Subject: Re: xennetback_ifstart: no mcl_pages crashes XEN2-dom0
To: Damian Lubosch <>
From: Manuel Bouyer <>
List: port-xen
Date: 06/29/2007 11:02:14
On Fri, Jun 29, 2007 at 09:35:55AM +0200, Damian Lubosch wrote:
> Hello!
> Again my dom0 crashed this night.
> The dom0 showed the same message as few weeks ago:
> Manuel Bouyer wrote:
> [...]
> >> I read on the list something about XEN-DOM0 needing 32MB for itself.
> >> (Date of posting: 4/15/2005 6pm from Manuel Bouyer) Does it mean that I
> >> have to increase my 2MB of free memory to 32? That's actually what I'd
> >> try next...
> >
> > I strongly suspect that there's not enough free ram for the hypervisor.
> Should the free ram be under dom0?

No, the hypervisor itself (i.e. 'free' in xm info)

> As I understand XEN, the hypervisor
> ist running in dom0, right?

No, the hypervisor is running on its own and control access to ressources
of all domains, including dom0. More technically, on i386 the hypervisor
runs in ring0, all domains kernels (including dom0) runs in ring1 and userland
programs in ring3. On plain NetBSD/i386 (and most other unix variants on i386),
the kernel runs in ring0 and userland in ring3, rings 1 and 2 are unused.

> The dom0 has 64MB of its own memory. Since
> it isn't doing anything else but providing XEN, I thought it would be
> enough. I could spare some more MB (up to 90) if it is necessary. Is it?

you can watch with top what's going on, but I suspect 64MB is enough.

> > I think the 32MB is not counted as part of "xm info", but it's something
> > that can't ever be allocated to domains anyway, it's internal to the
> > hypervisor.
> So nobody noticed problems having 2MB free in "xm info"? The free ram
> has no meaning but showing a waste of memory?

it's really free in the same sense as free in Unix: memory that can be
allocated to something if needed (it can be creation of new domains, or
page swapping needed as part of inter-domain communications)

> There is a lot of communication between the domUs. I resized the
> NMBCLUSTERS to 2048. Should I increase it even more?

You can watch then usage with netstat -m; but if you don't get the
"WARNING: mclpool limit reached; increase NMBCLUSTERS" message,
you're probably allocated enough.

> >
> > I don't think it will help.
> >
> Ok, that will save me some time. Should I maybe upgrade to XEN 3? Did it
> turn out to be more stable? My dom0 is running NetBSD Beta anyway so a
> switch won't cause many changes.

It won't change much I guess. I have important prodcution servers running
very well with Xen2. I also have some running Xen3; all of them are going
their job :)

Manuel Bouyer, LIP6, Universite Paris VI. 
     NetBSD: 26 ans d'experience feront toujours la difference