[ntp:questions] Can a clock drift be too big for ntpd?

Richard B. Gilbert rgilbert88 at comcast.net
Sat Oct 20 00:35:29 UTC 2007


Patrick Nolan wrote:
> On 2007-10-19, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> 
>>Some Linux systems have known problems with losing timer interrupts! 
>>During periods of heavy I/O load disk drivers may mask or disable 
>>interrupts for a little too long a time. . . .  Some Windows systems 
>>have also been known to exhibit similar behavior.
> 
> 
> I would like to know more about this.  How can I monitor the interrupts?
> 
> After my original post, I remembered that this machine has a unique
> feature.  I compiled a new kernel to add the Reiser file system.
> I don't think I changed anything else, but I don't have any previous
> experience with custom kernels.
> 
> 
>>What's the value stored in your drift file?
> 
> 
> Currently it's 74.080.  This morning it started out around 30.
> 
> 
>>DON'T use burst!  The burst keyword was intended for situations where 
>>ntpd has to make a phone call to NIST (or similar service) to get the 
>>time.  It is NOT suitable for general use over the internet.
> 
> 
> Without burst, it just drifts freely.  The size of the drift is even
> worse than I thought.  With burst, here are some lines from the log 
> file:
> 
> Oct 19 13:37:23 client ntpd[12595]: time reset +13.151972 s
> Oct 19 13:55:58 client ntpd[12595]: time reset +8.779090 s
> Oct 19 14:08:09 client ntpd[12595]: time reset +8.712040 s
> Oct 19 14:28:21 client ntpd[12595]: time reset +11.494533 s
> Oct 19 14:44:53 client ntpd[12595]: time reset +9.450835 s
> 
> If I ever get this situation under control I'll turn off burst.
> 
> 
>>Iburst is good.  It gets you a fast startup and then lets your system 
>>poll the server at normal intervals.
>>
>>Check the value of a Kernel variable called "HERTZ".  Some Linux systems 
>>set it to 1000 which is not good for NTP.  If yours is set to 1000 (or 
>>250) try changing it to 100.
> 
> 
> More ignorance on my part.  Where would I look for this?  I searched 
> the kernel source code and didn't find it.
> 
<snip>

Sorry, I've read about HERTZ but I'm not a Linux expert, have never 
experienced the problem, or actually done the fix.




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