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Date: Fri, 12 Mar 2004 13:52:09 -0800
To: linux-kernel@vger.kernel.org
From: Jonathan Lundell <linux@lundell-bros.com>
Subject: Re: nmi oddity
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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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