[ntp:questions] Single best NTP status indicator of clock accuracy?
david at ex.djwhome.demon.invalid
Thu Dec 10 22:33:06 UTC 2009
> I have several remote/embedded Linux devices that I am monitoring
> via SNMP and want to monitor a single variable to know how accurate
> the device thinks its time is.
The device doesn't think much about how accurate it is, only about how
> There seems to be several possibilities:
> - delay or offset or jitter (from ntpq -clpeers)
Offset is useless in isolation, as you may measure an outlyer. Delay is
only one hop, and represents a worst case factor. Jitter maybe, but
only if it is the dominant error term for your particular system.
> - sync distance (from ntptrace)
Gives you a near worst case figure, most of the time.
> - maximum error (from ntpdc)
Another attempt at worst case.
> - precision (from ntpq -crl)
Characteristic of the hardware/OS, not of the actual current time quality.
> - ?
> Below is the output of several NTP status monitoring commands...
> # ntpq -clpeers
> remote refid st t when poll reach delay
> offset jitter
> LOCAL(0) .LOCL. 10 l 43 64 377 0.000
> 0.000 0.015
(Should never be used on leaf nodes)
> *GPS_NMEA(0) .MAL. 0 l 4 16 377 0.000
> 0.156 0.015
> +ntp-2.gw.uiuc.e 22.214.171.124 2 u 41 64 377 71.652
> 2.432 0.528
> +ntp-1.gw.uiuc.e 126.96.36.199 2 u 21 64 377 72.009
> 2.373 1.389
> pll frequency: -27.934 ppm
> maximum error: 0.007856 s
> estimated error: 1.5e-05 s
This is probably the most useful single figure.
However, all except the ones I describe as maximum tell you about
repeatability, not error. E.g. if you are only using NMEA, and not PPS,
the error could be up to about a second, but, if only one sort of
sentence is coming from the GPS, you might have repeatability down in
the 15 microsecond range that these figures are suggesting. (The other
figures suggest that the overall error is no more than tens of ms.)
> status: 0001 pll
> pll time constant: 4
> precision: 1e-06 s
> frequency tolerance: 512 ppm
What confidence level do you need in the error bounds?
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