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

[Ed W <lists@xxxxxxxxxxxxxx>]

Hi

> That was a Linux problem in the intermediate Linux 2.6 kernels. I think,
> but
> am not sure, that this has been fixed relatively recently.

Do you think you could help dig up some reference to this?  I'm
struggling to find anything?

Note I have no problem with applying my own patches - the relevant file
looks like it has a fair number of tuneables, just not sure what I'm
fiddling with?


> No. IF chrony has a reasonable source of time, it will converge rapidly
> to the
> new drift rate. If it does not, it will drift due to the large change in
> the
> drift rate of the clock. Chrony cannot fix a broken clock if it has no
> outside
> source of time.

The only source I have (all the time) is my RTC?  The RTC is actually
showing variation in the 1-2ppm range range, so quite a chunk better
than my tsc estimate...


> OK, that is pretty recent. The problem is that the linux kernel people
> broke
> the initial rate calibration of the clocks.

The code seems to be in init/calibrate.c, however, a quick git log shows
very little activity in there and certainly nothing obvious for the last
few years?  Can you give me a hint on where to look?


> Chrony DOES use the rtc to initially set the clock. It also uses the drift
> rate of the rtc to try to correct that. It is not clear how useful that is,
> since the drift calculationof the rtc is done when the rtc is hot, and the
> actually drifting is done when the rtc is cold (computer switched off)

I understand that chrony uses the initial rtc time, but it doesn't seem
to use the RTC for anything else?  NTP allows you to use the RTC as an
input source, which would be useful in my case

I think the issue here is getting the processor TSC even vaguely
consistent from boot to boot.  I can set it through boot params, but
this isn't desirable..

I have just fiddled with a few of the tunables in calibrate.c, but I
don't seem to have made any improvement...

> Do you have any otehr possible time sources then TSC?

$ cat /sys/devices/system/clocksource/clocksource0/available_cloc
ksource
tsc


Ed W

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