libvirt users - Aug 2010 - performance with libvirt and kvm

Hi,

I am seeing a performance degradation while using libvirt to start my
vm (kvm). vm is fedora 12 and host is also fedora 12, both with
2.6.32.10-90.fc12.i686. Here are the statistics from iperf :
>From VM: [  3]  0.0-30.0 sec   199 MBytes  55.7 Mbits/sec
>From host : [  3]  0.0-30.0 sec   331 MBytes  92.6 Mbits/sec
libvirt command as seen from ps output :

/usr/bin/qemu-kvm -S -M pc-0.11 -enable-kvm -m 512 -smp 1 -name
f12kvm1 -uuid 9300bfe2-2b9c-d9f0-3b03-9c7fe9934393 -monitor
unix:/var/lib/libvirt/qemu/f12kvm1.monitor,server,nowait -boot c
-drive file=/var/lib/libvirt/f12.img,if=ide,bus=0,unit=0,boot=on,format=raw
-drive if=ide,media=cdrom,bus=1,unit=0,format=raw -net
nic,macaddr=52:54:00:51:7c:39,vlan=0,model=virtio,name=net0 -net
tap,fd=21,vlan=0,name=hostnet0 -serial pty -parallel none -usb -vnc
127.0.0.1:0 -k en-us -vga cirrus -balloon virtio


If I start a similar vm using qemu-kvm directly, the performance
matches with the host.
qemu kvm : [  3]  0.0-30.0 sec   329 MBytes  91.9 Mbits/sec
TCP window size is 64K for all the cases.

Command used : qemu-kvm vdisk.img -m 512 -net nic,model=virtio
macaddr=$macaddress -net tap,script=/etc/qemu-ifup

Any clues?

Thanks,
Nirmal

On Tue, Aug 3, 2010 at 11:04 PM, Nirmal Guhan <vavatutu at gmail.com>
wrote:
> I am seeing a performance degradation while using libvirt to start my
> vm (kvm). vm is fedora 12 and host is also fedora 12, both with
> 2.6.32.10-90.fc12.i686. Here are the statistics from iperf :
>
> From VM: [ ?3] ?0.0-30.0 sec ? 199 MBytes ?55.7 Mbits/sec
[...]
> qemu kvm : [ ?3] ?0.0-30.0 sec ? 329 MBytes ?91.9 Mbits/sec
Is this what you are comparing: 91.9 -> 55.7 Mbits/s?
> Command used : qemu-kvm vdisk.img -m 512 -net nic,model=virtio
> macaddr=$macaddress -net tap,script=/etc/qemu-ifup
Have you tried to bisect the command-lines by dropping options from
the libvirt command-line until you arrive at the manual qemu-kvm
command-line?

Try using your own tap networking instead of libvirt's but otherwise
keeping libvirt's command-line as much as possible.

Can you use /var/lib/libvirt/f12.img with the manual qemu-kvm instead
of the vdisk.img file?  In general, try to compare same with same and
eliminate variables.

Stefan