Re: [Xen-devel] [PATCH 11 of 11] Some automatic NUMA placement documentation

[Dario Faggioli <raistlin@xxxxxxxx>]

On Fri, 2012-06-08 at 15:01 +0100, Ian Jackson wrote:
> > Right, that is of couse an option, and I thought about going that way
> > for quite a while. However, what I think is best for libxl is to provide
> > a set of building blocks for its user to be able to implement the actual
> > heuristics.
> 
> The difficulty with this is that this is supposedly becoming a stable
> API.  If we change the approach later, we may want to change the API.
> Do we know clearly enough what building blocks are needed and what
> they should be like ?
> 
No, you all are right, not yet... More investigation is needed here and
it is not now the moment to do it.

> > Doing it the other way, i.e., one big function doing everything, would
> > mean that as soon as we want to change or improve the placement
> > heuristics, we need to modify the behaviour of that API call, which I
> > think it is suboptimal.
> 
> If we have a function whose documented behaviour is `try to do a
> roughly optimal thing' then improving its optimisation is not a change
> to the API semantics.
> 
> Specifically, existing code then will, when upgraded to a newer libxl,
> behaviour differently - better, we hope.  Is that not the intent ?
> 
It is. I'm changing this all into something like "when you do not say
anything, libxl will place the domain `sensibly`, +/- as xend was
doing". We'll then add all the necessary interface and API in place
during 4.3.

> > So, like it is right now, the actual heuristics is implemented in xl, on
> > top of these placement candidate generation and manipulation facilities,
> > which I finally decided it was the way I liked this whole thing
> > most. :-)
> 
> So how much of the code currently in libxl should be reproduced in
> (say) libvirt ?
> 
Again, I agree. Let's leave this for now, but I'll definitely
investigate this later.

Thanks a lot for looking into this, :-)
Dario

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

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