[ntp:questions] Flash 400 on all peers; can't get ntpd to be happy

Ralph ralph at depth.net
Mon Mar 14 16:14:59 UTC 2011


On Sunday, March 13, 2011 11:41:59 PM UTC-7, unruh wrote:
> 
> Sure. Teh Hw clock ( rtc) is on its own timer which does not depend on
> any of the system timers. However it typically has a rate that is many
> PPM out and that rate cannot be adjusted. This makes it completely
> unsuitable for the clock adjustment that ntp uses. Also setting that
> clock is tough and it is not very accurate ( only delivers time to the
> second-- and on modern system even that can be somewhat inaccurate since
> the rtc interrupt has been screwed up in modern versions of Linux. Also
> setting the clock only occurs .5 sec after the adjustment is actually
> made.). The rtc makes for a lousy clock. 
> 
> 
> Nope. The HW clock is a clock which is completely separate from the
> operating system. 
> 

You are thinking of HW clocks that run on hardware.  This is about a HW clock 
within a VM guest.  And a hardware clock within a VM is not hardware anymore, it 
is software on the HOST that is emulating the hardware.  And the software that 
is doing that emulation is doing it based on the [system] clocking on the host O/S. 
So if one has the HOST O/S clocking getting adjusted to fairly good accuracy, 
then the HW clock within the guest will be close to as accurate as the 
[system] clocking on the HOST.  If you believe that the HW clock on the guest 
is run otherwise, then find some evidence to that effect because everything 
I've found so far indicates that the guest HW clock is emulated just like all 
the other pieces of 'hardware' in the guest.

Now Linux's inability to read / set the HW clock accurately is something I 
can't speak to, but as Uwe points out, I'm not looking for super accuracy.

And I don't think you understood what I was describing... I wasn't advocating 
adjusting the HW clock the way ntpd adjusts the system clock.  I was advocating 
allowing the use of the HW clock to provide ntpd with a 'stable' clock for it 
to use in calculations.  Maybe the precision isn't as good, but it would still 
be a good enough level of precision for my purposes and would allow ntpd to 
determine the level of adjustment that needs to be made to the system clock 
in order to get it to run closer to reality.




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