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