On 2011-04-19, David Woolley <david@ex.djwhome.demon.invalid> wrote: > David J Taylor wrote: > >> >> From chronyc I would need to be able to use a simple Perl script to >> extract the numbers to be plotted - such as the Offset in the graphs >> above. An easy job if the format is standardised and machine readable. >> > > As I keep saying, offset does not accurately measure time keeping > accuracy. A low offset can mean that you are tracking traffic > variations on the LAN, or interrupt latency variations, rather than > local clock frequency variations.
I would say offset does measure it. However, if what you want is the rate, then you can also plot that. However, you could get a highly non-varying rate by never changing it. The only way you know whether or not your rate is right is by looking at the offsets. Note that it is ntpd that only looks at the offset. The rate is adjusted to decrease the offsets (+offset and you increase the rate, - offset and you decrease it). The way it gets "rid" of network fluctuations is by making the correction smaller than you would need to get rid of the offset by the next measurement. chrony fits the past, up to 64 data points to estimate the rate. That averages out the network fluctuations much more effectively than does ntpd. > > It is also a pessimistic estimate of accuracy when everything is locked > up and stable. How is it a pessimistic estimate-- it is the only estimate you have. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions