[ntp:questions] Linux 11-minute mode (RTC update)

Serge Bets serge.bets at NOSPAM.laposte.invalid
Fri Apr 4 10:45:20 UTC 2008


 On Thursday, April 3, 2008 at 16:25:17 +0200, Noob wrote:

> STA_UNSYNC (0x0040, clock unsynchronized) is 0. [...] Thus the kernel
> should write the system time to the RTC every 11 minutes; but it does
> not.

Fine! Don't touch anything, happy man, or it might well "tomber en
marche". Real men don't want the eleven-minutes mode. It is not only
extremely inaccurate by itself, but it also steps on the toes of those
tools that are able to manage the RTC properly.

I previously posted some figures, comparing the accuracy of writing the
RTC (not counting drift). Mean offset and dispersion:

 - eleven-minutes mode:           -2150 µs +-5000
 - hwclock util-linux-ng 2.13.1:  -2500 µs  +-170
 - hwclock 2.32 from BJH:             0 µs   +-10

Furthermore both hwclocks are able to evaluate and compensate the drift
of the RTC. The eleven-minutes mode cannot do that, and instead it can
perturbate hwclock's calculations.

Serge point Bets arobase laposte point net

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