Re: Back to the Future ? or some thing sinister ?

[john stultz]

On Thu, 2006-01-12 at 08:33 -0600, Ram Gupta wrote:
> On 1/11/06, john stultz <johnstul@xxxxxxxxxx> wrote:
> > On Sun, 2006-01-08 at 22:03 -0600, Nathan Lynch wrote:
> > > Chaitanya Hazarey wrote:
> > > >
> > > > We have got a machine, lets say X , make is IBM and the CPU is Intel
> > > > Pentium 4 2.60 GHz. Its running a 2.6.13.1 Kernel and previously,
> > > > 2.6.27-4 Kernel the distribution is Debian Sagre.
> 
> It may be BIOS related. But I feel it might be an overflow related
> issue. If the variable is signed int then there will be a transition
> from 0x7fffffff ns  to 0x80000000 ns which is basically from +2 sec to
> -2 sec which will result in 4 sec loss.

I'm pretty sure this is the BIOS issue. If your hesitant about updating
the BIOS, try booting w/ noapic, and see if that works around the issue.

The 4 second loss is the tv_nsec portion of the xtime timespec wrapping.
Since time is not accumulated (timer_interrupt isn't being called at the
normal HZ frequency), the TSC offset grows and grows (and finally will
wrap repeating the processes), causing the xtime.tv_nsec to wrap.

Thus you are correct that the symptom is overflow related, but the cause
is most likely the BIOS.

thanks
-john

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