| Re: [chrony-dev] Nanosecond timestamps |
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On Fri, Nov 04, 2016 at 11:52:47AM -0700, Denny Page wrote:
> The offset is being taken at 15 second intervals via a script that calls "chronyc -c sourcestats” and feeds the data to rrd. My polling was not quite as aggressive, at 2 for the device shown in the graphs I sent.
>
> I wouldn’t think I have much in the way of asymmetry, but anything’s possible. Both the monitoring system and the and the NTP servers under test are connected to the same switch. Here’s what my sources/sourcestats looked like:
In my testing on LAN the asymmetry in jitter doesn't seem to come from
network, but rather the kernel, drivers, or maybe HW.
Anyway, your results look much better than what I normally see, even
between machines connected directly without switch. Without HW
timestamping the std dev I see is at least few microseconds.
What HW and kernel do you use?
> 210 Number of sources = 7
> MS Name/IP address Stratum Poll Reach LastRx Last sample
> ===============================================================================
> ^+ 192.168.230.240 1 2 377 3 -218ns[ -240ns] +/- 23us
> ^x 192.168.230.241 1 2 377 0 -40us[ -40us] +/- 2930us
> ^x 192.168.230.242 1 2 377 0 -40us[ -40us] +/- 2930us
> ^* 192.168.230.244 1 2 377 0 -146ns[ -168ns] +/- 23us
> =x 192.168.230.1 2 3 377 1 -7027ns[-7049ns] +/- 1530us
> =x 192.168.230.2 2 2 377 4 +4821ns[+4799ns] +/- 86us
> =x 192.168.225.10 2 2 377 5 -24us[ -24us] +/- 206us
Hm, why are so many sources marked as falsetickers? Their measurements
look good.
> Back on the switch, if I bring the polling down to 1 then things tighten up a bit more. But still nowhere near the 151ns standard deviation that you are getting.
FWIW, from those 150 ns a large part seems to be instability of the
server's clock, which is synchronised to a GPS with PPS over serial
port. If I disable the PPS refclock, the std dev observed on the
client drops to about 50 ns. I have another server, which has clock
stable to maybe 20 ns (PPS over GPIO), but it doesn't have a NIC with
HW timestamping, so it's useless for these tests.
--
Miroslav Lichvar
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