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

David Hawkins david.j.hawkins at btinternet.com
Sat Oct 3 00:58:03 UTC 2009


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.
* 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.
* The processor card uses a PCI clock generator capable of spread spectrum 
output, this is always enabled and not controllable from the BIOS - the chip 
has two settings off and on with a -0.5% spread. Have verified with a 
spectrum analyser that the cards with good lock and bad lock, have the 
spread spectrum option enabled.
* The cards seem to be more lightly to exhibit the problem when they have 
been turned off for a day or so.
* Power saving modes of the processor are enabled, but understand that the 
timing is done using the counter timer in the Geode companion chip that runs 
at a constant 14.13MHz regardless of the power state:- also as all running 
exactly the same code why would some have problems and not others ?

 Sorry rather random thoughts but I have now run out of things to look at, 
have you ever seen a problem like this and even better found a solution ?

Dave 





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