Re: [Xen-devel] [PATCH 2/3] libxl: automatically set soft affinity after vnuma info

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Mar 24, 2015 at 03:06:24PM +0000, Dario Faggioli wrote:
> On Tue, 2015-03-24 at 14:32 +0000, Wei Liu wrote:
> > On Tue, Mar 24, 2015 at 02:42:15PM +0100, Dario Faggioli wrote:
> > > More specifically, vcpus are assigned to a vnode, which in
> > > turn is associated with a pnode. If a vcpu does not have any
> > > soft affinity, automatically build up one, matching the pcpus
> > > of the said pnode.
> > > 
> > 
> > What about hard affinity? Do we need to generate hard affinity map as
> > well? What's your thought on this?
> > 
> I think we should not do that. Hard affinity has a bigger impact on the
> system performance and overall configuration than soft, which is really
> just a preference, so I would let the user deal with that explicitly.
> 
> For example, setting the hard affinity to the same set of pcpus for
> multiple domains could, depending on the system load, prevent some of
> (the vcpus of) these domain to be able to execute for quite some time,
> while there are idle pcpus in the system. This may well be something the
> user wants, but I'd let him say so, if that is the case, rather than
> automatically setup things like that for everyone! By only affecting
> soft affinity, that doesn't happen, and we stay work conserving by
> default, which is important.
> 
> Also, only touching soft affinity is exactly what automatic NUMA
> placement already does, which in turn is consistent with using the
> result of placement to influence the domain's node affinity (i.e., what
> Xen 4.3 and 4.4 do, since soft affinity was not there), and it is the
> best solution, IMO.
> 
> Perhaps we can have another config option, telling whether one wants
> vnuma (and/or automatic placement) to affect soft or hard affinity (or
> both). I'm not sure how much this would be useful, while it would
> certainly make things more complex from both the implementation and the
> interface point of view (the latter being the more concerning aspect).
> In any case, we can add this at any point in future, if we feel it could
> be useful, and I still think the default setting --if such option is not
> specified-- should remain "affect soft affinity only".

Right. Thanks for the explanation. Regarding new option, we can add it
later if it turns out to be necessary.

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

> 
> > The code itself looks good to me.
> > 
> Thanks for looking at it. :-)
> 
> Regards,
> Dario



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