Poor network performance with cable modem assigned to guest

[Jon Fairbairn <jon.fairbairn@cl.cam.ac.uk>]

I'm experimenting with a virtual router. I did this a few years ago with
Xen and it worked well enough, but then fedora changed and it stopped
working, so I gave up for a while. Now I have a machine that supports
hardware virtualisation, I thought I'd try again.

The setup was done through virt-manager. The network between the host
and guest is a virtual bridge. What I've been trying to do is to assign
a USB cable modem to the guest, and connect to the internet through
that. I'd expect some degradation in performance, especially since
there's a firewall on both the virtual router and on the host. Here's
some figures wgetting a 12802500 byte file thrice from a nearby web
server:

Via hardware router: 1009K/s 1008K/s 1010K/s (12 or 13s)
Cable modem on host:   1.00M/s 1.00M/s 1.00M/s    (ditto)

(wait for it)

Via virtual router, assigned usb: 21.1K/s   (9m 58s!)

Now, as I said, I expected some performance hit doing it this way, but a
factor of fifty takes the biscuit.

What can be wrong?

 * * *

Details:

"Cable modem on host" above just means that I attached the cable modem
to the host and configured it as a network device in the usual way.

From the host to the guest I get about 12MB/s using scp, from the guest
to the host (initiated from the host) I get 7MB/s.

The host is AMD Athlon(tm) 64 X2 Dual Core Processor 4400+ with 6G of
RAM of which 256M is assigned to the guest (the hardware version only
has 188M) neither virtual nor hardware router has any swap.

The hardware router is the same kernel and nearly (modulo IP addresses
etc) the same configuration as the virtual router, running on an old ibm
pc (500MHz pentium III).

kernel on host: 2.6.29.6-217.2.16.fc11.x86_64
kernel on routers: 2.6.29.6-217.2.8.fc11.i586

All running fedora 11 (though the routers are very much cut down
installations).

qemu-kvm-0.10.6
libvirt-0.6.2-15.1.fc11.x86_64

libvirt uses this command to start the virtual machine:

LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin /usr/bin/qemu-kvm -S -M pc \
 -m 256 -smp 1 -name monogramme-virtual \
 -uuid [redacted] -monitor pty \
 -pidfile /var/run/libvirt/qemu//monogramme-virtual.pid \
 -boot d \
 -drive file=/[wherever]/livecd-fedora-monogramme.iso,\
if=ide,media=cdrom,index=2 \
 -net nic,macaddr=54:52:00:14:4f:18,vlan=0 \
 -net tap,fd=11,vlan=0 -serial pty -parallel none -usb -vnc 127.0.0.1:0 \
 -k en-gb \
 -usbdevice host:0bb2:6098 

0bb2:6098 = Ambit Microsystems Corp. USB Cable Modem (a usb 1.1 device)

Selinux is on on both machines.

I can't think of anything else relevant at the moment.

-- 
Jón Fairbairn                                 Jon.Fairbairn@cl.cam.ac.uk