[ntp:questions] Linux 2.6 and clock frequency

David Woolley david at djwhome.demon.co.uk
Fri Feb 9 23:23:27 UTC 2007


In article <%Z1zh.913733$1T2.858211 at pd7urf2no>,
shane-dated-1173632795.f2b9df at cm.nu wrote:

>             as I understand for the gps18lvc to work effectively, one needs
> a fairly accurate clock.  One thing I've found is with hz=100, I'm getting

Which Linux a achieves by interpolating the clock interrupts by reading
the CPU TSC register.  You do not need to increase the tick frequency
to improve accuracy, unless your PPS interface operates on a timer tick
poll, in which case you are never going to get the best accuracy.

> an ntp.drift of around 40 but with a fairly good rootdispersion at stratum 2
> of approx 15.  With hz=250, the frequency drift drops to 15 but dispersion

Let me guess. The clock gains when uncorrected.  The change in frequency
is due to the increased number of lost clock interrupts.  You do not
want any lost interrupts at all.

> So my question is should the drift be varying so much based on kernel hz and

It should be independent of frequency, except in that there might be a very
small increase in case temperature.

> why would there be increased jitter with a reduced frequency error?  Has

Firstly don't think of it as frequency error; it is a frequency
correction that is nulling out the original error in the motherboard
crystal frequency.  As long as this value is stable and less than about
+/- 450 ppm (to allow some head room for short term adjustments), it
doesn't matter what it is.  In fact, if the sign of the correction was
the opposite, and the lost interrupt hypothesis is valid, the magnitude
would have increased.

The reason the jitter increases is that every time you lose a clock
interrupt, you get a 4ms (at 250Hz) step in the apparent time.

> anyone else tried the hrtimers patch and had good results?

I don't know anything except what you have written, but from that, I would
think they would thoroughly confuse ntpd.  The near zero correction
looks very suspicious.  Getting a motherboard with a clock that is
accurate to 40 ppb would be very remarkable (probably less than 1% 
probability).




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