Re: hwclock can cause system lockup

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> On Thu, Oct 16, 2008 at 8:19 AM, Chris Mocock
> <[email protected]> wrote:
> I've got an hourly cron job on some machines that runs "/sbin/hwclock --utc
> --hctosys" to sync the hardware clock to the system clock. These machines
> were recently upgraded from an old custom RH7.3 installation to a custom
> spin of Fedora 9 and started to occassionally lock up - every day or three.
> I noticed that they would always lock up at 1 minute past the hour so
> tracked it down to this cron job.
What are you trying to do with this cron job? You are updating the
system clock from the hardware clock, and not the other way around,
as you say you are trying to do. The system does synchronize the
hardware clock to the system clock on shutdown. Also, /etc/adjtime
contains the information needed to  correct for hardware clock drift
that happens while the system is shut down. The offset information
is re-computed every time you set the hardware clock.

> I decided to test it on my standard Fedora 9 installation by running a
> simple script that runs the hwclock command every 3 seconds. Sure enough,
> the system ran for just over an hour before it locked up. By locked up I
> mean no activity, caps lock key doesn't work, can't ping, can't ssh in, but
> power is still on.
I never looked into how the hardware clocked is accessed. I wounder
if they are using the BIOS to access the clock, and the BIOS code
has a problem. From what I have read, it is also not a good idea to
change the system clock to rapidly, especially if you are adjusting
it backwards - but I do not know if that would cause lockups.


  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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