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



I repost this, this time by also including the libvirt mailing list.

Info on my libvirt: it's the version in Ubuntu 11.04 Natty which is 0.8.8-1ubuntu6.5 . I didn't recompile this one, while Kernel and qemu-kvm are vanilla and compiled by hand as described below.

My original message follows:

This is really strange.

I just installed a new host with kernel 3.0.3 and Qemu-KVM 0.14.1 compiled by me.

I have created the first VM.
This is on LVM, virtio etc... if I run it directly from bash console, it boots in 8 seconds (it's a bare ubuntu with no graphics), while if I boot it under virsh (libvirt) it boots in 20-22 seconds. This is the time from after Grub to the login prompt, or from after Grub to the ssh-server up.

I was almost able to replicate the whole libvirt command line on the bash console, and it still goes almost 3x faster when launched from bash than with virsh start vmname. The part I wasn't able to replicate is the -netdev part because I still haven't understood the semantics of it.

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


Which was taken from libvirt's command line. The only modifications I did to the original libvirt commandline (seen with ps aux) were:

- Removed -S

- 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.


Firstly I had thought that this could be fault of the VNC: I have compiled qemu-kvm with no separate vnc thread. I thought that libvirt might have connected to the vnc server at all times and this could have slowed down the whole VM. But then I also tried connecting vith vncviewer to the KVM machine launched directly from bash, and the speed of it didn't change. So no, it doesn't seem to be that.

BTW: is the slowdown of the VM on "no separate vnc thread" only in effect when somebody is actually connected to VNC, or always?

Also, note that the time difference is not visible in dmesg once the machine has booted. So it's not a slowdown in detecting devices. Devices are always detected within the first 3 seconds, according to dmesg, at 3.6 seconds the first ext4 mount begins. It seems to be really the OS boot that is slow... it seems an hard disk performance problem.

Thank you
R.


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