Timer message in MFC/Win32

Question

I was just trying the SetTimer method in Win32 with some low values such as 10ms as the timeout period. I calculated the time it took to get 500 timer events and expected it to be around 5 seconds. Surprisingly I found that it is taking about 7.5 seconds to get these many events which means that it is timing out at about 16ms. Is there any limitation on the value we can set for the timeout period ( I couldn't find anything on the MSDN ) ? Also, does the other processes running in my system affect these timer messages?

Answer 1

OnTimer is based on WM_TIMER message, which is a low message priority, meaning it will be send only when there's no other message waiting.

Also MSDN explain that you can not set an interval less than USER_TIMER_MINIMUM, which is 10.

Regardless of that the scheduler will honor the time quantum.

Answer 2

Windows is not a real-time OS and can't handle that kind of precision (10 ms intervals). Having said that, there are multiple kinds of timers and some have better precision than others.

Answer 3

You can alter the granularity of the system timer down to 1ms - this is intended for MIDI work.

Basically, my experiences on w2k are that any requested wait period under 13ms returns a wait which oscillates randomly between two values, 0ms and 13ms. Timers longer than that are generally very accurate. Your 500 timer events - some were 0ms, some were 13ms (assuming 13ms is still correct). You ended up with a time shortfall.

Answer 4

As stated - windows is not a realtime OS. Asking it to do anything and expecting it at a specific time later is a fools errand. Setting a timer asks windows nicely to fire the WM_TIMER event as soon after the time has passed as is possible. This may be after other threads are dealt with and done. Therefore the actual time to see the WM_TIMER event can't be realistically predicted - All you know is it's >the time you set.... Checkout this article on windows time