Win32 game loop that doesn't spike the CPU

Question

There are plenty of examples in Windows of applications triggering code at fairly high and stable framerates without spiking the CPU.

WPF/Silverlight/WinRT applications can do this, for example. So can browsers and media players. How exactly do they do this, and what API calls would I make to achieve the same effect from a Win32 application?

Clock polling doesn't work, of course, because that spikes the CPU. Neither does Sleep(), because you only get around 50ms granularity at best.

huysentruitw · Accepted Answer · 2012-04-18T11:51:23.753

5

They are using multimedia timers. You can find information on MSDN here

Only the view is invalidated (f.e. with InvalidateRect)on each multimedia timer event. Drawing happens in the WM_PAINT / OnPaint handler.

edited Apr 18 '12 at 11:51

answered Apr 18 '12 at 11:35

huysentruitw

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Looks like this has actually been obsoleted in favor of [queue timers](http://blogs.msdn.com/b/larryosterman/archive/2005/09/08/462477.aspx). – Rei Miyasaka Apr 18 '12 at 12:04
On the information page of [timeSetEvent](http://msdn.microsoft.com/en-us/library/windows/desktop/dd757634(v=vs.85).aspx) Ken Smith states that multimedia timer is still the only reliable, high resolution timer available in Windows. I think you should check out both and see what best fits your application. – huysentruitw Apr 18 '12 at 12:20
Bleh, I wish they'd make proper alternatives before they deprecate things. Then again, I think this would suffice for a game... – Rei Miyasaka Apr 18 '12 at 20:55

John Leidegren · Answer 2 · 2012-04-18T12:32:27.373

Actually, there's nothing wrong with sleep.

You can use a combination of QueryPerformanceCounter/QueryPerformanceFrequency to obtain very accurate timings and on average you can create a loop which ticks forward on average exactly when it's supposed to.

I have never seen a sleep to miss it's deadline by as much as 50 ms however, I've seen plenty of naive timers that drift. i.e. accumalte a small delay and conincedentally updates noticable irregular intervals. This is what causes uneven framerates.

If you play a very short beep on every n:th frame, this is very audiable.

Also, logic and rendering can be run independently of each other. The CPU might not appear to be that busy, but I bet you the GPU is hard at work.

Now, about not hogging the CPU. CPU usage is just a break down of CPU time spent by a process under a given sample (the thread schedulerer actually tracks this). If you have a target of 30 Hz for your game. You're limited to 33ms per frame, otherwise you'll be lagging behind (too slow CPU or too slow code), if you can't hit this target you won't be running at 30 Hz and if you hit it under 33ms then you can yield processor time, effectivly freeing up resources.

This might be an intresting read for you as well.

On a side note, instead of yielding time you could effecivly be doing prepwork for future computations. Some games when they are not under the heaviest of loads actually do things as sorting and memory defragmentation, a little bit here and there, adds up in the end.

Win32 game loop that doesn't spike the CPU

2 Answers2