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SubjectRe: [patch 1/5] sched: remove degenerate domains
Ingo Molnar wrote:
> * Nick Piggin <> wrote:
>>This is Suresh's patch with some modifications.
>>Remove degenerate scheduler domains during the sched-domain init.
> actually, i'd suggest to not do this patch. The point of booting with a
> CONFIG_NUMA kernel on a non-NUMA box is mostly for testing, and the
> 'degenerate' toplevel domain exposed conceptual bugs in the
> sched-domains code. In that sense removing such 'unnecessary' domains
> inhibits debuggability to a certain degree. If we had this patch earlier
> we'd not have experienced the wrong decisions taken by the scheduler,
> only on the much rarer 'really NUMA' boxes.

True. Although I'd imagine it may be something distros may want.
For example, a generic x86-64 kernel for both AMD and Intel systems
could easily have SMT and NUMA turned on.

I agree with the downside of exercising less code paths though.

What about putting as a (default to off for 2.6) config option in
the config embedded menu?

> is there any case where we'd want to simplify the domain tree? One more
> domain level is just one (and very minor) aspect of CONFIG_NUMA - i'd
> not want to run a CONFIG_NUMA kernel on a non-NUMA box, even if the
> domain tree got optimized. Hm?

I guess there is the SMT issue too, and even booting an SMP kernel
on a UP system. Also small ia64 NUMA systems will probably have one
redundant NUMA level.

If/when topologies get more complex (for example, the recent Altix
discussions we had with Paul), it will be generally easier to set
up all levels in a generic way, then weed them out using something
like this, rather than put the logic in the domain setup code.


SUSE Labs, Novell Inc.

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