RE: [Xen-devel] implementing memory ballooning in WIndows

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> > Do the Citrix Windows PV drivers implement memory ballooning? I've
been
> > looking at this a bit for GPLPV and the avenues I can see are:
> >
> > 1. Memory can be added via ACPI hot-add, but I assume that that
isn't
> > implemented in the GPL qemu code (or is it?) and only works for
Windows
> > Server Enterprise, not Vista, XP, or Server Standard.
> > 2. I can give memory back via just allocating a physical page and
giving
> > it back to Xen
> 
> Our drivers do implement ballooning (although it's not actually hooked
> up to the control tools, for mostly tedious reasons), and they do it
> using approach 2.
> 
> Approach 1 does have the advantage that you can later increase a
> domain's memory allocation beyond its boot-time size, but that's
> obviated to some extent by the new populate-on-demand support, and, as
> you say, memory hot add is quite heavily restricted by Microsoft's
> licensing.
> 

#2 is pretty much where I was going. My concern was that Windows says "I
have 4GB of memory, so I can use X amount for this and Y amount for that
and Z amount for something else", and when the memory is reduced down to
(say) 1GB, I was worried that Windows might get a bit unbalanced. Same
with Linux when it makes decisions about how much space to reserve for
network buffers etc.

But it seems that you have taken the stance that there is no such
problem, so I'll do the same.

The other thing I was concerned about is that the memory is only given
back once the PV drivers start, so in the situation where you have:
. 16G physical memory
. Dom0 with 2G of memory
. 6 DomU's with 4G of memory, ballooned down to 2G of memory

It would only work if all the DomU's successfully ran their PV drivers
and ballooned the memory down as requested, and you couldn't start them
all at once as on boot they'd actually need 4G of memory.

Did you ever have a tinker with the (mostly) undocumented
MmAddPhysicalMemory function at all? On face value it seems like it is
the sort of function that could be useful... Microsoft would not approve
of such a thing obviously which makes it unusable for production use,
but I am curious as to if it would actually work.

James


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