Re: nmi oddity

[Jonathan Lundell]

At 12:13 PM +0100 3/11/04, Mikael Pettersson wrote:
> Jonathan Lundell writes:
>  > Running smp 2.4.9 (don't ask) with updated nmi logic on a Dell 650
>  > (UP P4), I notice that NMI is running at about 1.08 Hz. Per the
>  > kernel logic, it should be running at HZ (100) Hz.
>  >
>  > I'm using NMI_LOCAL_APIC. check_nmi_watchdog() never gets called in
>  > this mode in an smp kernel, near as I can tell, so nmi_hz never gets
>  > set to 1.
>  >
>  > What's going on? Or does anyone else see it?
>
> This is a normal. The NMIs are generated from an in-CPU performance
> counter configured to increment once per CPU core clock cycle.
> (There is no wallclock-like event available, alas.)
>
> Whenever the kernel is idle, "hlt" is normally executed which
> stops the CPU until the next external interrupt (typically the
> timer). Hence, an idle machine will see a much lower NMI frequence
> than a non-idle one.

That's exactly what's happening; thanks.

If I run a compute-intensive task in the background (eg cat 
/dev/zero > /dev/null), the nmi frequency jumps up to 100 Hz.

Switching to NMI_IO_APIC is the best solution for me.

It makes NMI_LOCAL_APIC a v. questionable choice for a watchdog....

> This behaviour is not universal. Server Tualatins seem to run at
> full speed all the time; perhaps they ignore hlt?
>
> If you want NMI_LOCAL_APIC to be clock-like, upgrade your cooling
> solution and disable hlt.
>
> /Mikael
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--
/Jonathan Lundell.
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