[ntp:questions] Falseticker determination

David L. Mills mills at udel.edu
Thu Apr 5 18:56:49 UTC 2012

A C,

Before you take a hacksaw to code, you should see the How NTP Works 
collection in the online documentation, in particular the clock select 
algorithm page. It includes  advice on how to avoid falsetickers in 
cases like yours, including the use of tinker and/or tos commands. There 
should be no need of additional trace lines in the code, as there 
already are some that demonstrate the results of the clock select and 
clluster algorithms.


A C wrote:

> On 4/4/2012 18:52, E-Mail Sent to this address will be added to the 
> BlackLists wrote:
>> Dave Hart wrote:
>>> A C<agcarver+ntp at acarver.net>  wrote:
>>>> Where in the code of 4.2.7p270 is the determination that
>>>>   a peer is a falseticker?  I'm looking through ntp_proto.c
>>>>   but I don't think I'm fully grasping how the determination
>>>>   is made and the peer marked.
>>>> I want to put some debug lines in the area of the code
>>>>   where the falseticker is determined so I can figure out
>>>>   what conditions are causing the PPS to be marked as a
>>>>   false ticker.
>>> Line 2519 of ntp_proto.c (in clock_select):
>>>         peer->new_status = CTL_PST_SEL_SANE;
>>> All survivors to that point in the code get the x, fleetingly.  Those
>>> that keep it fail to survive to line 2688:
>>>         peers[i].peer->new_status = CTL_PST_SEL_SELCAND;
>> I would have thought 2835 - 2855 might be where he would
>>   want to take a closer look.
> Well that was a fun exercise.  The end result is that the PPS is no 
> longer a false ticker and I don't need a prefer peer either. :)
> Believe it or not, it's actually working better this way.  For 
> starters, the offset is staying within +/- 10 us of the PPS pulse.  
> Additionally, allowing the system to select the rest of the clocks 
> using the normal clock selection instead of a prefer peer has actually 
> quieted the sys_fuzz messages.  Usually I was seeing a sys_fuzz 
> message perhaps once every couple minutes.  Now I see one maybe once 
> in a few hours or more.  Plus if any of the peers explodes for 
> whatever reason it doesn't wipe out my clock because the averaged time 
> doesn't move much.
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