[CentOS] kvm/qemu and CPU load

Fri Jun 2 11:32:38 UTC 2017
hw <hw at gc-24.de>


I have a server using its 4 physical network interfaces
bonded, with the bonding interface added to a bridge.  The
bridge has the IP, and three VMs are using the bridge.  Two
of the VMs are running Debian, one is running Windoze 7.

CPU load caused by the qemu-kvm processes is way higher than
I´m happy with.  One of the Debian machine causes around
22% while it´s basically idle, the other one is around 3%,
and the Windoze one is around 50.  They are all mostly idle.

I can observe that when some network traffic is going on with
the Windoze machine, it causes a CPU load of 200%.  "Some network
traffic" means that virt-top is showing 1M/2M RX/TX.  Considering
that the bonding interface is theoretically capable of handling
4Gbit full duplex, the 3Mbit are neglectable. Virtio drivers are
being used.

Currently, virt-top shows 1.9% CPU for the Windoze machine and top
shows 22% CPU load for the corresponding qemu-kvm process.  There
is almost not network traffic.  The VM has 4 CPUs assigned.

What may cause the high CPU load?  Something must be seriously
wrong for an idle machine causing 22% CPU load and for the
same machine, still basically idle, with a some network traffic
to cause 200% CPU load on a Xeon 5690.