Re: [Xen-devel] domain creation vs querying free memory (xend and xl)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2012-10-04 at 14:20 +0100, Andres Lagar-Cavilla wrote:
> On Oct 4, 2012, at 6:17 AM, Ian Campbell wrote:
> 
> > On Thu, 2012-10-04 at 11:06 +0100, Tim Deegan wrote:
> >> but my question was really: what should xl do, in the presence of
> >> ballooning, sharing, paging and tmem, to
> >> - decide whether a VM can be started at all;
> >> - control those four systems to shuffle memory around; and
> 
> Are we talking about a per-VM control, with one or more of those sub-systems 
> colluding concurrently? Or are we talking about a global view, and how chunks 
> of host memory get sub-allocated? Hopefully the latter...
> 
> >> - resolve races sensibly to avoid small VMs deferring large ones.
> >> (AIUI, xl already has some logic to handle the case of balloon-to-fit.)
> >> 
> >> The second of those three is the interesting one.  It seems to me that
> >> if the tools can't force all other actors to give up memory (and not
> >> immediately take it back) then they can't guarantee to be able to start
> >> a new VM, even with the new reservation hypercalls.
> > 
> > There was a bit of discussion in the spring about this sort of thing
> > (well, three of the four), which seems to have fallen a bit by the
> > wayside^W^W^W^W^W^Wbeen deferred until 4.3 (ahem) e.g.
> > http://lists.xen.org/archives/html/xen-devel/2012-03/msg01181.html
> > 
> > I'm sure there was earlier discussion which led to that, but I can't
> > seem to see it in the archives right now, perhaps I'm not looking for
> > the right Subject.
> 
> IIRC, we had a bit of that conversation during the Santa Clara
> hackathon. The idea was to devise a scheme so that libxl can be told
> who the "actor" will be for memory management, and then hand-off
> appropriately. Add xl bindings, suitable defaults, and an
> implementation of the "balloon actor" by libxl, and the end result is
> the ability to start domains with a memory target suitably managed by
> balloon, xenpaging, tmem, foo, according to the user's wish. With no
> need to know obscure knobs. To the extent that might be possible.

That's right, I'd forgotten about that conversation. Yet some how the
mail I referenced seems to be a result of that conversation -- which is
a nice coincidence ;-)



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