[fedora-virt] Idle VM is hogging host cpu

[Dennis Jacobfeuerborn]

On 11/08/2010 05:23 PM, Richard W.M. Jones wrote:
> On Mon, Nov 08, 2010 at 05:14:33PM +0100, Dennis Jacobfeuerborn wrote:
>> What I'm saying is that the same guest is running on tickless
>> kernels in both cases (F11 and F14) yet on the F14 kernel it uses
>> 20% cpu whereas it uses just 1-2% on F11. So this leaves two
>> questions:
>>
>> What else besides tickless-ness in the host kernel is causing that
>> the identical guest uses much more cpu resources on the f14 kernel.
>>
>> Why is the "divider=10" for the guest necessary on f14 but not on f11?
>
> The point is that it is the tickless kernel in the *guest* which
> matters here.  By having either a tickless guest kernel or using
> divider=10 (for guest kernels which cannot support tickless), KVM has
> to inject fewer timer interrupts into the guest.
>
> A guest which is "idle" isn't really idle if it is demanding that the
> hypervisor sends it a timer interrupt 1000 times a second.  You can
> either reduce that number (divider=10 =>  100 times a second), or you
> make it so you only need a timer interrupt when there is something you
> need to schedule (tickless).
>
> In any case the host can't control what the guest requests.
>
> Why it is that Fedora 11 does so much better than Fedora 14 with these
> idle guests is an entirely separate question.  Please post the
> qemu-kvm command line from both cases.

Not that the attached measurements were done booting the VM with the 
"single" option and without the "divider=10" option in the guest. On the 
Fedora 14 host I removed the "nohz=off" option (which wasn't present in the 
F11 case in the first place). This should result in a comparison among equals.
(I edited the "top" header back in after it got remove by the grep for 
readability)

Regards,
   Dennis
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