Kernel with very low memory requirement (~12MB) for domU

["Rickard Borgmäster" <doktorn@sub.nu>]

Hello,

We are currently working on a master/bachelor level project, producing a 
xen + knoppix bootable CD for router simulation. Our task is to provide 
students as ourselves with a portable router lab that will run on any 
inexpesive PC, enabling students to exercise Cisco CCNA/CCNP level 
studies without access to physical equipment.

The goal is run 10x domU with nothing but the quagga/zebra daemons, on a 
P3/450MHz computer with 256MB ram.

So far, the CD and it's file systems is put together. We can boot it and 
run 6 routers in it's own domU before we run out of memory. And that's 
where we are now. We need to reduce the memory requirements for each 
domU, and possibly the dom0 later on.

Removing everything we're not dependent of (in make menuconfig) has 
reduced the vmlinuz kernel to just over 800KB. Still, it is not possible 
to start a domU with less than memory=17 in the config file. Although, I 
do think this is possible. Some tests we did last year (Xen 2.0.6 
installed in regular desktop system) showed that it was possible to boot 
a domU with 2.6.11-xenU kernel with just memory=12 (I hope I remember 
this correct).

I've spent a lot of time reading Xen documentation, wiki and lists. 
Sometimes I find "evidence" for my theories, like this post:

http://article.gmane.org/gmane.comp.emulators.xen.devel/11459

...where this config is present:

#-------configuration xmsec1-------------------------
kernel = "/boot/vmlinuz-2.6.11-xenU"
ramdisk="/boot/U1_ramdisk.img"
#security reference identifier
ssidref= 0x00010001
memory = 10
name = "xmsec1"
cpu = -1   # leave to Xen to pick
# Number of network interfaces. Default is 1.
nics=1
dhcp="dhcp"
#-----------------------------------------------------

As you can see, Reiner uses 10M for the each domU. Further down his 
post, it seems he is even running on less than that. I have no idea on 
what he is up to using ramdrives and such. We use an ext2 filesystem in 
a file, with cow_scripts from the xen 3 demo live cd.

To sum it up. We need to shrink the memory needed to start a domU down 
to somewhere around 12MB. And we do not have much time left to 
accomplish it. We'd seriuosly appreciate any feedback or tips on how to 
get going.

Thanks in advance,

An by the way, xen is a stunningly cool product.

Best regards
Rickard Borgmäster