Re: [PATCH] arm64: NUMA: Kconfig: Increase max number of nodes

[Robin Murphy]

On 2020-10-21 12:02, Jonathan Cameron wrote:
> On Wed, 21 Oct 2020 09:43:21 +0530
> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>
> > On 10/20/2020 11:39 PM, Valentin Schneider wrote:
> > > Hi,
> > >
> > > Nit on the subject: this only increases the default, the max is still 2¹⁰.
> >
> > Agreed.
> >
> > > On 20/10/20 18:34, Vanshidhar Konda wrote:
> > > > The current arm64 max NUMA nodes default to 4. Today's arm64 systems can
> > > > reach or exceed 16. Increase the number to 64 (matching x86_64).
> > > >
> > > > Signed-off-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> > > > ---
> > > >   arch/arm64/Kconfig | 2 +-
> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index 893130ce1626..3e69d3c981be 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -980,7 +980,7 @@ config NUMA
> > > >   config NODES_SHIFT
> > > >        int "Maximum NUMA Nodes (as a power of 2)"
> > > >        range 1 10
> > > > -	default "2"
> > > > +	default "6"
> > >
> > > This leads to more statically allocated memory for things like node to CPU
> > > maps (see uses of MAX_NUMNODES), but that shouldn't be too much of an
> > > issue.
> >
> > The smaller systems should not be required to waste those memory in
> > a default case, unless there is a real and available larger system
> > with those increased nodes.
> >
> > > AIUI this also directly correlates to how many more page->flags bits are
> > > required: are we sure the max 10 works on any aarch64 platform? I'm
> >
> > We will have to test that. Besides 256 (2 ^ 8) is the first threshold
> > to be crossed here.
> >
> > > genuinely asking here, given that I'm mostly a stranger to the mm
> > > world. The default should be something we're somewhat confident works
> > > everywhere.
> >
> > Agreed. Do we really need to match X86 right now ? Do we really have
> > systems that has 64 nodes ? We should not increase the default node
> > value and then try to solve some new problems, when there might not
> > be any system which could even use that. I would suggest increase
> > NODES_SHIFT value upto as required by a real and available system.
>
> I'm not going to give precise numbers on near future systems but it is public
> that we ship 8 NUMA node ARM64 systems today.  Things will get more
> interesting as CXL and CCIX enter the market on ARM systems,
> given chances are every CXL device will look like another NUMA
> node (CXL spec says they should be presented as such) and you
> may be able to rack up lots of them.
>
> So I'd argue minimum that makes sense today is 16 nodes, but looking forward
> even a little and 64 is not a great stretch.
> I'd make the jump to 64 so we can forget about this again for a year or two.
> People will want to run today's distros on these new machines and we'd
> rather not have to go around all the distros asking them to carry a patch
> increasing this count (I assume they are already carrying such a patch
> due to those 8 node systems)

Nit: I doubt any sane distro is going to carry a patch to adjust the 
*default* value of a Kconfig option. They might tune the actual value in 
their config, but, well, isn't that the whole point of configs? ;)

Robin.

> Jonathan
>
> > >    
> > > >        depends on NEED_MULTIPLE_NODES
> > > >        help
> > > >          Specify the maximum number of NUMA Nodes available on the target
> > >   
> >
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>
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