[ntp:questions] Bizzare half second disagreement between ntp hosts

Unruh unruh-spam at physics.ubc.ca
Fri Jan 11 05:12:38 UTC 2008

"Luis Colorado" <luis.colorado at hispalinux.es> writes:

>Just suppose you are downloading some great file what creates asymetries =
>in the data flow bandwidth.  When that happens, you observe discrepances =
>with external time sources in the form of sistematic offsets to  the =
>clocks outside.  When you lose a local time source, your server gets the =
>time reference from the couple of best stratum clocks disponible and can =
>skip even more than a second.

>If you need reliability and precission, you can mount two or more =
>servers with gps timesources and connect them over ethernet media.  =
>You'll have better than 1ms on the whole local net if you use PPS =

That is the machine that has the pps ( from a GPS 18LVM) signal. that gps
dropped for some reason, and then there seemed to be a discrepancy of about
half a sec between tick.usask.ca, and the other three level 2 or 3 ntp
sources. I realise that ntp would assume that the majority rules, but the
majority was wrong ( as seen on all of the other systems who got their time
from that system by chrony. They all suddenly saw a half sec jump in their
time-- ie they suddenly found themselves with a .48 sec offset).

So I did everything I thought I could to get reliability and precision, and
instead got a half second error.

Note that tick.usask.ca typically has a .2 ms offset, with a 40ms

The others were pool sources.

>"Unruh" <unruh-spam at physics.ubc.ca> escribi=F3 en el mensaje =
>news:FMfej.53259$5l3.36002 at edtnps82...
>>I have a very weird situation. I am running a GPS PPS (Garmin GPS18LVM)
>> with a few machines as a backup/initialization.=20
>> Sudeenly for about half and hour, my GPS failed for some reason ( =
>still do
>> not know what was wrong since it had come back on air by the time I =
>> something wrong). Every hour I run a ntpq -p just to check that my gps =
>> on air. I got this report.
>>     remote           refid      st t when poll reach   delay   offset =
>> =
>=3D =3D=3D
>> xtick.usask.ca   .GPS.            1 u 1003 1024  377   44.954    0.213 =
>> +sanrail.com      2 u  993 1024  377    1.486  -479.03 =
>> +raptor.tera-byt    2 u  322 1024  377   17.295  -480.35 =
>> *zeus.yocum.org    2 u  390 1024  377   70.415  -481.02 =
>> SHM(0)          .PPS.            0 l 1415   16    0    0.000   -0.002 =
>> Now I believe the tick.usask.ca result, since all of the machines =
>which use
>> mine as a source suddenly noticed a .48 second jump when my GPS =
>failed. But
>> why in the world would three systems all suddenly be out by .48 sec?=20
>> Doing a peers on them, one has a GPS as its source, one a .WWVB. and =
>one an
>> .ACTS. Why should all three suddenly be out by half a second?

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