Re: [Qemu-devel] [PATCH v3] Allow setting NUMA distance for different NUMA nodes

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Mar 30, 2017 at 05:26:26PM +0200, Andrew Jones wrote:
> On Thu, Mar 30, 2017 at 12:00:48PM -0300, Eduardo Habkost wrote:
> > On Wed, Mar 29, 2017 at 02:30:38PM +0200, Andrew Jones wrote:
> > > On Wed, Mar 29, 2017 at 01:44:58PM +0200, Igor Mammedov wrote:
> > > > On Wed, 29 Mar 2017 16:50:00 +0800
> > > > He Chen <he.chen@linux.intel.com> wrote:
> > > > > > > +static void default_numa_distance(void)
> > > > > > > +{
> > > > > > > +    int src, dst;
> > > > > > > +  
> > > > > > fill in defaults only if there weren't any '-numa dist' on CLI
> > > > > > and refuse to start if partial filled table were explicitly provided on CLI
> > > > > >   
> > > > > I am afraid that I may have a bad function name here, fill_numa_distance
> > > > > might be a better name?
> > > > > 
> > > > > Also, since the distance can be asymmetric, IMHO, providing a partial
> > > > > filled table looks fine. If we set many NUMA nodes e.g. 8 nodes for
> > > > > guest, the whole filled table would require many command lines which
> > > > > might be inconvenient and less flexible.
> > > > asymmetric doesn't imply sparse, so one has to specify full matrix
> > > > it might be inconvenient /long/ but is very flexible.
> > > 
> > > This makes me realize that a user only inputting one of A -> B or B -> A
> > > command line inputs doesn't imply symmetry.  It could be that the user
> > > just forgot to input the opposite.  To avoid the ambiguity we either need
> > > to force both to be input (as it was before I suggested otherwise), or
> > > add a '-numa symmetric' type of property to enable the shortcut.  I guess
> > > we should just avoid the shortcut, at least for now.
> > 
> > Is protecting the user from one very specific (and very rare[1])
> > mistake a good reason for making the automatic default less
> > useful?
> 
> Maybe not, but creating new interfaces with known ambiguities isn't ideal
> either.
> 
> > Requiring an explicit '-numa symmetric' option to enable
> > the automatic default seems to defeat the purpose of having an
> > automatic default, to me.
> 
> We could reverse it, '-numa asymmetric' could turn off the defaults
> and abort on an incomplete table.

I doubt anybody would ever use that option, either.

> 
> Or, we could leave it ambiguous, but improve the heuristic by requiring
> all directions be given if even one direction has an asymmetric reverse
> path given. With that, there's still a chance to let user error slip
> through, but much less. I could live with that.

Sounds good to me.

-- 
Eduardo