[libvirt] Qemu/KVM is 3x slower under libvirt (due to vhost=on)

Richard W.M. Jones rjones at redhat.com
Wed Sep 28 12:56:14 UTC 2011

On Wed, Sep 28, 2011 at 12:19:09PM +0200, Reeted wrote:
> On 09/28/11 11:53, Daniel P. Berrange wrote:
> >On Wed, Sep 28, 2011 at 11:49:01AM +0200, Reeted wrote:
> >>YES!
> >>It's the vhost. With vhost=on it takes about 12 seconds more time to boot.
> >>
> >>...meaning? :-)
> >I've no idea. I was always under the impression that 'vhost=on' was
> >the 'make it go much faster' switch. So something is going wrong
> >here that I cna't explain.
> >
> >Perhaps one of the network people on this list can explain...
> >
> >
> >To turn vhost off in the libvirt XML, you should be able to use
> ><driver name='qemu'/>  for the interface in question,eg
> >
> >
> >     <interface type='user'>
> >       <mac address='52:54:00:e5:48:58'/>
> >       <model type='virtio'/>
> >       <driver name='qemu'/>
> >     </interface>
> Ok that seems to work: it removes the vhost part in the virsh launch
> hence cutting down 12secs of boot time.
> If nobody comes out with an explanation of why, I will open another
> thread on the kvm list for this. I would probably need to test disk
> performance on vhost=on to see if it degrades or it's for another
> reason that boot time is increased.

Is it using CPU during this time, or is the qemu-kvm process idle?

It wouldn't be the first time that a network option ROM sat around
waiting for an imaginary console user to press a key.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

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