[ntp:questions] Strange NTP problem on AMD Geode LX cards.

Richard B. Gilbert rgilbert88 at comcast.net
Sat Oct 3 01:31:45 UTC 2009


David Hawkins wrote:
> Hi
> I'm using a number of XTX form factor AMD Geode LX (500Mhz) cards at work.
> (Cannot get to news at work, and have left memory stick with details at work 
> ! so apologies for missing info !)
> They are running Sues Linux from a read only flash drive, all identical 
> clones other than host names and IP addresses.
> 
> Most of the time ntp runs with no problems and will lock to a local server 
> with less than 5ms offset, and the drift file comes out at between about -20 
> and -40.
> 
> But now and again a system will not get a stable lock, and on investigation 
> the drift file is at the maximum of -500.
> When I first encountered this I assumed it was a hardware problem with the 
> processor card, just a one off, but now have seen this on around 10 systems 
> out of 30 or so I have tested.
> When a system shows this fault, powering the unit on and off will almost 
> always solve it, the unit synchronising to the server after a couple of 
> hours with a drift file setting of -20 to -40 like the others.
> I'm more of a hardware engineer than software, but have now run out things 
> to look at to solve this problem.
> 
> I have considered / done the following
> 
> * The drift file is stored in the ram drive /dev/shm so always starts at 
> 00.000 when the system is started.

Then I can't see that a drift file does anything useful in your 
situation.  Normally the drift file gives NTPD a "leg up" when NTPD is 
restarted.  It SHOULD be zero for a "cold start".  For a warm start; 
e.g. a reboot, it should give NTPD a close approximation to a value that 
NTPD might spend 30 minutes to ten hours finding without this help.

> * On a system not locking stopping ntp and restarting having set the drift 
> file to -28, results in the drift going back to -400 over a couple of 
> hours - so not some odd start-up state that confuses the control loop.

This suggests that your local clock is defective!  Most properly working 
hardware will generate an absolute value that is less than 100 and many 
will have an absolute value less than 50.

<snip>




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