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

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Mon Jun 9 02:29:27 UTC 2008


In article <484ba1f1$0$661$5a6aecb4 at news.aaisp.net.uk>,
 David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>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.

Interesting suggestion.  Thanks.

Something changes when I turn off the tickless stuff in the kernel.
I haven't sorted out what's going on yet.


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

I'm using a recent ntp-dev with no changes from any distribution channels.

>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 
>drivers.

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

Yes.  It's a simple/dumb user program.  It pokes a serial port
a lot and occasionally writes a line to a log file.


-- 
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