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

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

Am 20.07.2012 10:26, schrieb Andre Przywara:
> 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?
Hmm. I wouldn't be sure about this. I would guess there is a reason a guest has
more than 1 vcpu. Normally this is because the guest needs more power.

Having only 1 vcpu means this is enough. It probably would need only 0.1 vcpus.

A guest with many vcpus suffers much more from a heavy loaded node, as lock
contention in the guest is increasing with number of vcpus and waiting for a
vcpu holding a lock but being preempted.


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
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Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
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