[ntp:questions] Re: Ntp-4.1.2: tinker step 0;always slew and kernel time discipline

David L. Mills mills at udel.edu
Fri Feb 18 15:29:20 UTC 2005


Nikolaev,

As I said in my response to your message in my inbox, all claims on 
stability are cancelled in the configuration you selected. I am not 
surprised that you see large overshoots, instability and incorrect 
behavior. There are all kinds of reasons for this, including violations 
of the expected Unix behavior, unexpected transients between the kernel 
and daemon feedback loops and who knows what else. The engineered 
parameters are not designed for your application. The best choice, as I 
said in my reply to your message, is to forget ntpd altogether and use 
your wristwatch. You will have to analyze each and every case on your own.

Dave

Nikolaev, Sergey wrote:

> Dave,
> 
> Thanks for your replies. Actually, I have just got data from my tests
> and this is what I am observing.
> If "tinker step 0" but the "kernel" is not disabled explicitly, a
> 20-second offset 
> (set manually via the date command) is corrected at a rate of about 0.5
> millisecond/s as expected, 
> but overshoots the right time by about the same absolute value of
> offset. This happens repeatedly. 
> For example, from -20 sec it goes all the way to 20 sec offset and then
> back to -20 and so on many times. 
> I did not wait for it to settle down. 
> 
> When the "kernel" is disabled, it overshooted only by less than 0.5s and
> then slowly settled to the correct time.
> Actually, it settled to the range of +-120ms around the peer's time.
> 
> It looks to me that "disable kernel" offers a saner behavior in this
> case.
> Any comments?
> 
> Regards,
> Sergey
> 
> -----Original Message-----
> From: questions-bounces at lists.ntp.isc.org
> [mailto:questions-bounces at lists.ntp.isc.org] On Behalf Of David L. Mills
> Sent: Thursday, February 17, 2005 9:24 PM
> To: questions at lists.ntp.isc.org
> Subject: [ntp:questions] Re: Ntp-4.1.2: tinker step 0;always slew and
> kernel time discipline
> 
> 
> Nikolaev,
> 
> In truth, the kernel is disabled only if the actual correction is 
> greater than 0.5 s no matter what the tinker. However, there could be 
> grief if the discipline switches from one to the other. There are a 
> rather large number of untested and probably dangerous things that might
> 
> happen with uncharted combinations of tinkers and I'm not thrilled when 
> spending lots of time fixing the broken combinations. You are on your 
> own, but a "diable kernel" command might be appropriate.
> 
> Dave
> 
> Nikolaev, Sergey wrote:
> 
>>We have a requirement to always slew the clock even if it means long 
>>periods of time to correct the offset.
>>
>>The NTP documentation mentions this tinker option:
>>
>>step step
>>The argument is the step threshold, by default 0.128 s. It can be set
> 
> to any positive number in seconds. If set to zero, step adjustments will
> never occur. 
> 
>>Note: The kernel time discipline is disabled if the step threshold is
> 
> set to zero or greater than the default.
> 
>>However when I set "tinker step 0" and restart ntpd, I see in "ntpdc 
>>-c sysinfo" that the kernel flag is still enabled. Obviously, it is 
>>not disabled automaticly just by setting "tinker step 0".
>>
>>Do I need to disable it explicitly?
>>
>>Thanks in advance,
>>Sergey
>>
>>
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