Message-ID: <4F6A5D42.3030108@t-online.de>
Date: Wed, 21 Mar 2012 23:59:14 +0100
From: Christian Franke <Christian.Franke@t-online.de>
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Subject: clock_getres(CLOCK_REALTIME, .) may return an outdated and too high resolution
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clock_getres(CLOCK_REALTIME, &ts) queries the actual resolution through
NtQueryTimerResolution (&coarsest, &finest, &actual) during its first 
call and returns this value unchanged afterwards.

This returns a global Windows setting which may be temporarily modified 
by other applications by using e.g. timeBegin/EndPeriod(). For example 
playing a flash video in browser sets the resolution to 1ms. It is reset 
to default ~15ms when the browser is closed.

As a consequence the actual resolution might be much lower than reported 
by clock_getres() when clock_gettime() is used for measurements later. 
It would IMO be better to return the 'coarsest' instead of the 'actual' 
value.

If clock_setres() is used, this setting should be returned instead of 
the 'actual' value at the time of the setting.

BTW: GetSystemTimeAsFileTime() apparently provides the same resolution 
(at least on Win7 x64). So the more complex use of 
SharedUserData.InterruptTime may have less benefit than expected.

Christian


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