Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c -bisected

[Dave Jones]

On Tue, Aug 26, 2008 at 12:09:46PM -0700, Linus Torvalds wrote:

 > If you want the default kernel to support 4k cores, we'll need to fix the 
 > stack usage.  I don't think that is impossible, but IT IS NOT GOING TO 
 > HAPPEN for 2.6.27.
 > 
 > And quite frankly, if some vendor like RedHat enables NR_CPUS=4096 by 
 > default, they are totally and utterly crazy.

heh.  *picks through Fedora changelog*

* Thu Aug 14 2008 Dave Jones <davej@xxxxxxxxxx>
- Bump max cpus supported on x86-64 to 4096. Just to see what happens.

I never did get to find out unfortunatly, because of the security fiasco
in Fedora infrastructure the last week or two.
 
 > But some SGI-specific binary that is meant for SGI machines only, and has 
 > been extensively tested with the setup used on SGI machines is a different 
 > thing.

Every extra kernel image a distro vendor ends up shipping has an associated cost.

* build time: It currently takes about 2 hours for a set of Fedora RPMs.
  For RHEL it'll be even worse due to the extra archs).
  Killing off -smp specific builds was a big win for us in this regard.
  Adding extra flavours is always painful.

* diskspace (distro kernels aren't small. With the associated debugging symbols,
   they take up a shitload of disk space really fast).

* Having everyone running the same kernel makes it much easier to test/debug.
  Our QA guys hate adding extra columns to their test matrix.

But yes, for this to be even remotely feasible, there has to be a negligable
performance cost associated with it, which right now, we clearly don't have.
Given that the number of people running 4096 CPU boxes even in a few years time
will still be tiny, punishing the common case is obviously absurd.

	Dave

-- 
http://www.codemonkey.org.uk
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