Re: [patch 2/2] x86_64: Configure stack size

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 9 Nov 2007 13:19:11 -0800 (PST)
Christoph Lameter <clameter@sgi.com> wrote:

> On Fri, 9 Nov 2007, Andrew Morton wrote:
> 
> > I'm talking about software, not hardware.  I'd expect that you'll have
> > trouble talking RH/suse/etc into general shipping of an NR_CPUS=16384
> > kernel.
> 
> Yeah that is one reason why I removed all percpu arrays from the kernel 
> with the cpu_alloc patchset. I think we can get to a point where this does 
> not hurt that much. We want to be as close as possible to a distro kernel 
> as possible.
>  
> > > If I'm correct than I'd have thought that this will be a significant
> > problem for SGI, so we should find other solutions.
> 
> Maybe a special kernel from the distros is unavoidable but then they have 
> done that in the past for us too. Certainly we do not want to have the 
> kernel patches just for HPC apps. This is an option after all and not a 
> default. Mike Travis is working on reducing the per cpu overhead in the 
> x86_64 arch code. So we should be getting to a pretty good situation even 
> if we have to leave the cpumasks alone.
> 
> > > > So I'm wobbly.  Could we please examine the alternatives before proceeding?
> > > 
> > > This works fine with a 32k stack on IA64 with 4k processors.
> > 
> > yeah, but that's an order-1 allocation on ia64, not an order-3.
> 
> Well the default is also an order-1 allocation on x86_64. The order-3 
> alloc is not going to be that much of a problem if you have a system with 
> several terabytes of RAM.

Fair enough.

Did you consider making the stack size a calculated-in-Kconfig-arithmetic
thing rather than an offered-to-humans thing?  Derive it from CONFIG_NR_CPUS?

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