Re: [Xen-devel] [PATCH RFC v2 5/7] libxl/vNUMA: VM config parsing functions

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Dario Faggioli writes ("Re: [Xen-devel] [PATCH RFC v2 5/7] libxl/vNUMA: VM 
config parsing functions"):
> On ven, 2013-10-11 at 12:18 +0100, Ian Jackson wrote:
> > Aren't there some constraints that need to be imposed ?  For example,
> > distance[X,Y]==distance[Y,X] ?  What about the triangle inequality
> > (distance[A,B] + distance[B,C] >= distance[A,C]) ?
> > 
> I think Linux does some sanity checking of the distance table (and I
> think it disables NUMA if finding out something weird, Elena?). However,
> this (what Linux expects/checks for) shouldn't really be the only
> criterion, since although Linux is the only current implementation,
> there is no reason why this can't be implemented by other OSes.

I wasn't really asking about Linux.  I was talking about NUMA systems
in general.  The thing which prompted me to ask is simply that I want
to be sure that the set of checks we perform has been deliberately
chosen, to reflect the actual potential nature of NUMA systems.

You have chosen to call this parameter "distance".  In mathematics, a
distance would normally have both the symmetry and triangle
inequality, as well as a number of other properties:
    https://en.wikipedia.org/wiki/Metric_space#Definition

What subset of those properties are true of NUMA distances ?  (I
assume that these NUMA distances are estimates to be used by
heuristics, rather than actual measurements of a specific underlying
property.)

> > If we do need this I think the nested lists are probably a better
> > syntax for specifying the whole array.  
> 
> Agreed. I actually wanted to say the same. Would something like this be
> ok?
> 
> distances = [ [10, 20], [20, 10] ]

Right.  I don't think there is anything preventing us implementing
this in the parser, although the existing code probably doesn't
support it.  I can help with the parser.

...
> distances = [ [10, 20, 30, 40],
>                   [10, 20, 30],
>                       [10, 20],
>                           [10] ]
> 
> (I.e., "distances = [ [10, 20, 30, 40], [10, 20, 30], [10, 20], [10] ])

I'd suggest that we should expect the user to specify the lower left
half, rather than the upper right.  That avoids the counterintuitive
offset of the indices in subsequent rows.

(Also I observe that your example violates distance[A,B] == 0.)

Ian.

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