Re: [PATCH] RFC: initial libxl support for xenpaging

[Olaf Hering <olaf@aepfle.de>]

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? 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 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