Keeping time synchronized
Todd.Denniston at ssa.crane.navy.mil
Tue Apr 1 14:14:36 UTC 2008
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
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
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.
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