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

[Eduardo Habkost]

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 <address@hidden> 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? Requiring an explicit '-numa symmetric' option to enable
the automatic default seems to defeat the purpose of having an
automatic default, to me.

I bet people would just specify the distances for both
directions, instead of having to read the documentation for
"-numa symmetric" to be sure if that's exactly what they want.

[1] I mean: forgetting an option isn't rare, but asymmetric NUMA
    distances are so rare that most people who reviewed those
    patches were surprised when they learned that the specs allow
    asymmetric distances.

-- 
Eduardo