[libvirt] Qemu/KVM is 3x slower under libvirt

Reeted reeted at shiftmail.org
Wed Sep 28 09:49:01 UTC 2011

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 -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.

It's the vhost. With vhost=on it takes about 12 seconds more time to boot.

...meaning? :-)

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