CPU throttling in C++

Question

I was just wandering if there is an elegant way to set the maximum CPU load for a particular thread doing intensive calculations. Right now I have located the most time consuming loop in the thread (it does only compression) and use GetTickCount() and Sleep() with hardcoded values. It makes sure that the loop continues for a certain period of time and than sleeps for a certain minimal time. It more or less does the job i.e. guarantees that the thread will not use more than 50% of CPU. However behavior is dependent on the number of CPU cores (huge disadvantage) and simply ugly (smaller disadvantage :)). Any ideas?

Answer 1

Sounds like a job the OS's scheduler should be doing.

Answer 2

I can't think of any cross platform way of what you want (or any guaranteed way full stop) but as you are using GetTickCount perhaps you aren't interested in cross platform :)

I'd use interprocess communications and set the intensive processes nice levels to get what you require but I'm not sure that's appropriate for your situation.

EDIT: I agree with Bernard which is why I think a process rather than a thread might be more appropriate but it just might not suit your purposes.

Answer 3

On linux, you can change the scheduling priority of a thread with nice().

Answer 4

I can't think of any cross platform way of what you want (or any guaranteed way full stop) but as you are using GetTickCount perhaps you aren't interested in cross platform :)

Indeed :). I am happy with Windows based solutions.

I'd use interprocess communications and set the intensive processes nice levels to get what you require but I'm not sure that's appropriate for your situation.

Yes, changing priority is good but it will not help if CPU is mostly idle and that is the case. The issue is that a watchdog monitoring the process periodically logs not nice warnings about CPU load of the process owning mentioned thread. For me it is rather a case of having easier to analyse watchdog logs than a real problem.

Answer 5

What visible behavior do you want to achieve?

That is, what does this watchdog want from your threads? Should they by no means use more than, say, 80% of the CPU? Can setting process base priority to Idle possibly calm the WD down?

Answer 6

I am not aware of any API to do get the OS's scheduler to do what you want (even if your thread is idle-priority, if there are no higher-priority ready threads, yours will run). However, I think you can improvise a fairly elegant throttling function based on what you are already doing. Essentially (I don't have a Windows dev machine handy):

Pick a default amount of time the thread will sleep each iteration. Then, on each iteration (or on every nth iteration, such that the throttling function doesn't itself become a significant CPU load),

1. Compute the amount of CPU time your thread used since the last time your throttling function was called (I'll call this dCPU). You can use the GetThreadTimes() API to get the amount of time your thread has been executing.
2. Compute the amount of real time elapsed since the last time your throttling function was called (I'll call this dClock).
3. dCPU / dClock is the percent CPU usage (of one CPU). If it is higher than you want, increase your sleep time, if lower, decrease the sleep time.
4. Have your thread sleep for the computed time.

Depending on how your watchdog computes CPU usage, you might want to use GetProcessAffinityMask() to find out how many CPUs the system has. dCPU / (dClock * CPUs) is the percentage of total CPU time available.

You will still have to pick some magic numbers for the initial sleep time and the increment/decrement amount, but I think this algorithm could be tuned to keep a thread running at fairly close to a determined percent of CPU.

Answer 7

The problem is it's not normal to want to leave the CPU idle while you have work to do. Normally you set a background task to IDLE priority, and let the OS handle scheduling it all the CPU time that isn't used by interactive tasks.

It sound to me like the problem is the watchdog process.

If your background task is CPU-bound then you want it to take all the unused CPU time for its task.

Maybe you should look at fixing the watchdog program?