On Wed, Nov 10, 2021 at 03:45:06PM +0200, Andriy Gapon wrote:
> On 10/11/2021 11:30, Andriy Gapon wrote:
> > On 09/11/2021 17:56, Andriy Gapon wrote:
> > > So, as I was saying, when the delta is large the calculations in
> > > tc_windup and bintime_off give slightly different results and that
> > > can lead to a discontinuity of the time when timehands are switched.
> >
> > A quick follow-up.
> > I think that both tc_windup and bintime_off have fundamentally correct
> > calculations but with different precision. Both seem to produce values
> > slightly greater than a "true" value where the bintime fractional delta
> > would be calculated as tc_delta * 2^64 / tc_frequency. That's because
> > of how th_scale is calculated.
> >
> > When the timecounter delta is greater than the frequency then the value
> > in tc_windup is closer to the true value because it accounts for whole
> > seconds precisely: a tc_frequency number of timecounter ticks is equal
> > to one second. bintime_off, however, converts both whole seconds and
> > fractions using th_scale. So, its result is consistently greater when
> > the delta is longer than a second.
> >
> > E.g., in my environment: tc_frequency = 14318180, th_scale = 1288420532460.
> > For a delta of 14318180 (== tc_frequency) tc_windup calculates a one
> > second advance, bt = { 1, 0 }.
> > bintime_off for the same delta will produce bt = { 1, 1093027638570944 }.
> > The difference is minuscule, just 59 ppm in relative terms.
> > But it's 59 microseconds of "jumping back in time".
> >
> > I think that the precision of bintime_off is sufficient and its
> > calculations are faster, so I think that it's better to use the same
> > calculations in tc_windup as well. Especially given that they are
> > identical for sub-second deltas and longer deltas should be extremely
> > rare.
> >
> > I am working on patch to implement this.
>
> The promised patch: https://people.freebsd.org/~avg/kern-tc-add-delta.diff
The patch looks fine to me.
