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18日 16:10
> To: Wei Chen <Wei.Chen@xxxxxxx>
> Cc: nd <nd@xxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; George
> Dunlap <george.dunlap@xxxxxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau Monné
> <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Julien Grall
> <julien@xxxxxxx>
> Subject: Re: [PATCH v2 9/9] xen: introduce a Kconfig option to configure
> NUMA nodes number
> 
> >>>>> Sent: 2022年7月12日 22:34
> >>>>>
> >>>>> 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.
> >>>
> >>> The defaults are fine. But does it make sense to have 255 nodes when
> >>> just 32 CPUs were selected? I'm afraid kconfig is going to get in the
> >>> way, but I think I'd like the upper bound to be min(NR_CPUS, 255).
> >>
> >> Looking around NUMA nodes with 0 CPUs exists. So I don't think we
> should
> >> tie the two.
> >>
> >
> > Yes, some nodes can only have RAM and some nodes can only have CPUs.
> > So is it ok to use 2-255 for node range?
> 
> Personally I think we shouldn't allow unreasonably high node counts,
> unless proven by real hardware existing. Which goes hand in hand with
> me wanting the upper bound to be a power of 2 value, if for nothing
> else than a hint that using power-of-2 values here is preferable.
> Hence I'd like to propose 64 or 128 as upper bound, in this order of
> (my personal) preference.
> 

Thanks, I will use 64 in next version.

Wei Chen

> Jan