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

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On Thu, Jun 08, 2017 at 05:09:49PM -0500, Michael Schwager wrote:
> Hello,
> Is it possible to get clock accuracy in the microseconds using chrony and a
> local time server?

Yes, that should be possible.

> 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.

What polling interval have you tried? ntpstat reports the NTP root
distance, which includes delay and dispersion accumulated from the
stratum 0 time source down to the client. The problem may be with the
server. If it is reporting a large root delay or dispersion, it
doesn't matter how stable and symmetric is the network delay or what
the client does, the distance reported by the client will be always

You can check the root delay and dispersion of the server with
"ntpq -c as -c 'rv &1'" or "chronyc ntpdata".

The distance gives you the maximum estimated error. If it's 5
milliseconds, it doesn't mean the actual error of the clock is 5
milliseconds. It's more likely just few tens or hundreds of
microseconds. With a decent server and a sufficiently short polling
interval it should be closer to the former.

BTW, PTP doesn't have anything like that and generally PTP slaves
cannot estimate their maximum error.

> 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.

Note that in 7.4 will be chrony-3.1, which supports HW timestamping.
If you don't have switches with PTP support and the NIC on the machine
can timestamp all packets (not just PTP packets), NTP might work
better for you.

Miroslav Lichvar

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