Keeping time synchronized

[Todd Denniston]

Konstantin Svist wrote, On 03/31/2008 07:21 PM:
> Hi all,
> 
> Is there a good ["Fedora"] way to keep time synchronized on a laptop?
> 
> I've checked around and ntpd seems to be a bad idea because it counts
> CPU cycles - but modern CPUs switch frequencies all the time to save
> power. Some tutorials say this causes NTP to not work properly.
> 

Been a while since I looked at the kernel code that ntpd hooks into, but...

1) in the kernel, the kernel keeps system time using timer interrupts NOT 
clock frequency.
2) in the kernel where the kernel does it's 11 minute write to the TOY clock 
it uses system time and IIRC jiffy transitions which take into account any 
frequency transitions.
3) ntpd runs as a user space daemon, and does it's comparisons against system 
time, not CPU cycles (which it does not have access to).
Also this is on Linux, I never looked at it on other OS's.

Now, I have seen Linux system time get a little squirrelly when the PCMCIA and 
PATA IDE buses conspired to turn off interrupts a large part of the time, thus 
messing up system time. In those cases however ntpd still kept the time within 
1 second of right, it just took steps, instead of slews, quite often.


-- 
Todd Denniston
Crane Division, Naval Surface Warfare Center (NSWC Crane)
Harnessing the Power of Technology for the Warfighter