Re: Linux time code

[john stultz <johnstul@us.ibm.com>]

On Thu, 2006-08-17 at 15:11 -0700, Jesse Barnes wrote:
> On Thursday, August 17, 2006 2:58 pm, john stultz wrote:
> > On Thu, 2006-08-17 at 13:43 +0200, Roman Zippel wrote:
> > > On Wed, 16 Aug 2006, john stultz wrote:
> > > > > For example there is a POSIX-like sys_clock_gettime() intended
> > > > > to server the end-user directly, but there's no counterpart
> > > > > do_clock_gettime() to server any in-kernel needs.
> > > >
> > > > Hmmm.. ktime_get(), ktime_get_ts() and ktime_get_real(), provide
> > > > this info. Is there something missing here?
> > >
> > > What is missing is the abiltity to map a clock to a posix clock, so
> > > that you would have CLOCK_REALTIME/CLOCK_MONOTONIC as NTP controlled
> > > clocks and other CLOCK_* as the raw clock.
> >
> > Is there a use case for this (wanting non-NTP corrected time on a
> > system running NTPd) you have in mind?
> 
> Isn't this what CLOCK_MONOTONIC[_HR] is for?  It's not supposed to jump 
> around at all, so the basic usage model is to use this source for 
> timestamping purposes...

Well, CLOCK_MONOTONIC is not affected by calls to settimeofday() so it
will never go backward, however it does get frequency correction if
provided by NTP (thus a second will be a correct second and you won't
accumulate error).

Also the _HR clocks have always been out of tree, so there isn't the
binary compatibility worry.

thanks
-john


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