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

Unruh unruh-spam at physics.ubc.ca
Sat Oct 3 01:53:20 UTC 2009


"David Hawkins" <david.j.hawkins at btinternet.com> writes:

>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.

Yes, it could be. ntpd is very very bad at homing in on a suitable
drift. It takes a long time ( many hours) and the drift tends to wander
off to very high values.  ntpd is terrible at settling down, and is good
for systems which are (almost) always on.  Now the drift in those
machines could be bad, but it could also be a bad response of ntp.
How long have you waited to see what it settles down to? If you look at
the offsets, what  drift do you estimate yourself from those offsets (
esp the first few offsets).


>* 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 

No idea what "spread spectrum " means for a clock. 

>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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