Very slow network performance with KVM using virtio and tap.

[Robert Goley]

I have tried a KVM setup 2 different ways.  The first was using a straight bridge without using tap at all.  The WinXP VM is using the latest known version of the virtio drivers.  The bridge is on a gigabit nic.  The second setup was using a bridge containing a tap0 interface and using the tap0 interface in the KVM config.  I have tested network speeds from the host with other machines in the network using iperf.  Bridge interface to other machine on the network is fast with with speeds you would expect from using 1G nics.  Using the same iperf tests, I get about 1/100 of the speed when using the bridge or tap0 interfaces with the virtio nic.  WinXP shows the virtio nic as being 1 Gbps device. I am including some of my config below.  Anyone have suggestions on how to get greater than 1MBs transfer rates when using KVM in a bridged setup? ####interfaces#### iface br0 inet static         address 192.168.0.155         network 192.168.0.0         netmask 255.255.255.0         broadcast 192.168.0.255 #        gateway 192.168.0.1         bridge_ports eth3 tap0         bridge_stp off         bridge_maxwait 0         bridge_fd 0         pre-up /usr/sbin/tunctl -u libvirt-qemu -t tap0         pre-up ifconfig tap0 up         post-down ifconfig tap0 ####interfaces#### ###libvirt xml file### <domain type='kvm'>   <name>WINXPVM</name>   <uuid>f1340442-6088-11e0-a43a-001b214871e8</uuid>   <memory>524288</memory>   <currentMemory>524288</currentMemory>   <vcpu>1</vcpu>   <os>     <type arch='x86_64' machine='pc-0.12'>hvm</type>     <boot dev='cdrom'/>   </os>   <features>     <acpi/>     <apic/>     <pae/>   </features>   <clock offset='utc'/>   <on_poweroff>destroy</on_poweroff>   <on_reboot>restart</on_reboot>   <on_crash>restart</on_crash>   <devices>     <emulator>/usr/bin/kvm</emulator>     <disk type='file' device='disk'>       <driver name='qemu' type='raw'/>       <source file='/kvm/WINXP/WINXP.img'/>       <target dev='hda' bus='ide'/>       <address type='drive' controller='0' bus='0' unit='0'/>     </disk>     <disk type='file' device='cdrom'>       <driver name='qemu' type='raw'/>       <source file='/kvm/iso/WINXP-PRO-SP3-VIRTIO.iso'/>       <target dev='hdc' bus='ide'/>       <readonly/>       <address type='drive' controller='0' bus='1' unit='0'/>     </disk>     <controller type='ide' index='0'>       <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>     </controller>     <interface type='ethernet'>       <mac address='de:ad:be:ef:9a:9e'/>       <target dev='tap0'/>       <model type='virtio'/>       <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>     </interface>     <serial type='pty'>       <target port='0'/>     </serial>     <console type='pty'>       <target type='serial' port='0'/>     </console>     <input type='tablet' bus='usb'/>     <input type='mouse' bus='ps2'/>     <graphics type='vnc' port='29005' autoport='no' keymap='en-us'/>     <video>       <model type='cirrus' vram='9216' heads='1'/>       <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>     </video>     <memballoon model='virtio'>       <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>     </memballoon>   </devices> </domain> ###libvirt xml file###      -- Robert Goley FOSS Implementation Specialist Toll Free: (800) 338-4984 Local: (770) 479-7933 Fax: (770) 479-4076 www.openrda.com America's only Free & Open Source fund accounting software company.