Re: [Xen-devel] [PATCH 1 of 3] libxl: take node distances into account during NUMA placement

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Dario Faggioli writes ("Re: [Xen-devel] [PATCH 1 of 3] libxl: take node 
distances into account during NUMA placement"):
> On Thu, 2012-10-18 at 16:17 +0100, George Dunlap wrote:
> > And now we're doing N^2 for each
> > candidate? 
> 
> Again, yes, but that is turning it from Ndoms*Nvcpus to
> Ndoms*Nvcpus+Nnodes^2, which is still dominated by the first term. IIRC,
> Andre tried to start >50 domains with 2 vCPUs on a 8 nodes system, which
> means 50*2 vs 8*8.

Are you sure about this calculation ?  It seems to me that 
doing Nnodes^2 for each candidate multiplies the cost by the number of
candidates, rather than adding Nnodes^2.

There are in the worst case nearly 2^Nnodes candidates (combinations
of nodes).  So the cost is
  <= 2^Nnodes * Nnodes^2

Your algorithm runs with up to 16 nodes.  Your change here increases
the worst-case cost from
  2^16 = 65556
to
  2^16 * 16^2 = 16777216

I don't think that's a good idea.

Ian.

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