is worse performance).

Answer 2

do all tasks in thread pool run in the same priority regardless the calling process priority?

They have to. The only thing dropped off at the pool is a delegate. That holds a reference to an object but not to the Thread that dropped it off.

Answer 3

The ones that are currently running have the same priority. But there's a queue for the ones that aren't running yet - so in practice, there is a "priority". Even more confusingly, thread priorities can be boosted (and limited) by the OS, for example when two threads in the threadpool depend on each other (e.g. one is blocking the other). And of course, any time something is blocking on the threadpool, you're wasting resources :D

That said, you shouldn't really change thread priorities at all, threadpool or not. You don't really need to, and thread (and process) priorities don't work the way you probably expect them to - it's just not worth it. Keep everything at normal, and just ignore that there's a Priority property, and you'll avoid a lot of unnecessary problems :)

You'll find a lot of nice explanations on the internet - for example, http://blog.codinghorror.com/thread-priorities-are-evil/. Those are usually dated, of course - but so is the concept of thread priorities, really - they were designed for single-core machines at a time when OSes weren't all that good at pre-emptive multitasking, really.