kernel clock drifts

ravspratapsingh · 08-03-2010, 08:12 AM

Hi,

I am running 2.6.27 kernel on my arm AT91SAM9G20. I am not using ntpd. I sync my kernel time with the reference time and then leave the system for 24 hours. I see that the kernel clock drifts by 8 seconds. Can anyone help me in figuring out the reason behind this. And how to fix this in kernel?

druuna · 08-03-2010, 09:03 AM

Hi,

Too my knowledge this cannot be fixed in the kernel.

The time comes from a crystal on the motherboard. These crystals are good enough to show a roughly correct time, but hardly ever good enough to keep perfect time over longer periods. I've seen big differences in the drift that can occur (from a second over a few days to more the 15 seconds in one day).

Syncing your time with a reliable outside source is the only way to go. If you are already syncing once every 24 hours, you might consider changing the config to do it more often. I personally would use ntpd.

Hope this helps.

ravspratapsingh · 08-04-2010, 02:57 AM

Quote:

Originally Posted by druuna

Hi,

Too my knowledge this cannot be fixed in the kernel.

The time comes from a crystal on the motherboard. These crystals are good enough to show a roughly correct time, but hardly ever good enough to keep perfect time over longer periods. I've seen big differences in the drift that can occur (from a second over a few days to more the 15 seconds in one day).

Syncing your time with a reliable outside source is the only way to go. If you are already syncing once every 24 hours, you might consider changing the config to do it more often. I personally would use ntpd.

Hope this helps.

Thanks for the reply.

We have ntpd running on the board. But some of these boards are not connected on network and there we face this drfit issue. Can't we modify in kernel to workaround this issue?

druuna · 08-04-2010, 03:15 AM

Hi,

Quote:

Can't we modify in kernel to workaround this issue?

Like I said before: No you cannot. See comment below.

michaelk · 08-04-2010, 07:46 AM

Yes, you can adjust system time without ntp and I would assume is available for the ARM.
http://linux.die.net/man/8/adjtimex

You can also compensate for hardware clock drift too. http://linux.die.net/man/8/hwclock

ravspratapsingh · 08-18-2010, 11:50 PM

Does anyone know how to apply correction factor for drift in kernel?