[ntp:questions] Drift from serial activity, VIA system, LInux

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Jun 8 09:10:55 UTC 2008

Hal Murray wrote:

> The quirk is that the drift changes when I run the monitoring
> program.  It changes more if I poll faster.  It changed by
> 20 ppm when I was polling with a 10 ms delay and it's more
> than 80 ppm and still climbing when I reduced the delay to 1 ms.

My first guess would be a bug in the tickless clock in some recent 
Linuxes.  If possible, disable this and see if it improves things.  If 
it does take it up with the Linux kernel developers (I think this one 
came from Red Hat), but let us know as well, as this is something that 
was introduced without consulting the NTP community.

Note that tickless clock processing also led to some unofficial changes 
in ntpd, in Linux, which again are the responsibility of the Linux 

Note, to operate with 1ms delays in a non-tickless system, you need a 
1000Hz clock and that is vulnerable to lost interrupts from slow device 

I'm assuming that the UPS monitoring program runs in user space, 
otherwise it could be causing lost interrupts, itself.

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