Re: [Xen-devel] NUMA TODO-list for xen-devel

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.08.12 at 18:36, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Thu, Aug 2, 2012 at 2:34 PM, Dario Faggioli <raistlin@xxxxxxxx> wrote:
>> On Thu, 2012-08-02 at 10:43 +0100, Jan Beulich wrote:
>>> >>> On 01.08.12 at 18:16, Dario Faggioli <raistlin@xxxxxxxx> wrote:
>>> >     - Virtual NUMA topology exposure to guests (a.k.a guest-numa). If a
>>> >       guest ends up on more than one nodes, make sure it knows it's
>>> >       running on a NUMA platform (smaller than the actual host, but
>>> >       still NUMA). This interacts with some of the above points:
>>>
>>> The question is whether this is really useful beyond the (I would
>>> suppose) relatively small set of cases where migration isn't
>>> needed.
>>>
>> Mmm... Not sure I'm getting what you're saying here, sorry. Are you
>> suggesting that exposing a virtual topology is not a good idea as it
>> poses constraints/prevents live migration?
>>
>> If yes, well, I mostly agree that this is an huge issue, and that's why
>> I think wee need some bright idea on how to deal with it. I mean, it's
>> easy to make it optional and let it automatically disable migration,
>> giving users the choice what they prefer, but I think this is more
>> dodging the problem than dealing with it! :-P
>>
>>> >        * consider this during automatic placement for
>>> >          resuming/migrating domains (if they have a virtual topology,
>>> >          better not to change it);
>>> >        * consider this during memory migration (it can change the
>>> >          actual topology, should we update it on-line or disable memory
>>> >          migration?)
> 
> I think we could use cpu hot-plug to change the "virtual topology" of
> VMs, couldn't we?  We could probably even do that on a running guest
> if we really needed to.

Hmm, not sure - using hotplug behind the back of the guest might
be possible, but you'd first need to hot-unplug the vCPU. That's
something that I don't think you can do on HVM guests (and for
PV guests, guest visible NUMA support makes even less sense
than for HVM ones).

Jan


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