Re: System Clock is updated before the TAI-UTC offset is applied for PHC source in chrony 6.4.1
Bill Unruh <[email protected]> Fri, 16 Jan 2026 13:16:22 -0800 (PST)
| Newsgroups | gmane.comp.time.chrony.user |
|---|---|
| Message-ID | <[email protected]> |
This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --173737085847408931517685981822492483 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE I guess I do not know how you demonstrated that it was in the 10s of ns range. That chrony tells you the offset is is in that range does not, to = me, demonstrate that it is accurate to that level. William G. Unruh __|__Hagler Fellow, Distinguished |_Tel:UBC +1(604)822-327= 3 Physics&Astronomy _|__ Research Prof, IQSE |__ US +1(979)7399950 UBC, Vancouver,BC _|_ TAMU4242, 578 University Dr |_ [email protected] Canada V6T 1Z1 ____|__College Stn, Tx, USA 77843 _|_www.theory.physics.ubc= =2Eca I cannot reply to emails from outlook or hotmail or other microsoft domains= =2E On Sat, 17 Jan 2026, Chris Hodgetts wrote: > [CAUTION: Non-UBC Email]=E2=80=9CAgreed in the general case. In my setup = the PHC is disciplined via > hardware-timestamped PTP from a GPS-locked grandmaster, so the system clo= ck is aligned to a local hardware > reference rather than software interrupt timing. I=E2=80=99m not claiming= absolute UTC accuracy at single-digit > nanoseconds, but the relative system-to-PHC and system-to-GM alignment is= demonstrably in the tens-of-ns > range.=E2=80=9D > > On 17 Jan 2026, at 09:35, Bill Unruh <[email protected]> wrote: >=20 > Well, the scatter is 9ns. The problem is that the interrupt program takes= a > while to load, and to adjust the clock. Ie, without an independent clockt= hat > the computer can quickly read, knowing the precesion is fraught. >=20 >=20 >=20 > William G. Unruh __|__Hagler Fellow, Distinguished |_Tel:UBC +1(604)822-3= 273 > Physics&Astronomy _|__ Research Prof, IQSE =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0|__ =C2=A0US +1(979)7399950 > UBC, Vancouver,BC _|_ TAMU4242, 578 University Dr =C2=A0|_ unruh@physics.= ubc.ca > Canada V6T 1Z1 ____|__College Stn, Tx, USA 77843 =C2=A0_|_www.theory.phys= ics.ubc.ca > I cannot reply to emails from outlook or hotmail or other microsoft domai= ns. >=20 > On Fri, 16 Jan 2026, Chris Hodgetts wrote: > > [CAUTION: Non-UBC Email] > > Why use the PHC =E2=80=9C > > chronyc tracking > Reference ID =C2=A0=C2=A0=C2=A0: 50484330 (PHC0) > Stratum =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0: 1 > Ref time (UTC) =C2=A0: Fri Jan 16 08:40:40 2026 > System time =C2=A0=C2=A0=C2=A0=C2=A0: 0.000000007 seconds fast of N= TP time > Last offset =C2=A0=C2=A0=C2=A0=C2=A0: -0.000000014 seconds > RMS offset =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0: 0.000000029 seconds > Frequency =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0: 14.710 ppm slow > Residual freq =C2=A0=C2=A0: +0.000 ppm > Skew =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0: 0.017 ppm > Root delay =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0: 0.000000001 seconds > Root dispersion : 0.000000544 seconds > Update interval : 1.0 seconds > Leap status =C2=A0=C2=A0=C2=A0=C2=A0: Normal >=20 > > Because why not=E2=80=A6. > Also I am playing with PTP so it just is what it is and I need the = accuracy. >=20 >=20 > I know the feeling. I got interested in ntp and chrony for the same reaso= n. My > own need for time precision and accuracy is at best in the seconds range,= not > ns. but it is nice to have bragging rights. >=20 > > But look at that system time=E2=80=A6. It=E2=80=99s in the nanoseco= nds of accuracy - just because - why not > have the best if you can get the best. >=20 > > On 16 Jan 2026, at 19:56, Bill Unruh <[email protected]> w= rote: > > Although this is certainly a pain, I wonder why you are using= the PHC source > at all. Use a couple of network sources plus the gps (yes, gp= s under chrony > can get down to sub-micro second accuracy so it would always = be the primary > source, except if it loast tracking, in which case the fallba= ck would be the > time from the network source, which would be in the microseco= nds, or 10s of > microseconds accuracy. > What accuracy do you need? I know that 1 second in the life o= f the universe > would give nice bragging rights, but do you really need that?= (and no gps > would not give that). > > Also, on startup the clock should be assumed to be bad for so= me time. Why are > you switching it off at all?If you want accurate time, let th= e clock and > chrony run for as long as possible. > > William G. Unruh __|__Hagler Fellow, Distinguished |_Tel:UBC = +1(604)822-3273 > Physics&Astronomy _|__ Research Prof, IQSE =C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0|__ =C2=A0US +1(979)7399950 > UBC, Vancouver,BC _|_ TAMU4242, 578 University Dr =C2=A0|_ un= [email protected] > Canada V6T 1Z1 ____|__College Stn, Tx, USA 77843 =C2=A0_|_www= =2Etheory.physics.ubc.ca > I cannot reply to emails from outlook or hotmail or other mic= rosoft domains. > > On Fri, 16 Jan 2026, Simon Plackett wrote: > > [CAUTION: Non-UBC Email] > > Hi, > > Before I send a lot of detail I wondered if anyone had = seen this > phenomenon. I have just upgraded my local chrony server= to Trixie on > RPI 5 with chrony 4.6.1. > > I had previously been using the ethernet NIC clock as a= time source > with ptp4l and phc2sys, which worked fine. > > During the upgrade I changed to leapseclist from leapse= ctz as the > latter does not work on Trixie. > > The issue is that on startup the system clock is set by= the PHC > source > before the TAI-UTC adjustment has been applied which me= ans it jumps > 36+ seconds one way and then back again during the star= tup. This > obviously trashes the PHC as a source, and it didn't ha= ppen > previously. > > Essentially PHC is chosen as the source, updates system= clock (wrong > by ~37s) then the TAI-UTC adjustment is found which cau= ses system > time > to jump again in the opposite direction by ~37s > > I can see this using systemctl status chrony or using j= ournalctl. > > I have currently turned PHC off again - but happy to tu= rn back on > and > send any logs etc that might be useful. > > The other source is PPS from GPS, which is even more ac= curate on the > new versions :-) > > Thanks > Simon > > -- > To unsubscribe email [email protected]= ily.org > with "unsubscribe" in the subject. > For help email chrony-users-request-kWFZVVI9zxvPqho9SqqRMim6D+HspMUB@public.gmane.org= g > with "help" in the subject. > Trouble? =C2=A0Email [email protected] >=20 >=20 > > -- > To unsubscribe email chrony-users-request-kWFZVVI9zxvPqho9SqqRMim6D+HspMUB@public.gmane.org= g with > "unsubscribe" in the subject. > For help email chrony-users-request-kWFZVVI9zxvPqho9SqqRMmD2FQJk+8+b@public.gmane.org with= "help" in the > subject. > Trouble? =C2=A0Email [email protected] >=20 >=20 >=20 >=20 > --173737085847408931517685981822492483-- -- To unsubscribe email chrony-users-request-kWFZVVI9zxvPqho9SqqRMmD2FQJk+8+b@public.gmane.org with "unsubscribe" in the subject. For help email chrony-users-request-kWFZVVI9zxvPqho9SqqRMmD2FQJk+8+b@public.gmane.org with "help" in the subject. Trouble? Email [email protected]