On Mon, 2010-09-13 at 22:17 +0100, Dan Magenheimer wrote:
> > From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
> > Cc: Dan Magenheimer; Daniel Kiper; Stefano Stabellini; Konrad Rzeszutek
> > Wilk
> > Subject: Ballooning up
> >
> > I finally got around to implementing "ballooning up" in the pvops
> > kernels. Now if you start a domain with "memory=X maxmem=Y", the
> > domain
> > will start with X MB of memory, but you can use "x[ml] mem-set" to
> > expand the domain up to Y.
>
> Nice!
>
> > As a side-effect, it also works for dom0. If you set dom0_mem on the
> > Xen command line, then nr_pages is limited to that value, but the
> > kernel
> > can still see the system's real E820 map, and therefore adds all the
> > system's memory to its own balloon driver, potentially allowing dom0 to
> > expand up to take all physical memory.
> >
> > However, this may caused bad side-effects if your system memory is much
> > larger than your dom0_mem, especially if you use a 32-bit dom0. I may
> > need to add a kernel command line option to limit the max initial
> > balloon size to mitigate this...
>
> I would call this dom0 functionality a bug. I think both Citrix
> and Oracle use dom0_mem as a normal boot option for every
> installation and, while I think both employ heuristics to choose
> a larger dom0_mem for larger physical memory, I don't think it
> grows large enough for, say, >256GB physical memory, to accommodate
> the necessarily large number of page tables.
FWIW XenServer statically uses dom0_mem=752M and then balloons down on
smaller systems where so much domain 0 memory is not required, the
minimum is 128M or 256M or something.
A 32on64 domain 0 kernel fails to boot if dom0_mem is more than around
~56G because it runs out of lowmem for the page array. I suspect that
for some period before that the system isn't terribly usable due to low
amounts of available lowmem, even if it does manage to boot.
> So, I'd vote for NOT allowing dom0 to balloon up to physical
> memory if dom0_mem is specified, and possibly a kernel command
> line option that allows it to grow beyond. Or, possibly, no
> option and never allow dom0 memory to grow beyond dom0_mem
> unless (possibly) it grows with hot-plug.
>
> Dan
>
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