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



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


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