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Re: Debian stock kernel config -- CONFIG_NR_CPUS=32?

On 2010-10-22 03:15 +0200, Andrew Reid wrote:

>   I recently deployed some new many-core servers at work, with
> 48 cores each (4x 12 core AMD 6174s), and ran into an issue where
> the stock Debian kernel is compiled with CONFIG_NR_CPUS=32,
> meaning it will only use the first 32 cores that it sees.

For the record, CONFIG_NR_CPUS has been increased to 512 (the maximum
supported upstream) in Squeeze.

>   For old Debian hands like me, this is an easy fix, I just built 
> a new kernel configured for more cores, and it works just fine.
>   But I'm curious if anyone on the list knows the rationale for
> distributing kernels with this set to 32.  Is that just a 
> reasonable number that's never been updated?  Or is there some
> complication that arises after 32 cores, and should I be more
> careful about tuning other parameters?

Basically, 32 is chosen a bit arbitrarily.  But there are some problems
with high values of CONFIG_NR_CPUS:

- each supported CPU adds about eight kilobytes to the kernel image,
  wasting memory on most machines.

- On Linux 2.6.28 (maybe all kernels prior to 2.6.29), module size blows
  up: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=516709.


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