Hello virtualists,

Avi asked me to bring this issue to the ML, so here it is:

Searching for solutions to a persistant problem with our KVM hosts, I
stumbled across Avis post

[REGRESSION] High, likely incorrect process cpu usage counters with
kvm and 2.6.2[67]

dated "Sun, 31 Aug 2008 08:43:41 -0700".

Ok, long story short, we've been experiencing the 100% CPU (sometimes
of course even 200% or 9999%) in top on the KVM host, where there was
2-3% before. I think, this happened after 2.6.25. We've been running
2.6.29 for some time now.

Now we updated the host machine again and are running Gentoo with
2.6.30-r6 - the latest linux kernel (as of 2009-09-07). We're using
the KVM that comes
shipped with the kernel. The host machines are identical hardware
configuration, built around two Xeon 5148 (Core2 with 2.33Ghz)

The problem is still persistent. In the guest, the load is 0.0, idle
100%. However, top of the corresponding kvm process on host shows 100%
(or more) CPU from time to time. I do NOT believe, that this is only
an accounting problem, as the host (4 cores machine), starts to assign
less CPU time to processes if all CPUs are fed with these KVM
processes. The hosts are running 8-12 KVM processs on average.

The guests are also Gentoo Linux machines, also kernel 2.6.30, some
are 32bit some 64bit, problem seems independent of that. No guest is

I'm somewhat puzzled, that the problem seems to persist over one year
after you discovered this or something that looks identical to me.

The only betterment we've seen with the latest kernel update is, that
the kvm processes fall to 0% CPU usage for some time, so the load of
the host machine was lowered. Still, when they pop up with 100% CPU
for 20 seconds or so, no reason within the guest can be observed.

I'm willing to assist in any more qualified reportings and (eventual)
bughunting, should you consider this worthwhile pursuing.


As for means to reproduce this: I have no idea. Tried on my notebook
(Intel T7200 @ 2.00GHz running 2.6.30-tuxonice-r5 - also Gentoo) the
problem is not there.

So to sum up the problem:

* was not present until and including 2.6.25, since 2.6.26
* is only present on our Xeon 5148 machines
* is independent of the guest being 32bit or 64bit (also Gentoo Linux)
* is independent whether we took the kvm included in the kernel or external (88)

So if anyone has ideas how to treat this, I'd be glad to hear them.

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