Mail Archives: cygwin/2012/03/26/04:52:52
On Mar 22 18:47, Christian Franke wrote:
> Corinna Vinschen wrote:
> >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.
>
> No. Yes.
>
> No, clock_getres(CLOCK_REALTIME, .) returns gtod.resolution() which
> calls hires_ms::resolution().
>
> Yes, I mean this function which returns the 'actual' value from
> NtQueryTimerResolution(:-).
>
>
> >>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 :)
>
> Yes, it is not POSIX and does not exist on Linux, FreeBSD, ...
>
> It IMO is useful as CLOCK_REALTIME does only provide a rather coarse
> default resolution on Cygwin.
I see your point, but what bugs me a bit is the fact that
clock_getres(CLOCK_REALTIME) and clock_setres(CLOCK_REALTIME) will
always return the same value coarsest, regardless what value has been set.
I added this comment to clock_setres at one point:
/* Convert to 100ns to match OS resolution. The OS uses ULONG values
to express resolution in 100ns units, so the coarsest timer resolution
is < 430 secs. Actually the coarsest timer resolution is only slightly
beyond 15ms, but this might change in future OS versions, so we play nice
here. */
So, what if the OS really returns bigger values in coarsest at one
point? I know, I know, that's unlikely to the point of non-existence.
> - Unlike on e.g. Linux, CLOCK_REALTIME does not provide a better
> resolution than gettimeofday().
We can only use what the OS provides. Starting with Windows 8 there
will be a new function call GetSystemTimePreciseAsFileTime:
http://msdn.microsoft.com/en-us/library/windows/desktop/hh706895%28v=vs.85%29.aspx
> >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.
>
> If this is also the case for older Windows versions, it would be
> probably better to revert to GetSystemTimeAsFileTime().
Yes, I'm going to do that. I think you're right on all accounts, I'm
just feeling a bit uncomfortable with clock_setres always returning the
coarsest resolution.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat
--
Problem reports: http://cygwin.com/problems.html
FAQ: http://cygwin.com/faq/
Documentation: http://cygwin.com/docs.html
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
- Raw text -