Question

Every n*x milliseconds I perform an action where n = 0, 1, 2, …; x is some increment.

Example – every 25 milliseconds I perform a calculation.

This action can take fewer than x seconds for each increment. As a result, I need a way in C# to wait the remaining (x – actual_time) milliseconds.

Example – if the calculation only takes 20 milliseconds, I need to wait 5 more milliseconds before re-running the calculation.

Please advise.

Thanks,
Kevin

Answer 1

I need a way in C# to wait the remaining (x – actual_time) milliseconds.

I presume that is C# running on Windows.

And there is your problem. Windows is not a “realtime” operating system.

The best you can do if you need millisecond-grade timing precision is to set the thread priority of your thread extremely high, and then busy-wait while querying the high performance timer (Stopwatch).

You cannot yield to another thread; the other thread could run for as much as 16 milliseconds before the operating system context switches it, and of course unless you are the highest priority thread, you have no guarantee that control is coming back to you after those 16 milliseconds are up.

Now, setting thread priority high and then busy waiting is one of the most rude things you can possibly do; essentially you will be taking control of the user’s machine and not allowing them to do anything else with it.

Therefore what I would do is abandon this course of action entirely. Either, (1) consider obtaining an operating system designed for realtime process control if that is in fact your application, rather than an operating system designed for multitasking a bunch of line-of-business applications. Or (2) abandon your requirement that the action happen exactly every 25 milliseconds. Just perform the calculation once and yield the remainder of your quantum to another thread. When you get control back, see if more than 25 ms has passed since you yielded; if it has not, yield again. If it has, start over and perform the calculation.