[fedora-virt] Idle VM is hogging host cpu

[Richard W.M. Jones]

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.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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