On 26/11/2012 23:59, David Woolley wrote:
David Taylor wrote:
server reached over the LAN to have an offset of -0.028 milliseconds,
whereas its own PPS source is reported to have an offset of -0.001
milliseconds. Is that the right sign and amount for the offsets you
might expect comparing the LAN input and the GPIO pin interrupt for
the PPS source?
Offsets should be randomly distributed between + and - signs. If they
are all of one sign, you are not locked up or there is a bug. They
should be of s similar magnitude to the jitter.
They may not be evenly distributed, because of limitations of the
network, but they must be spread across zero.
David, for clarification:
On the card which is synced purely to LAN servers, over a day the mean
offset of the FreeBSD stratum-1 server (the selected sync source) is
0.000 ms, and that of two Windows stratum-1 servers is 0.014 ms. The
mean jitter is 0.041 ms (FreeBSD) and 0.048 and 0.280 ms (Windows). The
offset of WAN-based pool servers was in the region of 2 - 8 ms.
On the card with the PPS source, the mean offset of the other servers is
-0.020 ms (FreeBSD) and -0.008 ms (both Windows). Mean jitter is 0.230
ms (FreeBSD) and 0.036 ms and 0.230 and 0.047 ms (Windows). The offset
of WAN-based pool servers was in the region of 2 - 14 ms.
The WAN is via a cable modem connection, rather asymmetrical at 30 Mb/s
down and 3 Mb/s up.
Looking at the graph, the offsets of the LAN servers are very stable,
and much less variable than the WAN servers. They also are clustered
around zero. I conclude that there's no bug and the NTP is locked.
What I may well be seeing is the differences in interrupt latency
between the different hardware and operating systems. It's not a
problem for me.
--
Cheers,
David
Web: http://www.satsignal.eu
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