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

On Fri, 2012-02-17 at 14:25 +0000, Olaf Hering wrote:
> On Fri, Feb 17, Ian Campbell wrote:
> > IOW at the libxl layer we have the full semantics available to callers
> > but at the xl layer we only expose one "target memory" value to users
> > which we expect the guest to use ballooning to reach but which we
> > "enforce" with paging if they don't comply.
> So if I understand that right there should be a new boolean, like
> xenpaging=yes/no?

Yes. Although you can probably omit the "xen" prefix in a Xen
configuration file.

> Or it could be like this (in which case the pager currently cant start
> due to lack of PoD support):
> memory=N
> maxmem=N+X
> xenpaging=yes

This is the style I was thinking of.

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).

> In both cases "xl mem-set" will adjust both ballooning and paging values?


> 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.

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.

> Regarding the tuning knobs, right now I can only think of the policy mru
> size and the number of evicts before checking for new events.
> So you propose to have something like "xl xenpaging domid knob value"?

The other option would be
        xl paging-knob-set domid value

We seem to have commands in both forms already.


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