[Qemu-discuss] Running several ARM VMs with a multicast-based VLAN results in extremely slow forwarded connections

[Alex Rønne Petersen]

Hi folks,

I'm using a script like this:

#!/usr/bin/env bash
qemu-system-arm \
    -machine versatilepb \
    -kernel vmlinuz-3.2.0-3-versatile \
    -hda hda$1.img \
    -initrd initrd.img-3.2.0-3-versatile \
    -append "root=/dev/sda1" \
    -m 640 \
    -k da \
    -localtime \
    -net nic,vlan=0,macaddr=52:54:00:12:34:$((55 + $1)) \
    -net user,vlan=0 \
    -net socket,mcast=230.0.0.42:6045 \
    -redir tcp:$((6049 + $1))::23 \
    -vnc :$((0 + $1)) \

(The first argument is simply a VM identifier.)

Given this script, I start up 6 VMs. They're able to see each other
and seem to communicate just fine and very responsively.

However, if I try to SSH into any of the machines (note the -redir
argument which sets up the forwarding), the connection is *extremely*
slow. It can literally take 40+ seconds for a command to go through.
This happens regardless of whether I SSH from outside or inside the
host machine. Now, if I remove the multicast line entirely (thus of
course having no connectivity between VMs), SSH connections into each
VM are perfectly responsive. VNC connections to the QEMU instances are
responsive regardless of network settings, FWIW.

The host system is Ubuntu 12.04 (x86_64) running QEMU 1.0.50.

Does anyone know what's going awry here?

Thanks in advance,
Alex