[Sussex] NTP Weirdness

[Steve Dobson]

Jon

On Thu, 2008-04-24 at 09:59 +0100, Jon Fautley wrote:
> On Thu, 24 Apr 2008 09:00:48 +0100
> Steve Dobson <steve.dobson at syscall.org.uk> wrote:
> 
> > All:  Yesterday afternoon I disabled NTP on the box and reset the
> > time. This morning (less than 12 hours later) the clock was ~10
> > minutes fast. As the system clock is run of the software timer
> > interrupt my thoughts are now turning to overclocking as being the
> > prim suspect.
> 
> Highly unlikely. You're probably just using an unsyncronised
> timesource. Multi-CPU systems can often exhibit this problem as
> the timestamp counters aren't synced between the different CPU cores,
> so depending on which core gets the TSC query when a gettimeofday()
> call is executed means that you can see jumping time. Have you noticed
> time going "backwards" as well as forwards?

No, the time always jumps forward.

> A quick check would be to boot with "clock=pmtmr" on the kernel command
> line. This will force the kernel to use the "Power Management Timer"
> chip to track system time. This is the most stable, but also the most
> "expensive" clock to use in terms of performance. On a home system,
> it's unlikely you'll see any major performance difference, but on big
> end systems that make lots of gettimeofday() calls - i.e Oracle
> databases - you can notice a performance hit and slightly increased CPU
> utilisation.

I can certainly give this a try and will.

> Depending on the hardware, you might also want to see if there is the
> option to enable the "High Performance Event Timer" (HPET) - this is
> the "best" timesource to be using, but only newer systems have them
> installed, and you often need to tweak the BIOS to enable it (on an HP
> DL585 system I think the option is "Enable Linux Event Timer" or
> somesuch).
> 
> A bit of googling or prodding the kernel source code should reveal the
> other types of timer you can query - I can't remember the others off
> the top of my head I'm afraid.

Thanks for the info.
Steve

-- 
Steve Dobson

I guess it was all a DREAM ... or an episode of HAWAII FIVE-O ...

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