Re: [PATCH 1 of 3 v5/leftover] libxl: enable automatic placement of guests on NUMA nodes

[Dario Faggioli <raistlin@linux.it>]

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On Thu, 2012-07-19 at 16:22 +0200, Dario Faggioli wrote:
> Interesting. That's really the kind of testing we need in order to
> fine-tune the details. Thanks for doing this.
> 
> > Then I started 32 guests, each 4 vCPUs and 1 GB of RAM.
> > Now since the code prefers free memory so much over free CPUs, the 
> > placement was the following:
> > node0: guests 2,5,8,11,14,17,20,25,30
> > node1: guests 21,27
> > node2: none
> > node3: none
> > node4: guests 1,4,7,10,13,16,19,23,29
> > node5: guests 24,31
> > node6: guests 3,6,9,12,15,18,22,28
> > node7: guests 26,32
> > 
> > As you can see, the nodes with more memory are _way_ overloaded, while 
> > the lower memory ones are underutilized. In fact the first 20 guests 
> > didn't use the other nodes at all.
> > I don't care so much about the two memory-less nodes, but I'd like to 
> > know how you came to the magic "3" in the formula:
> > 
> > > +
> > > +    return sign(3*freememkb_diff + nrdomains_diff);
> > > +}
> > 
>
> That all being said, this is the first time the patchset had the chance
> to run on such a big system, so I'm definitely open to suggestion on how
> to make that formula better in reflecting what we think it's The Right
> Thing!
> 
Thinking more about this, I realize that I was implicitly assuming some
symmetry in the amount of memory each nodes comes with, which is
probably something I shouldn't have done...

I really am not sure what to do here, perhaps treating the two metrics
more evenly? Or maybe even reverse the logic and give nr_domains more
weight?

I was also thinking whether it could be worthwhile to consider the total
number of vcpus on a node instead than the number of domain, but again,
that's not guaranteed to be any more meaningful (suppose there are a lot
of idle vcpus)...

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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