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Re: [Xen-devel] [PATCH] RFC: initial libxl support for xenpaging

On Fri, 2012-02-17 at 15:24 +0000, Olaf Hering wrote:
> On Fri, Feb 17, Ian Campbell wrote:
> > Paging needs to either be compatible with or replace PoD IMHO (or
> > perhaps simply be mutually exclusive with it as an interim solution,
> > i.e. paging=yes disables pod).
> This has to be fixed in hypervisor at some point. PoD as such is very
> useful, and I expect adding the cooperation with paging is not very
> hard. It just takes time.
> > > Should there be xl commands to adjust just ballooning and/or paging?
> > 
> > Perhaps as debuging aids or something but I wouldn't expect these to be
> > commands which we would suggest end users needed to touch. The
> > interactions between these different knobs and the semantics of changing
> > just one or the other would be pretty complex.
> Where do you see the complexity?

For users in determining what changing a given value will actually do,
what values they need to use to get the behaviour which they want, what
happens if they change first one value and then the other, how do they
interact with mem-set and memmax, what are the various invalid or
meaningless combinations etc.

There's also complexity for us in trying to decide what the right answer
is to each of those questions and trying to implement it in a consistent
way and explain it all to users.

>  The balloon driver does not notice that
> the guest is paged, and xenpaging itself can cope with ballooning.
> Giving the host admin the choice to page more (at the expense of some IO
> and slight slowdown) without caring about the current memory constraints
> within the guest sounds useful to me.

The "minmem" suggestion allows exactly this without exposing the user to
too much of the underlying implementation details.

> > The only additional nob I can see being useful would be a minmem option
> > which is the smallest amount of memory which the guest should think it
> > has, i.e. it would be a lower bound on the ballooning target but not the
> > paging target. The default would be some fraction of maxmem (similar to
> > the minimum_target stuff in linux-2.6.18-xen.hg:drivers/xen/balloon/).
> > This would be used to reduce the memory used by a domain past the point
> > at which it would start OOMing etc.
> Sounds useful to prevent accidents.
> Olaf

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