Re: [chrony-users] Inaccurate jiffy calculation at boot (x86 & 2.6.37)

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Tue, Apr 05, 2011 at 05:14:16PM +0100, Ed W wrote:
> 1) Is there an expectation that the lpj should be fixed for a given
> piece of hardware? ie once it's been determined then it should really
> remain the same for the rest of it's life? (my limited testing suggests
> yes?)

I think it should be fixed for a given piece of hw and sw.

> 2) Is there a direct correlation between altering lpj and adjusting
> drift file parameters? (Assume everything else stays the same)
> 
> *Iff* 1) and 2) hold then it would seem that the drift params are not
> actually useful without correcting for the clock mismeasurement which
> occurs at boot?  Further it would seem that we can correct for it if we
> also track the lpj used to measure a given set of drift params?

Yes.

> Out of curiousity, what do other people see as their range of initial
> lpj values and correspondingly how much does chrony vary in terms of
> tracking estimates between reboots on your hardware?  Does this whole
> problem go away on faster hardware and/or HPET/PM timers?

The range I usually see is few hundreds of ppm. I can't confirm what
Bill said about some kernel version fixing the problem, I think I've
always had it. Maybe it's not possible to get better resuls from
250ms measurement interval. The hpet and acpi_pm clocksources don't
seem to have this issue.

-- 
Miroslav Lichvar

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