* Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, Nov 19, 2013 at 08:09:04PM +0100, Ingo Molnar wrote:
> > The actual value of the limit - here's the on-stack cpumask sizes of
> > the candidate range:
> >
> > 128 CPUs: 16 byte cpumasks
> > 256 CPUs: 32 byte cpumasks
> > 512 CPUs: 64 byte cpumasks
>
> So 512 / 64bytes is a single cacheline and feels like a nice cut-off
> before requiring an extra indirection and more cachelines.
>
> 64 bytes also doesn't sound _that_ big to have on-stack.
The cacheline size itself isn't necessarily super meaningful for
on-stack variables: they are rarely cacheline aligned so they will
take part in two cachelines.
> So I'd go for having the cut-off on >512, unless of course theres
> evidence 64bytes is already too much.
I'm fine with that in any case, for the other reason I outlined: it's
the highest one and we can iterate down if it proves to be bad. If we
start out too low we'll probably never know it was too low.
Thanks,
Ingo
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