Re: [PATCH 08 of 10 v2] libxl: enable automatic placement of guests on NUMA nodes

[Dario Faggioli <raistlin@linux.it>]

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On Tue, 2012-06-26 at 12:03 +0100, Ian Jackson wrote:
> George Dunlap writes ("Re: [Xen-devel] [PATCH 08 of 10 v2] libxl: enable automatic placement of guests on NUMA nodes"):
> > The only potential concern is that it seems likely that your
> > comparison function above will not actually generate an ordering,
> > because the relationship it generates isn't transitive.  That is, you
> > could have a situation where A > B, B > C, but C > A.  For example:
> > A: freemem 1000, domains 1
> > B: freemem 1090, domains 2
> > C: freemem 1110, domains 3
> > 
> > In this case, A>B, because memory is within 10% but has fewer domains.
> >  B > C, because B is within 10%, and has fewer domains.  But C > A,
> > because C has more than 10% more memory than A (so its domains are not
> > counted against it).
> 
> The conventional approach to this kind of thing is to invent a score
> for sorting etc.  Something like
>     score = tuning_parameter * log(freemem) - number_of_domains
> which does sort of roughly the same as your 10% rule if you squint.
> 
Right, I thought about this, but then got a bit scared about properly
mixing apple, oranges and melons (as I have number of nodes, amount of
free memory and number of domains), giving the proper "weight" to each
of them.

Anyway, I see the potential issue George is reporting, so I'll try to
come up with some formula...

Thanks and regards,
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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