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

[Andre Przywara <andre.przywara@xxxxxxx>]

On 07/20/2012 10:20 AM, Dario Faggioli wrote:
> 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 replaced the 3 with 1 already, that didn't change so much. I think we 
should kind of reverse the importance of node load, since starving for 
CPU time is much worse than bad memory latency. I will do some 
experiments...
> 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)...
Right, that was my thinking on the ride to work also ;-)
What about this: 1P and 2P guests really use their vCPUs, but for bigger 
guests we assume only a fractional usage?
Andre

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12


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