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Re: [Xen-users] best practice NUMA config for dom0 ?

On Wed, 2016-05-18 at 16:29 +0100, Wei Liu wrote:
> CC Dario who implemented numa placement.
Thanks Wei.

> On Tue, May 10, 2016 at 05:40:44PM +0200, Håkon Alstadheim wrote:
> > 
> > It has been my understanding, without any documentation to back it
> > up,
> > that on a Xen server the hypervisor does all then NUMA handling,
> > and
> > that linux in dom0 or domU should keep its hands off. 
About the lack of documentation, there is something but I agree it is
not sufficient. I'll find some time to improve things.

About dom0 and domU keeping their hands off from NUMA handling, well, that's 
more than just true, as neither of them _can_, right now, do much about the 
NUMA-ness of the server.

The only thing that dom0 is in control of, wrt NUMA placement of guests, is 
whether or not to specify one in guests' config files (e.g., by specifying an 
hard or soft vcpu affinity). But that's it.

If no such hint is provided, xl and libxl will figure out an automatic 
placement for the new domain on the host NUMA nodes.

> > Now I notice that
> > xl is complaining:
> > 
> > libxl: notice: libxl_numa.c:499:libxl__get_numa_candidate: NUMA
> > placement failed, performance might be affected
> > 
Mmm.. well, the first thing to figure out is why placement is failing.
If you use `xl -vvv create ...` it should tell you more.

Also, it would be helpful if also you tell us the characteristics of
both the host and the guests, such as:
 - number of pCPUs
 - number of NUMA nodes
 - amount or RAM per node
 - amount of free RAM in each node, as reported by `xl info -n'
 - number of vCPUs of the guest
 - amount of RAM of the guest

> > So, given that dom0 is running a fairly recent kernel, and Xen is
> > the
> > latest stable (4.6.1), how should I configure the linux kernel for
> > best
> > numa handling ? I have two cpu-dies with ram that will benefit from
> > NUMA
> > aware allocation.
> > 
No special configuration is necessary. As a matter of fact, for now,
since we don't fully support virtual NUMA topology for dom0 and domUs,
you may well disable NUMA from both the dom0 and domU kernels.

But even if you don't (i.e., you leave it enabled/compiled in), that
should not hurt, and I don't think is in any way what is responsible
for the placement issue you're facing.

<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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