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Re: [Xen-devel] [Hackathon minutes] PV frontends/backends and NUMA machines

On ven, 2013-05-24 at 17:00 +0100, George Dunlap wrote:
> On 21/05/13 14:43, Dario Faggioli wrote:
> > However, to do that, we, I think, need to be able not only to specify
> > that we want 1G worth of memory on one specific node, but also to
> > request explicitly for some of Dom0's PFN to be here and for some others
> > to be there, as we were saying earlier in the thread with Tim.
> One thing that I wanted to add to this discussion -- unless there's some 
> way for the toolstack to figure out, for each node, how much memory is 
> currently free *and* how much memory could be freed by dom0 on that 
> node, 
"A way to tell how much free memory there is on a node", yes, there is
already. "A way to tell how much memory could be freed by dom0 on a
node", no, there isn't anything like that.

This has to do with NUMA aware ballooning, which is another big bullet
on my NUMA roadmap/TODO list, although it hasn't been mentioned in the
hackathon (or at least, I don't remember it being mentioned).

I proposed it for GSoC, and got mails from several people interested in
working on it. Among them, there is one that seems to be keen on doing
the job, even outside of GSoC, but he can't start right away, as he's
otherwise engaged for the forthcoming weeks. I can double check his
availability and investigate better his commitment...

Also, I think NUMA aware ballooning requires (or, at least, would be
easier to implement with) NUMA awareness in the guest (dom0 in this
case), so we have sort of a circular dependency here. :-D

> and a way to ask dom0 to free memory from a specific a node, then 
> booting with dom0 having all the memory is basically going to make all 
> of our NUMA work a noop.
Booting *without* dom0_mem=XXX already hurts quite a bit, since the
current automatic NUMA placement code needs to know how much free memory
we have on each node, and having them completely filled by dom0 is all
but the best situation for it to reach a decent solution! :-(

> We may end up having to switch from defaulting to giving dom0 and 
> autoballooning to giving dom0 a fixed amount.
Well, from a NUMA only point of view, that would be really desirable....
But I'm not sure we can go that far, especially because I see it very
hard to find a value that would make everyone happy.


<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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