[Xen-users] Re: Minimal kernel for domU

[Rickard Borgmäster <doktorn@xxxxxx>]

Petersson, Mats wrote:
> I believe that current Xen (3.0, testing, unstable, etc) do not
> support Kernel 2.4. It's probably not A LOT of work to make it work,
> but it's still some work.

The Xen 3 documentation doesn't mention 2.4 kernels either, so maybe 
you're right.
> However, I have a feeling that the difference in Kernel size wouldn't
> be THAT great - if you minimize the 2.6 kernel, how much bigger is it
> compared to a 2.4 kernel? I don't know the answer, but I would think
> that the KERNEL itself isn't that much difference - 2.6 has MORE
> FEATURES that you can turn on (or off), but once you've got the same
> feature-set, I would think [I'm guessing!] that the size difference
> isn't very much.
That's why I'm asking :-) The reason for me coming up with the idea is 
that the kernel vmlinuz-2.6.12.6-xenU is about 1,1MB while the 
vmlinuz-2.4.29-xenU (from demoCD) is just 816KB. So I thought it would 
be worth asking about.
> Are you planning on running VERY many domains with small memory
> footprint?

I don't know what VERY many domains means to you, but yes, sort of, 
since everything is relative.
What we are trying to accomplish is to build a bootable LiveCD with Xen 
(this is sort of done already). In the booted knoppix environment, a 
number of virtual machines should be started up, to run nothing but a 
routing daemon (quagga / zebra).
The goal is to have a Cisco router simulator on a bootable CD-Rom, for 
students to practice routing scenarions without any physical Cisco 
routers. Furthermore, the goal is to run 10 virtual routers on a 
computer with only 256MB ram.
This could be done. I know that. Tests with Xen 2.0.7 before Christmas 
showed that domU's could boot with only 12MB Ram. Now we're going for 
Xen 3, and today the minimum domU memory that will still boot domU, is 
17MB. But that is too big. The host can only run 8 domU's before crashing.
> Consider that the page-tables for just mapping the memory once in a
> 1GB system takes up one megabyte [or two megabytes if you use PAE].
> Obviously, if you're running multiple tasks, each task would have some
> amount of page-table entries, possibly including duplicate ones for
> user-mode mappings of shared memory regions and in some cases also
> duplicates for the kernel space memory.
I do know some about computers, but the above was a bit too complicated 
for me I think... Maybe you can translate it to more understandable text 
now that you know the background. I suspect it might be useful 
information...
Best regards
Rickard Borgmäster


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