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



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? :-)


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