[ntp:questions] ntpd sensitivity to ordering of servers in ntp.conf?
michael.cook at sfr.fr
Fri Feb 26 08:07:51 UTC 2016
> Le 25 févr. 2016 à 14:13, Miroslav Lichvar <mlichvar at redhat.com> a écrit :
> On Thu, Feb 25, 2016 at 02:49:48AM -0800, Weber wrote:
>> ntp.conf is specifies both servers with minpoll 4/maxpoll 4. Peer and loop
>> statistics are enabled.
>> By just changing the order of servers in ntp.conf the delay and offset
>> values in peerstats are swapped. Now it is A with 60us delay and B has 85us.
>> Similarly, A's offset is not -5us and B is showing +5us.
>> It appears there is something in ntpd where measurements on server A in
>> ntp.conf come out slightly different depending its ordering in ntp.conf.
> When A is specified as first in the config, the interval between
> polling of A and B will be 1 second and the interval between B and A
> will be 15 seconds. When you swap the servers, the intervals will be
> swapped too. I think there could be a lot of things than would happen
> in 15 seconds, but not in 1 second. Maybe some power saving feature is
> activated or maybe some cache entry expires.
> You could try adding B manually via ntpq -c config:, timing the
> command so that the polling is exactly between two polls of B, and
> see what happens with the delays. Or you could run ping against the
> servers to keep the link "up".
> The direction in which the offset changed suggests it's the processing
> of the server packet that has the extra delay.
I have also run some tests with identically configured (hardware (ARM uc)/LAN segment/OS( linux Debian variant )/NTP version 4.2.8p4) servers/client and replicate Weber’s findings. However loading the network as suggested by Miroslav does not improve the situation. Neither does saturating the cpu with other work. Agreed that this is just usecs and would be washed out over the WAN, but it is an order of magnitude. Curious.
So what’s up?
> Miroslav Lichvar
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