Question

There is programs that is able to limit the CPU usage of processes in Windows. For example BES and ThreadMaster. I need to write my own program that does the same thing as these programs but with different configuration capabilities. Does anybody know how the CPU throttling of a process is done (code)? I’m not talking about setting the priority of a process, but rather how to limit it’s CPU usage to for example 15% even if there is no other processes competing for CPU time.

Update: I need to be able to throttle any processes that is already running and that I have no source code access to.

Answer 1

Very simplified, it could work somehow like this:

1. Create a periodic waitable timer with some reasonable small wait time (maybe 100ms). Get a “last” value for each relevant process by calling GetProcessTimes once.
2. Loop forever, blocking on the timer.
3. Each time you wake up:
   • if GetProcessAffinityMask returns 0, call SetProcessAffinityMask(old_value). This means we’ve suspended that process in our last iteration, we’re now giving it a chance to run again.
   • else call GetProcessTimes to get the “current” value
   • call GetSystemTimeAsFileTime
   • calculate delta by subtracting last from current
   • cpu_usage = (deltaKernelTime + deltaUserTime) / (deltaTime)
   • if that’s more than you want call old_value = GetProcessAffinityMask followed by SetProcessAffinityMask(0) which will take the process offline.

This is basically a very primitive version of the scheduler that runs in the kernel, implemented in userland. It puts a process “to sleep” for a small amount of time if it has used more CPU time than what you deem right. A more sophisticated measurement maybe going over a second or 5 seconds would be possible (and probably desirable).

You might be tempted to suspend all threads in the process instead. However, it is important not to fiddle with priorities and not to use SuspendThread unless you know exactly what a program is doing, as this can easily lead to deadlocks and other nasty side effects. Think for example of suspending a thread holding a critical section while another thread is still running and trying to acquire the same object. Or imagine your process gets swapped out in the middle of suspending a dozen threads, leaving half of them running and the other half dead.

Setting the affinity mask to zero on the other hand simply means that from now on no single thread in the process gets any more time slices on any processor. Resetting the affinity gives — atomically, at the same time — all threads the possibility to run again.

Unluckily, SetProcessAffinityMask does not return the old mask as SetThreadAffinityMask does, at least according to the documentation. Therefore an extra Get... call is necessary.