Keeping time synchronized

[Konstantin Svist]

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.

[Mauriat M]

On Mon, Mar 31, 2008 at 7:21 PM, Konstantin Svist <fry.kun(a)gmail.com&gt; wrote:
>  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.
>  
Just out of curiosity, can you point to some such tutorials? url?

-Mauriat

[Adalbert Prokop]

George N. White III wrote on Tuesday 01 April 2008:

>  For many people, a simple "sudo rdate -s ..." may be good
> enough. 
Or "sudo ntpdate ..." which uses the NTP protocol to get time.

-- 
bye,
Adalbert

Small animal kamikaze attack on power supplies

[George N. White III]

On Mon, 31 Mar 2008, Konstantin Svist wrote:

>  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. 
I think those tutorials are outdated, but there are other reasons ntpd may 
not be suitable for laptop use:

1.  the usual "reference" implementation prefers stable, long-lived 
connections.  Recent versions have the ability to deal with interfaces 
going up and down, but were never intended for typical laptop usage 
patterns (long periods off net, intermittent connections, assorted 
networks that may each provide different NTP servers, ...).

2.  the algorithms assume symmetric network delays which not be true
of some links (e.g., VPN) encountered by laptops

3.  ports used by NTP are often blocked, so if you rely on networks
you don't control, you may encounter problems

Chrony <http://chrony.sunsite.dk/> uses the same wire protocols, but has 
algorithms designed for systems with transient network access (dialup, 
laptops that move around).  Another option is a GPS receiver designed for 
time service (cheap, but they need something simpler than USB -- normally 
a real serial or parallel port).

For many people, a simple "sudo rdate -s ..." may be good enough.


-- 
George N. White III  <aa056(a)chebucto.ns.ca&gt;

[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

[Tim]

On Mon, 2008-03-31 at 16:21 -0700, Konstantin Svist wrote:
>  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.  
I'm using ntpd on a laptop, with a CPU that changes frequencies as
required, and I can't say that I've noticed any time keeping problems.

-- 
(This computer runs FC7, my others run FC4, FC5 & FC6, in case that's
 important to the thread.)

Don't send private replies to my address, the mailbox is ignored.
I read messages from the public lists.

[Cameron Simpson]

On 31Mar2008 16:21, Konstantin Svist <fry.kun(a)gmail.com&gt; wrote:
>  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. 
Ntpd is the normal way. Why not try it out?

Ntpd (and any ntp daemon in general) does watch "drift", to keep an idea
of the discrepancy between your system clock and a correct clock, and
uses it the schedule time checks and general correction.

I do not have much knowledge about how CPU speed changes affect ntpd but I
would have thought: not much! The OS is supposed to keep "correct" time
(subject to drift) across these changes and no normal UNIX program counts CPU
cycles - in a multitaksing OS there's no guarentee of how much CPU your
getting anyway and on a modern CPU "cycles" are a loose and dodgy idea
anyway. Ntpd _should_ be looking at the system clock periodically, not
counting CPU cycles.  

I suspect you will find ntpd works pretty well.
-- 
Cameron Simpson <cs(a)zip.com.au&gt; DoD#743
http://www.cskk.ezoshosting.com/cs/

There are two excuses for poor proof-reading. The first is incompetence, for
which the author should be killfiled. The second is indifference, for which
the author should be killed.    - Dan Hillman <dcah100(a)cus.cam.ac.uk&gt;