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

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

James --

I'm completely ignorant about Windows but have often wondered
if self-ballooning scripts (which today only run in a RedHat-ish
environment) could be implemented on Windows.

If you might be interested, look in the tree (starting
around Xen 3.3) at tools/xenballoon, with more info
on the Xen Summit Summer 2008 site.

Dan

> -----Original Message-----
> From: Steven Smith [mailto:steven.smith@xxxxxxxxxxxxx]
> Sent: Wednesday, June 03, 2009 8:58 AM
> To: James Harper
> Cc: Steven Smith; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] implementing memory ballooning in WIndows
> 
> 
> > > > 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.
> We've gotten away with it so far, certainly.
> 
> > 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.
> Well, the second problem is solved by populate-on-demand mode.  The
> first is much harder, and almost certainly requires some kind of
> emergency hypervisor paging.
> 
> Of course, if you do populate-on-demand and then *can't* balloon down
> then the failure mode is extremely bad.  It can hopefully be avoided
> by a sufficiently paranoid toolstack, but it's still rather
> unfortunate.
> 
> > 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.
> I've not looked at it at all, sorry.
> 
> Steven.
>

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