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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/22/2015 11:49 AM, Dario Faggioli wrote:
> On Wed, 2015-07-22 at 11:32 -0400, Boris Ostrovsky wrote:
> > On 07/22/2015 10:50 AM, Dario Faggioli wrote:
> > > Yep. Exacty. As Boris says, this is a generic scheduling issue, although
> > > it's tru that it's only (as far as I can tell) with vNUMA that it bite
> > > us so hard...
> > I am not sure that it's only vNUMA. It's just that with vNUMA we can see
> > a warning (on your system) that something goes wrong. In other cases
> > (like scheduling, or sizing objects based on discovered cache sizes) we
> > don't see anything in the log but system/programs are making wrong
> > decisions.
> I'm not questioning that the guest scheduler is put in a position where
> it may make weird and/or wrong choices... Let's put it like this: on
> that very test box, or any other NUMA box I've worked on, I've never
> seen performance affected _so_much_ as in the vNUMA case.
>
> In fact, of course there are other issues (like the ones you're
> mentioning, caused by this), but it's only with vNUMA that I see 2 out
> of 4 vcpus completely lost! :-/
My guess would be that scheduling domains are not set up properly. And 
perhaps because your test has lots of IO the load doesn't cross 
thresholds that are needed to switch to another domain?
Does it work the same way if you run a compiled busy loop (i.e. 
'main(){while(1);}')?
> > (And your results above may well be the example of that)
> Right. BTW, the example was meant at investigating what you suggested,
> i.e., to have the guest topology follow the host topology. I tried, but
> could not achieve it. Do you think I'm doing something wrong? May it be
> that it's not always doable (ISTR Andrew saying that it is also the pcpu
> where the vcpus are created that matters)?
I don't think this is currently doable with what we have for CPUID 
support in xl syntax. I am pretty sure we need to at least be able to 
specify all leaf 4's indexes. And we can't.
BTW, irrespective of this particular problem, adding support for indexed 
CPUID leaves would be a good idea.
-boris

> I'm (re)asking because, if you think that's feasible, I can work on
> that. I fit's not, well... :-/

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