On Wed, Aug 9, 2017 at 12:38 PM, Miroslav Lichvar <mlich...@redhat.com> wrote: > On Tue, Aug 08, 2017 at 06:40:29PM +0300, Horia Muntean wrote: >> How would one compute the estimated error of the system time from UTC >> at any point in time from information given by 'chronyc tracking' ? >> Is 'System Time' + 'Root Delay' / 2 + 'Root Dispersion' the correct formula >> ? > > Yes, that would be a good estimate of the maximum error. When the > clock is synchronized and the system time offset is just noise which > is already included in the dispersion, I guess the first term can be > ignored. > >> If yes then then all these 3 values would be helpful to be put in >> tracking.log (or maybe other log) regardless >> they are before or after an update as long as they are consistent >> (they refer to the state of time from the same moment). > > The value of the "System time" offset right after the update can be > calculated by adding (or substracting?) "Offset" and "Rem. corr.". It > decreases over time at a rate which can be calculated from "Offset sd" > and corrtimeratio in chrony.conf. > >> For consistency maybe they should be the values 'chronyc tracking' >> would display after the last update. > > The trouble is that the dispersion is not constant. It's increasing at > a rate of MAX(skew, maxclockerr) and the formula could change in a > future version of chrony. > > If the log had root dispersion right before clock update, one could > get a (slightly more pessimistic) estimate of the maximum error simply > by adding values from the log (root delay and dispersion from one > line, and offset and remaining correction from the previous line). > > -- > Miroslav Lichvar
The root dispersion just after a clock update contains what the server reports right ? In my opinion the values (root delay end dispersion) taken just after the clock update along with a short update interval (less than few seconds) would give a good max. error estimate. In a short amount of time between the updates the root dispersion should not increase too much. And for a good synchronization I don't think one can afford large update intervals. -- To unsubscribe email chrony-users-requ...@chrony.tuxfamily.org with "unsubscribe" in the subject. For help email chrony-users-requ...@chrony.tuxfamily.org with "help" in the subject. Trouble? Email listmas...@chrony.tuxfamily.org.
