| Re: [chrony-dev] DRIFT_REMOVAL_INTERVAL value |
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On Fri, Jul 24, 2015 at 06:17:33PM +1200, Bryan Christianson wrote:
> I have the MacOS X drift removal patch just about ready. Just a question on an appropriate value for DRIFT_REMOVAL_INTERVAL
>
> 1.0 - gives mean System Time offset of about 9 usec
> 0.5 - gives mean offset of about 5 usec
> 0.125 - gives mean offset of under 2 usec
Very interesting. It probably means the kernel internally applies the
adjtime offset in shorter interval than one second unlike on Linux and
other systems.
If you call adjtime() with a large value and observe the remaining
offset as reported by adjtime(NULL, &tv) in a short interval, what is
the shortest interval (and step in the offset) in which you see the
offset changing?
> There is a corresponding cpu penalty for the shorter intervals, but even at 0.125 secs, chronyd is using < 0.1 % cpu on my Mac Mini. (i7, 4 cores)
>
> Thats on my lan with a raspberry pi stratum 1 server - results would probably vary for more remote servers.
>
> However - I was wondering if the interval should be configurable, but for now I'm happy to set it to whatever you think is most appropriate.
For now I think 1 second is good. It could be configurable. I'm
wondering how difficult it would be to make it adaptive, keep the
interval long enough so the offset stays below the offset sd value.
--
Miroslav Lichvar
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