[virt-tools-list] KVM virtual server CPU utilization shown on virt-top implies more than 100% of virtual CPUs in use

[Dennis Jacobfeuerborn]

On 08/10/2011 03:45 PM, Yuksel Gunal wrote:
> I have been playing with KVM virtual servers on Red Hat and used the
> virt-top tool to monitor CPU utilization. I am puzzled by an observation
> that I was wondering if someone could explain: I have a 32-CPU blade and a
> KVM virtual server with 2 virtual CPUs on it. I am running some tight loops
> in this virtual server and pushing the CPU usage on it. What I see on
> virt-top is 7% utilization for this virtual server. The way I interpret it
> is 7% of 32 CPUs, which is 2.24 CPUs in use (7 * 32 / 100). What I am
> puzzled about is that I would not have expected this virtual server to use
> more than 2 CPUs since it only has 2 virtual CPUs assigned. Is it expected
> that KVM virtual servers could consume more than the number of virtual CPUs
> assigned to them or is this a known Linux accounting problem?

I've noticed that if you run Centos 6 guests on a Centos 6 host the guest 
automatically gets a tablet device installed that causes the VM to use 10% 
of a dual core cpu even when the guest is booted with init=/bin/bash. 
Removing that device fixes the issue.
The same thing apparently happens on RHEL and SL systems so it's really an 
upstream issue. I consider this to be a pretty huge bug as it renders the 
default install practically useless as a virtualization platform.

Regards,
   Dennis