[ntp:questions] NTP + kernel frequency

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sat Nov 10 10:35:11 UTC 2007

In article <47356F94.5080901 at skynet.be>,
Jan Ceuleers <janspam.ceuleers at skynet.be> wrote:

> In other words, this method doesn't seem to be entirely reliable, 
> possibly as a side effect of frequency scaling.

I'm not familiar with frequency scaling, but I would suggest that if
the kernel interrupt rates are not constant, ntpd also won't be entirely
reliable.  Dave Mills complains from time to time that the Linux kernel
doesn't adjust the kernel PLL parameters correctly when the clock rate is
varied from 100 Hz to another fixed rate, so I very much doubt that the
kernel PLL code has been correctly modified to cope with a dynamically
variable interrupt rate.

There is also an issue that the fixed speed code rounds things in a way that
only certain rates are safe.

The kernel interrupt rate is a better indication of what really matters
(except that it will read low if you are losing interrupts) than HZ,
if there is any systematic difference between them.

If a developer for that code doesn't raise their head here, now, I think
it pretty much safe to assume that the machine is not suitable for ntpd
use for anything except a pure client with low time quality expectations.
Basically, if the developers are not monitoring this news group, it is
almost certain that they not properly taken into account ntpd's needs.

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