[ntp:questions] Re: simple time server

Serge Bets serge.bets at NOSPAM.laposte.invalid
Fri Aug 4 15:25:35 UTC 2006


Hello David,

 On Friday, August 4, 2006 at 7:50:55 +0100, David Woolley wrote:

> bryanh at giraffe-data.com (Bryan Henderson) wrote:
>> Philosophically, the Linux kernel has no business messing with the
>> hardware clock.
> In engineering terms, it is the most sensible place to do it, because
> the kernel is in the position to do this at the most accurate time
> without excessive overheads.

The method used by Linux kernels to set the RTC, last time I checked,
was indeed cheap in terms of processor cycles. But in terms of accuracy,
it's also cheap, dead cheap. The kernel sets the RTC in a frame of
plus/minus 5 milliseconds (with HZ=100) around the intended time, and
without compensation for the exact restart delay of the RTC chip (can
add up to 3 ms of error).

A roughly calibrated hwclock sets the RTC with a 10 or 20 times better
accuracy _at least_. A hundred of microseconds is repeatably achievable.
With such figures, my choice is all done. Even if I must admit that the
processing overhead is *huge* (but I don't care much).


Serge.
-- 
Serge point Bets arobase laposte point net




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