Re: idle linux guest takes up to 8.5% of host CPU

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Adam Monsen wrote:
> I have a Fedora 7 host with two guests:
> lnx01: Fedora 7
> win01: Windows XP Pro
>
> When they are both active, the host CPU usage by the Linux guest
> hovers around 8 to 9 percent, while the host CPU usage by the Windows
> guest hovers around 1 to 1.5 percent. These values are seen in the
> "virt-manager" application.
>
> Any idea why an idle Fedora 7 guest would use more host CPU than an
> idle Windows XP guest?
>
>   

Several reasons:

- Linux userspace sucks up more cpu than Windows.  That's especially 
true if you have X running.
- If running a 64-bit kernel, Linux is running at 250Hz or 300Hz whereas 
Windows runs at 100Hz unless you're doing multimedia. F7 32-bit kernels 
adjust the timer dynamically.
- If your machine is idle, it scales down the clock frequency, making 
guest idle usage seem higher (that doesn't account for the difference 
between Linux and Windows, though).

That said, I usually see much lower numbers.  Maybe you have an older 
cpu.  Also do look at 
http://kvm.qumranet.com/kvmwiki/Windows_ACPI_Workaround for possibly 
reducing Windows cpu usage further.

When the apic work is merged, I expect idle cpu usage to drop dramatically.

-- 
error compiling committee.c: too many arguments to function


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