Question

Last week I needed to test some different algorithmic functions and to make it easy to myself I added some artifical sleeps and simply measured the clock time. Something like this:

start = clock();
for (int i=0;i<10000;++i)
   {
   ...
   Sleep(1);
   ...
   }
end = clock();

Since the argument of Sleep is expressed in milliseconds I expected a total wall clock time of about 10 seconds (a big higher because of the algorithms but that’s not important now), and that was indeed my result.

This morning I had to reboot my PC because of new Microsoft Windows hot fixes and to my surprise Sleep(1) didn’t take 1 millisecond anymore, but about 0.0156 seconds.

So my test results were completely screwed up, since the total time grew from 10 seconds to about 156 seconds.

We tested this on several PC’s and apparently on some PC’s the result of one Sleep was indeed 1 ms. On other PC’s it was 0.0156 seconds.

Then, suddenly, after a while, the time of Sleep dropped to 0.01 second, and then an hour later back to 0.001 second (1 ms).

Is this normal behavior in Windows?
Is Windows ‘sleepy’ the first hours after reboot and then gradually gets a higher sleep-granularity after a while?
Or are there any other aspects that might explain the change in behavior?

In all my tests no other application was running at the same time (or: at least not taking any CPU).

Any ideas?

OS is Windows 7.

Answer 1

I’ve not heard about the resolution jumping around like that on its own, but in general the resolution of Sleep follows the clock tick of the task scheduler. So by default it’s usually 10 or 15 ms, depending on the edition of Windows. You can set it manually to 1 ms by issuing a timeBeginPeriod.