[ntp:questions] Single best NTP status indicator of clock accuracy?

David Woolley david at ex.djwhome.demon.invalid
Thu Dec 10 22:33:06 UTC 2009


kjans wrote:
> 
>   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 
repeatable.

> 
>   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 128.118.25.5     2 u   41   64  377   71.652
> 2.432   0.528
> +ntp-1.gw.uiuc.e 128.174.38.133   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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