slow responsiveness of kvm guests

[Kai Zimmer <kai@zimmer.net>]

Hi,

i have three guests running via qemu-kvm. All three are webservers using apache2/django and mysql. System load is very low on all guests and the host, too. Guest 1 reaction times (web server, ssh) are fine as long as no other guest is getting busy. When i ssh to guest 2 it takes 5-15 seconds until i get a login. When i run top or htop on one of the guests the other guests don't react for 5 seconds (webservers or typing in ssh shell). My colleagues already try to convince me to use xen instead...

already
- disabled apm and acpi in the guests
- turned Hyperthreading off
- tried different i/o schedulers: cfq vs. deadline. Result is the same.
- nailed down guest process via taskset to separate cpu cores

host is
OpenSuSe 11.1 x86_64
kvm-78-6.5.1
2xXeon 5550 Nehalem (Quadcores, Speedstep-enabled)
48 GB RAM
1 GB Ethernet as tap interface

all three guests are
Debian 5.0 x86_64
two virtual cpus
4GB RAM (hugepage-backed)
started via
qemu-kvm -hda xyz.qcow2 -daemonize -net nic,model=virtio -net tap -k de -m 3584 -smp 2 -mem-path /hugepages -vnc :2

Any ideas what goes wrong here?
Kai