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Re: High Volume of Syslog Messages on Fedora 16 Dom0

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On Fri, Oct 07, 2011 at 10:31:37PM +0300, Marko Ristola wrote:
> 
> Hi.
> 
> I found out that disabling AMD Cool and Quiet from BIOS
> will remove the flood of these messages under Xen Dom0:
> If BIOS support for switching CPU frequencies is turned off,
> Linux kernel won't try to alter the frequency.
> 
> My computer has Fedora 16, basic desktop computer.
> The hardware is old, and thus I haven't bothered to request features
> for this old hardware. Only problem with the hardware is, that
> XEN Hypervisor scheduler doesn't use HPET. I think hypervisor scheduler
> does busy looping, instead of using HALT instruction.

The default operation is to use 'hlt' (from arch/x86/domain.c in
Xen 4.1.1 hypervisor):

static void default_idle(void)
{
    local_irq_disable();
    if ( cpu_is_haltable(smp_processor_id()) )
        safe_halt();
    else 
        local_irq_enable();
}

HPET does not have anything to do with HALT. HPET is used to
figure out the time.

> This was a problem on last hot Summer, but now it is cold Autum.
> 
> Without Xen Hypervisor on Linux kernel, I use "hpet=force".
> Linux Kernel finds out by the knowledge of old hardware,
> that HPET is propably there, even though ACPI doesn't advertice it.
> 
> "hpet=force" was on XEN Hypervisor a few years ago, but the code has been removed.
> I don't know if it is coming back.

Unless somebody posts a patch - then no.
> 
> I can test old hardware so that XEN hypervisor and DOM0 messages go into a serial cable.

That is appreciated. The Xen ACPI cpufreq patches that would push up the _P states to
the hypervisor are not yet ready - but when they are ready it would be very good to try
it out on your hardware.

Note, the ACPI cpufreq patches enable the hypervisor to use different
methods that just doing 'hlt'. They can use 'mwait' or some other form
of putting the CPU to sleep.. and naturally also change the frequency and those
fancy things.
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