Re: [Xen-devel] Uses of &frame_table[xfn]

[Ewan Mellor <ewan@xxxxxxxxxxxxx>]

On Fri, Dec 23, 2005 at 05:05:29AM -0800, Magenheimer, Dan (HP Labs Fort 
Collins) wrote:

> > > On a related note, I'm not clear on the relationship between
> > > the "memory" variable in xmdefconfig and the two reservation
> > > variables needed for ballooning (not sure what they are named,
> > > but call them max_reservation and initial_reservation).  Which
> > > of these latter correspond to "memory" in xmdefconfig and how
> > > does the other get determined?
> > 
> > For unprivileged guests, Xend sets both the maximum and 
> > current reservation to
> > the configured "memory" value, when a domain is created.  
> > Subsequent calls to
> > xm mem-set set the current reservation, and leave the maximum 
> > alone.  Calls to
> > xm mem-max set the maximum (and reducing the maximum without 
> > thought is a good
> > way to crash your guest).
> > 
> > For dom0, the initial values are set from the kernel command 
> > line, and the
> > current reservation is altered if memory is needed for domain 
> > creation,
> > migration, etc.
> 
> Thanks for the reply.  This is starting to make sense.
> 
> With the balloon driver still unimplemented, I have seen
> messages (from xend?) that /proc/xen/balloon can't be
> opened.  This occurs when I try to create a domain such that
> the sum of memory of all launched domains (including dom0)
> exceeds the total machine memory on the machine.
> 
> So when I get this message, is xend trying to balloon memory
> from dom0?  Or is this the point where xend(?) tries
> to do the xm mem-set to balloon some memory away from a
> domU (and, if so, how does it decide which one(s) to steal
> memory from)?

The first of those.  It will balloon dom0 down as far as is necessary, stopping 
at dom0-min-mem, as configured in /etc/xen/xend-config.sxp.  You should see
a message in /var/log/xend.log saying "Balloon: need X MB, free Y MB." when it
does this, and it will try to balloon dom0 to release the difference.

Ewan.

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