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

On Friday 22 October 2010 03:22:02 Sven Joachim wrote:
> 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.

  This is good to know.  When I built my own 2.6.26, I also
noticed that the maximum value offered by the configurator
was 256 -- we'll likely be seeing systems with that many cores
within a few years, if current trends continue.

> 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.

				-- A.
Andrew Reid / reidac@bellatlantic.net

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