Re: Recommendation for chrony with 1PPS setup

On Wed, 21 Aug 2024, Abhijith Sethuraj wrote:

> [CAUTION: Non-UBC Email]I spoke too soon, it failed over to "foobar" right now. What's the
> recommended way to make this happen faster?

Why should it? It has no idea whether pps disappeared because of death, or
because it is was temporarily indiposed. It has trained the system clock so
that it is tracking UTS with high accuracy. Ie your system clock is better
than almost any other clock you have. Why should it transfer to a possibly
worse clock just because of a temporary glitch in the better clock?

YOu might know that you put a bullet through the PPS. chrony does not and
will wait a while to see if it comes back to life before going to someone
else.
> 
> 
> Thanks,
> Abhijith
> 
> On Wed, Aug 21, 2024 at 3:13 AM Abhijith Sethuraj <[email protected]> wrote:
>       Thanks, that's very helpful.
> 
> Here's what I see:
> 
> S Name/IP Address        Auth COpts EOpts Last Score     Interval  Leap
> =======================================================================
> * 1PPS                      N -P--- -P---    1   1.0    -7ns   +31ns  N
> D foobar                    N -P--- -P---    3   1.0 -1550ns +2091ns  N
> 
> Going through the man pages, "foobar" is in "D" state when its root distance (in a recent
> measurement) is greater than combinelimit * root distance of current best source's root
> distance. So, I tried increasing combinelimit to a higher value and it started combining.
> The 1PPS signal is coming from a time server appliance that has a good clock and uses a GNSS
> ref clock as well; "foobar" is another good source in the same site. In such a case, is it
> best to combine both 1PPS and "foobar" sources; or should I just use 1PPS (if available) and
> fallback to other sources if 1PPS isn't available (that is, don't forcefully combine them)?
> 
> Also, one thing that I noticed is that if I stop the 1PPS signals, it's not falling back to
> "foobar"; 1PPS remains as the best source. Should I add something in the configs to make
> this work?
> 
> S Name/IP Address        Auth COpts EOpts Last Score     Interval  Leap
> =======================================================================
> * 1PPS                      N ----- -----  228   1.0  -200us  +200us  N
> D foobar                    N ----- -----    1   1.0 -2650ns +1040ns  N
> 
> 
> Thanks in advance,
> Abhijith
> 
> 
> On Tue, Aug 20, 2024 at 3:45 PM Miroslav Lichvar <[email protected]> wrote:
>       On Tue, Aug 20, 2024 at 03:23:03PM +0530, Abhijith Sethuraj wrote:
>       > Hello,
>       >
>       > We have a 1PPS input to the NIC and we're planning to run chrony to use
>       > 1PPS as an additional refclock, in addition to getting time of day via NTP.
>       > When we tried this out, we're not entirely sure if things work fine as the
>       > source that gives time of day shows up as "unusable" in `chronyc sources`.
>
>       The "?" character in the sources means a different state than those
>       previously listed. Run chronyc selectdata -v to get the actual
>       selection state.
>
>       --
>       Miroslav Lichvar
> 
>
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> 
>