RE: [PATCH v2 9/9] xen: introduce a Kconfig option to configure NUMA nodes number

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Jan,

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 2022年7月12日 22:34
> To: Wei Chen <Wei.Chen@xxxxxxx>
> Cc: nd <nd@xxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; George
> Dunlap <george.dunlap@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Stefano
> Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau Monné
> <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2 9/9] xen: introduce a Kconfig option to configure
> NUMA nodes number
> 
> On 08.07.2022 16:54, Wei Chen wrote:
> > --- a/xen/arch/Kconfig
> > +++ b/xen/arch/Kconfig
> > @@ -17,3 +17,14 @@ config NR_CPUS
> >       For CPU cores which support Simultaneous Multi-Threading or
> similar
> >       technologies, this the number of logical threads which Xen will
> >       support.
> > +
> > +config NR_NUMA_NODES
> > +   int "Maximum number of NUMA nodes supported"
> > +   range 1 255
> > +   default "64"
> > +   depends on NUMA
> 
> Does 1 make sense? That's not going to be NUMA then, I would say.
> 

Ok, we need at least 2 nodes to be a real NUMA.

> Does the value being (perhaps far) larger than NR_CPUS make sense?
> 

Arm has 128 default NR_CPUS (except some platforms) and x86 has 256.
So I am not very clear about your comments about far larger? As my
Understanding, one node has 2 or 4 cores are very common in a NUMA
System.

> Why does the range end at a not-power-of-2 value? (I was actually
> going to suggest to have a shift value specified here, until
> spotting that NODES_SHIFT isn't used anywhere else, and hence
> you're rightfully deleting it.)
> 

I think we have discussed about the 255 in v1. Because Xen is using
u8 as nodeid_t, so 255 may be a upper bound.

And if use a shift value, from a user perspective, I don't like it.
It needs to be converted, not intuitive enough. It also limits my
input range, even though my numerical values are reasonable. Yes,
if a machine has 15 node, we can ask them to input 16, but why not
let the users decide? instead of being forced to enter 16 by the program?

> > +   help
> > +     Controls the build-time size of various arrays and bitmaps
> > +     associated with multiple-nodes management. It is the upper bound
> of
> > +     the number of NUMA nodes that the scheduler, memory allocation and
> > +          other NUMA-aware components can handle.
> 
> Nit: indentation.
> 

Ok.

Cheers,
Wei Chen

> Jan