Re: [chrony-users] Possible to get sub-millisecond accuracy?

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On Thu, 8 Jun 2017, Michael Schwager wrote:

Hello,Is it possible to get clock accuracy in the microseconds using chrony
and a local time server?

With its averaging over the past N polls, chrony can get to a precision that
is significantly less than 1usec. However that is not the same as accuracy.
The problems are twofold. It takes time to service an interrupt, and it takes
time for that interrupt process to read the system clock. Those service
intervals are around 1usec and can vary (eg if other interrups are being
serviced, and interrupt handling is switched off by them, that adds a few usec
) This will give an accuracy of around 1usec. (usec=1/1000000 of a second) Note that this is ONLY if you use PPS. If you do not use PPS from a gps say,
then you are down to the 10s of usec range. If you use GPS with NMEA, not PPS,
then you are down to the many msec range.

I have tried with ntpd and the best I have seen is 5 ms accuracy (according
to ntpstat). I have tweaked polling intervals but still the best is 5 ms.

I have no idea what you mean here by ms. I assume you mean milliseconds.

ntpd will give you tens of usec, not msec. even from a local stratum 1
source, not GPS. So it is not clear what you are doing, or what kind of system
you are runnint ntpd on.

I have a local GPS NTP/PTP time appliance, and on the same switch, a few
servers. One of the servers has a Solarflare PTP-enabled network card and we
are running sfptpd. On that host, our delay to our appliance varies
generally by less that 50 ns, with some jitter into the 150 ns range. In

How are you measuring that?

short: little jitter in our network, and only one (local) hop between time
server and client.. 

I have another server that I need to discipline using NTP. This is because
it's CentOS 6.3, which lacks the facilities to support
ptp.(see, "In July of 2011, the PTP
Hardware Clock (PHC) subsystem was merged into Linux version 3.0...").
Upgrading to CentOS 7 is not on option right now, due to work priorities for
our development crew.

shrony will run on it. Chrony will,
 give a lot lot better than ms accuracy.

In short: The business wants microsecond accuracy today and PTP will have to
wait on CentOS 7 which will be a couple of weeks down the road due to our
development priorities.

-Mike Schwager

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