Re: clock_getres(CLOCK_REALTIME, .) may return an outdated and too high resolution

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Mar 21 23:59, Christian Franke wrote:
> 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.

clock_getres already returns the coarsest time.  Did you mean the
setting in hires_ms::resolution, by any chance?  It's using the
actual setting right now.

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

Well, I'm not overly concerned about clock_setres, given that it's
probably not used at all :)

> 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.

On pre-Vista, the accuracy of GetSystemTimeAsFileTime is 15.625 ms,
fixed.  On Vista and later it seems to be 15ms or better, but its
resultion is not constant anymore.

But I'm not shure either, if the timeGetTime_ns shuffle has any positive
effect.  Given that we allow a jitter of 40 ms, we''re potentially worse
off than by just calling GetSystemTimeAsFileTime and be done with it.
Also, all processes would be guaranteed to be on the same time, not only
the processes within the same session sharing gtod.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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