[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

David Woolley david at ex.djwhome.demon.co.uk.invalid
Wed Jan 23 23:24:46 UTC 2008

Unruh wrote:

> Chrony is also  a server. The key detraction for me is that it cannot use hardware clocks. 

That would be a specification violation (should level, I think), as 
chrony is only an SNTP implementation.  In some "off list" 
correspondence with Dave Mills be points out one reason for this is that 
the end to end response of an NTP network depends on all steps 
implementing the protocol properly.  My impression is that, even though 
the clock discipline algorithm isn't normative in NTPv3, that includes 
using the specified discipline algorithm.  I suspect this may be 
enforced in the latest version of the specification.

I strongly suspect that the pool server people wouldn't want chroony as 
a pool server, although it turns out that someone was running optenntpd 
as one.

> are better than NTP's are. The key question is how close to the real time
> is the time that the system clock delivers. Chrony is closer by factors of
> at least 2 and probably if run at high priority as my ntp is, much better

You cannot say that except when the system is clearly out of lock, as 
you are not measuring the necessary parameter, because to do so would 
require special instrumentation.

> I have seen this both with a chrony controlled clock and an NTP controlled
> clock. It is just that the NTP response is not good. 

As noted in another article, I suspect what might be needed is a mixed 
approach, using the chrony approach to gain or regain lock (whilst 
signalling alarm and stratum 16 downstream) with the ntpd approach used 
when the loop is locked.  However Dave Mills can be particularly stubborn.

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