Re: [Xen-devel] [PATCH v3 3/3] xl: Improve xl documentation in regards to guest memory management

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Tue, Apr 09, 2013 at 03:33:58PM +0100, Ian Jackson wrote:
> Daniel Kiper writes ("Re: [Xen-devel] [PATCH v3 3/3] xl: Improve xl 
> documentation in regards to guest memory management"):
> > On Mon, Apr 08, 2013 at 05:19:00PM +0100, Ian Jackson wrote:
> > > Presumably hotplugged memory can be ballooned too.  So perhaps we need
> > > another column ?
> >
> > I think that it is better to add comment that after
> > hotplugging memory could be ballooned down.
>
> I'd prefer not to make the diagram wrong.  Perhaps it would be better
> to annotate the "static maximum" to say that it might be increased by
> memory hotplug.

No, memory hotplug should not change "static maximum" in any case.
"static maximum" should define memory limit for a given guest.
If "static maximum" is above "build maximum" then memory hotplug
in the guest should be involved (if it is available).

> > > It might be worth adding:
> > >
> > >   "Guests whose actual allocation exceeds the target may experience
> > >    memory allocation failures."
> >
> > ... or "Guests which attempts to exceed the target may experience
> > memory allocation failures."
>
> No, because a guest which is being asked to shrink will experience
> memory allocation failures if it doesn't shrink monotonically.

IIRC if you shrink memory for a given guest and it does not have
memory balloon driver installed nothing happens. I did not tested
it but it would be crazy when host could steal random page from
the guest without its acceptance. It would lead to crash almost
immediately. That is why I wrote "attempts" because memory allocation
failures may happen when you increase reservation only.

Daniel

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