RE: [Xen-users] Can we run more virtual machine than the memory avaible?

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Rachid Abdoun
> Sent: 22 June 2006 14:57
> To: Harald Schioeberg
> Cc: Xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Can we run more virtual machine than 
> the memory avaible?
> 
> It's a big problem for me if you create a virtual network with 100  
> hosts (30MB per host) you need 3 Go per host!!  I think this is the  
> disadvantage  of xen

It may be a disadvantage, but it happens to be the design choosen for Xen. Xen 
has limitations, which may cause it to be a disadvantage in some respects. 

But I'm sure having 100 machines with 30MB of memory each would be even more of 
a disadvantage, right?

Also, if you're trying to run 100 virtual machines, I'm sure you don't REALLY 
want to do that on a single CPU, so we're not talking about a very basic 
machine... 

I'm actually not aware of any virtual machine system where you are allowed to 
overcommit memory... But I haven't really been looking for them either... 

Can you suggest how this should be solved? 

There are LOTS of highly complicated situations to solve. 

We could obviously use Dom0 for the swapping process (at least in the sense of 
using it's disk drivers), so that's not a big issue. 

However, which pages can be swapped and which can't isn't so easy to determine 
unless you are aware of what the OS knows about the memory - for example, if 
the hypervisor decides that the page we've just given to the network card 
should be swapped out and replaced with some other page, it wouldn't work very 
well - receiving a packet would destroy the new contents of the page, sending a 
packet would send incorrect data. The hypervisor has NO KNOWLEDGE of what's in 
which page, so it's not able to choose a page correctly. Of course, the OS has 
some special call that is called when the system is to "lock" a page in memory, 
and in a para-virtualized environment, this could be intercepted and used to 
prevent the pages from being paged out. But in the case of a unmodified OS, 
this would definitely not work, as there's not a single special operation to 
track this information - the OS would just have some internal table of some 
sort (binary tree, hash-list, linear array, bitmap or whatever the OS designer 
decided was the best way to track a bunch of memory pages...)

I don't think this will be implemented in Xen for a LONG time. 

--
Mats
> 
> 
> Le 22 juin 06 à 15:22, Harald Schioeberg a écrit :
> 
> > -----BEGIN PGP SIGNED MESSAGE-----
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> >
> > Thats not possible, you'll need at least 664MB + 32 MB for the
> > hypervisor + some overhead, xen won't use exactly 30MB but 
> a little  
> > bit
> > more.
> >
> > one thing: if you want to use the thing to simmulate some network
> > behaviour: xen currently supports only 3 virtual interfaces 
> per domU,
> > that cripples my virtual network as well.
> >
> > harald
> >
> >
> > Rachid Abdoun wrote:
> >> Hi,
> >>
> >> I'd like to create a virtual network with xen who have more tha 20
> >> hosts. Every host has 30MB in his configuration file. The total of
> >> memory is
> >>
> >> 20X30 = 600MB
> >>
> >> +
> >>
> >> 64MB for the dom0
> >> --------------------------
> >> = 664MB
> >>
> >> but I only have 512MB on my workstation. Can this virtual network
> >> possible with xen? Or you need to have enough ram!?
> >>
> >> Thank you
> >>
> >> _______________________________________________
> >> Xen-users mailing list
> >> Xen-users@xxxxxxxxxxxxxxxxxxx
> >> http://lists.xensource.com/xen-users
> >>
> >
> >
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> 
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