[ntp:questions] Loopstats jitter field mostly zero?

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Nov 27 18:42:48 UTC 2012


On 27/11/2012 16:24, gabs wrote:
[]
> The displayed jitter in loopstats or the peer list can be different from the
> actual jitter of the source. This case is the reverse: low actual jitter,
> 15 us displayed jitter.
>
> root at OpenWrt:/tmp# tail /tmp/ntp/loops
> 56257 76888.554 -0.000000080 6.553 0.000015259 0.000053 3
> 56257 76896.554 -0.000000098 6.553 0.000015259 0.000052 3
> 56257 76904.554 -0.000000071 6.553 0.000015259 0.000051 3
> 56257 76912.554 -0.000000061 6.553 0.000015259 0.000049 3
> 56257 76920.554 -0.000000091 6.553 0.000015259 0.000049 3
> 56257 76928.554 -0.000000008 6.553 0.000015259 0.000046 3
> 56257 76936.554 -0.000000048 6.553 0.000015259 0.000043 3
> 56257 76944.554 -0.000000014 6.553 0.000015259 0.000041 3
> 56257 76952.554 0.000000039 6.553 0.000015259 0.000038 3
> 56257 76960.554 0.000000016 6.553 0.000015259 0.000036 3
>
> This is normal for this platform since ntpd seems to use the precision as the
> lower bound for reported jitter. The precision is low as it takes 12 us to read
> the clock and convert to NTP timestamp format.
>
> For the RPi, I have no idea but I'll be getting one before the year ends :)

Oh, that's interesting, gabs.  I know that Dave Hart has done some work 
in that area, so in case it helps:

System with PPS signal showing zero jitter in the loopstats:
C:\>ntpq -c rv raspi-1
associd=0 status=011d leap_none, sync_pps, 1 event, kern,
version="ntpd 4.2.7p324 at 1.2483 Mon Nov 19 16:03:37 UTC 2012 (1)",
processor="armv6l", system="Linux/3.2.27-pps-g965b922-dirty", leap=00,
stratum=1, precision=-19, rootdelay=0.000, rootdisp=1.180, refid=PPS,
reftime=d45f851e.54f1a9c6  Tue, Nov 27 2012 18:29:50.331,
clock=d45f852a.b6780c11  Tue, Nov 27 2012 18:30:02.712, peer=5855, tc=4,
mintc=3, offset=-0.000842, frequency=-42.566, sys_jitter=0.001907,
clk_jitter=0.000, clk_wander=0.000

System working purely off LAN sync, non-zero jitter:
C:\>ntpq -c rv raspi-2
associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync,
version="ntpd 4.2.7p321 at 1.2483 Fri Nov 16 15:06:21 UTC 2012 (1)",
processor="armv6l", system="Linux/3.2.27-pps-g965b922-dirty", leap=00,
stratum=2, precision=-19, rootdelay=0.505, rootdisp=3.978,
refid=192.168.0.3,
reftime=d45f84b4.981fcb01  Tue, Nov 27 2012 18:28:04.594,
clock=d45f8537.eb89aef6  Tue, Nov 27 2012 18:30:15.920, peer=7910, tc=5,
mintc=3, offset=-0.013495, frequency=-22.812, sys_jitter=0.036904,
clk_jitter=0.024, clk_wander=0.013

-- 
Cheers,
David
Web: http://www.satsignal.eu



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