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Re: [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest

On Fri, 2015-07-17 at 07:09 +0100, Jan Beulich wrote:
> >>> On 16.07.15 at 18:59, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > And in general (both for PV and HVM) --- is there any reason to expose 
> > CPU topology at all? I can see it being useful if VCPUs are pinned but 
> > if they are not then it can make performance worse.
> Indeed 
Indeed indeed. :-)

And in fact, this is even independent from vNUMA. Yet, I remember we
were discussing about this since the beginning of vNUMA work, back when
it was Elena doing it, but the it seems we all forgot... Sorry for
that! :-/

I seriously think we should do something about this as, while in a non
vNUMA setup it can certainly cause weird/inconsistent performance, in a
vNUMA one, as shown, it's quite a hige mess.

> - that's what our kernels have been doing for years, and
> it seems like someone over here is now looking into whether this
> could be done in pv-ops too (without too much uglification).
That would be great, IMO. I'd be up for helping with this, but I know
next to nothing about CPUID, so that would require some setup time. If,
at least, you could keep me in the loop it would be great.

In the meanwhile, what should we do? Document this? How? "don't use
vNUMA with PV guest in SMT enabled systems" seems a bit harsh... Is
there a workaround we can put in place/suggest?

<<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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