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

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Sep 28, 2011 at 11:49:01AM +0200, Reeted wrote:
> On 09/28/11 11:28, Daniel P. Berrange wrote:
> >On Wed, Sep 28, 2011 at 11:19:43AM +0200, Reeted wrote:
> >>On 09/28/11 09:51, Daniel P. Berrange wrote:
> >>>>This is my bash commandline:
> >>>>
> >>>>/opt/qemu-kvm-0.14.1/bin/qemu-system-x86_64 -M pc-0.14 -enable-kvm
> >>>>-m 2002 -smp 2,sockets=2,cores=1,threads=1 -name vmname1-1 -uuid
> >>>>ee75e28a-3bf3-78d9-3cba-65aa63973380 -nodefconfig -nodefaults
> >>>>-chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/vmname1-1.monitor,server,nowait
> >>>>-mon chardev=charmonitor,id=monitor,mode=readline -rtc base=utc
> >>>>-boot order=dc,menu=on -drive file=/dev/mapper/vgPtpVM-lvVM_Vmname1_d1,if=none,id=drive-virtio-disk0,boot=on,format=raw,cache=none,aio=native
> >>>>-device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
> >>>>-drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw,cache=none,aio=native
> >>>>-device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0
> >>>>-net nic,model=virtio -net tap,ifname=tap0,script=no,downscript=no
> >>>>-usb -vnc 127.0.0.1:0 -vga cirrus -device
> >>>>virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5
> >>>This shows KVM is being requested, but we should validate that KVM is
> >>>definitely being activated when under libvirt. You can test this by
> >>>doing:
> >>>
> >>>     virsh qemu-monitor-command vmname1 'info kvm'
> >>kvm support: enabled
> >>
> >>I think I would see a higher impact if it was KVM not enabled.
> >>
> >>>>Which was taken from libvirt's command line. The only modifications
> >>>>I did to the original libvirt commandline (seen with ps aux) were:
> >
> >>>>- Network was: -netdev tap,fd=17,id=hostnet0,vhost=on,vhostfd=18
> >>>>-device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:05:36:60,bus=pci.0,addr=0x3
> >>>>Has been simplified to: -net nic,model=virtio -net
> >>>>tap,ifname=tap0,script=no,downscript=no
> >>>>and manual bridging of the tap0 interface.
> >>>You could have equivalently used
> >>>
> >>>  -netdev tap,ifname=tap0,script=no,downscript=no,id=hostnet0,vhost=on
> >>>  -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:05:36:60,bus=pci.0,addr=0x3
> >>It's this! It's this!! (thanks for the line)
> >>
> >>It raises boot time by 10-13 seconds
> >Ok, that is truely bizarre and I don't really have any explanation
> >for why that is. I guess you could try 'vhost=off' too and see if that
> >makes the difference.
> 
> 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>

Regards,
Daniel
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