[ntp:questions] drift value very large and very unstable

Rob Neal hundoj at comcast.net
Sat Mar 8 19:30:28 UTC 2008



On Fri, 7 Mar 2008, Andy Helten wrote:

>
> Richard Gilbert wrote:
>>
>>
>> A comment in ntp.conf and/or the startup file, explaining WHY stepping
>> is enabled should go a long way toward solving the "dumbass" problem.
>>
>>
>
> Yes, I know, but that requires someone to actually read the comments in
> the conf files and there is always the '-x' argument that could be added
> to the command line.  Adding to the fun, we use 'include' to build up
> ntp.conf so we actually have three conf files that are combined.
>
> At any rate, reading through other "step disable" problems in the
> mailing list, I'm getting the impression that this option isn't really
> recommended nor supported.  This, of course, begs the question of why
> provide an option that is known to cause problems?  How about a comment
> in the docs that states it plainly:  Don't use this option.  Yes, it
> does say "The kernel time discipline is disabled with this option", but
> that didn't mean anything to me when I started this work and still means
> very little.  Certainly nothing I've read implies this option is
> guaranteed to cause problems.  In fact, I ran for weeks with a different
> processor board and different linux kernel with stepping disabled and
> achieved excellent performance.
>
> Andy
>
> _______________________________________________

http://www.eecis.udel.edu/~mills/ntp/html/miscopt.html

Note the preface to the tinker description. Fair warning.

There are links on that website that explain the gritty details of the 
ntpd algorithms. The book is well worth the read, and the money. Save
yourself some time and buy a copy.

On 4.2.5p110 on an embedded system I ran a test with step=panic=0,
1.4 second offset. The frequency went to 500, amortized the offset
and overshot zero by a snit, then dropped the frequency while maintaining
reasonable offsets. This continued until the frequency was at normal
for that box. Took several hours. Nothing broken there.

'reasonable' in this case was sub 120ms, I'm ok with that. YMMV.

There is a 'ntp-wait' in the 'script' directory in the distro that may be 
of use to you, given the constraints you wish to assume.

Rob








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