[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
> Oct 19 13:37:23 client ntpd: time reset +13.151972 s
> Oct 19 13:55:58 client ntpd: time reset +8.779090 s
> Oct 19 14:08:09 client ntpd: time reset +8.712040 s
> Oct 19 14:28:21 client ntpd: time reset +11.494533 s
> Oct 19 14:44:53 client ntpd: 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.
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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