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Re: [Xen-devel] Fwd: [tip:x86/urgent] x86, sched: Allow topologies where NUMA nodes share an LLC

On Tue, 2018-04-17 at 15:08 +0100, Andrew Cooper wrote:
> On 17/04/18 15:02, Juergen Gross wrote:
> > Is this something we should be aware of in Xen, too?
> If we had something close to a working topology representation,
> probably.
True, as far as letting the guest know about these details (in
appropriate circumnstaces) goes.

About using it _within_ Xen, well:

"Intel's Skylake Server CPUs have a different LLC topology than
generations. When in Sub-NUMA-Clustering (SNC) mode, the package is
into two "slices", each containing half the cores, half the LLC, and
memory controller and each slice is enumerated to Linux as a NUMA
node. This is similar to how the cores and LLC were arranged for the
Cluster-On-Die (CoD) feature."

This looks similar (but not identical) to, e.g., AMD EPYC. If Xen also
sees each slice as a NUMA node as well, we already are doing something
(although, of course, we can always improve).

If not, I see ways of taking advantage of the information that not all
the cores in the socket equally share the LLC, e.g., in Credit2, by
providing a per-LLC runqueue option and/or by taking that into account
in the 'migration resistance' mechanisms (there is already work in that

<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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