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clock_getres(CLOCK_REALTIME, .) may return an outdated and too high resolution
- From: Christian Franke <Christian dot Franke at t-online dot de>
- To: cygwin at cygwin dot com
- Date: Wed, 21 Mar 2012 23:59:14 +0100
- Subject: clock_getres(CLOCK_REALTIME, .) may return an outdated and too high resolution
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'
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.
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