CPU usage wrt kernel mode

[Christian Theune <ct@gocept.com>]

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Hi,

we're setting up a couple of machines with KVM and are really happy in
general.

However, we have a consistently appearing issue with a high percentage
of CPU time spent in kernel (aka system) mode.

We're using kernel 2.6.25 on the host and 2.6.25 in the guest. KVM 73 is
compiled manually on the host.

We eliminated virtio as a problem source already, as well as specific
disk storages (local LVM device, AoE device, qcow and raw images).

We use a Linux kernel compilation run to produce the behaviour by
issuing `make -j(N+1)` (for N CPUs) and comaring various virtual machine
configurations as well as the host.

The hosts have 4 to 8 cores and the guests were run with 1 or 2 virtual
CPUs.

In a guest with a single CPU the percentage of system mode time goes up
to 70-80% grinding the VM almost to a halt.

On the host system load stays around 10%.

Latencytop seems to indicate file operations that might cause the CPU
time. The filesystem is ext3 on the host and the guests.

We're kind of stuck after a couple of days of debugging and wonder
whether someone knows this behaviour (we assume this isn't supposed to
be this way) and might know how to debug it further (or even solve it).

Best regards,
Christian

-- 
Christian Theune · ct@gocept.com
gocept gmbh & co. kg · forsterstraße 29 · 06112 halle (saale) · germany
http://gocept.com · tel +49 345 1229889 7 · fax +49 345 1229889 1
Zope and Plone consulting and development

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