linux-kernel - Re: [PATCH 2/2] x86: Modify Kconfig to allow up to 4096 cpus

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Date:	Wed, 26 Mar 2008 18:09:24 +0200
From:	Adrian Bunk <bunk@...nel.org>
To:	Mike Travis <travis@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86: Modify Kconfig to allow up to 4096 cpus

On Tue, Mar 25, 2008 at 06:41:39PM -0700, Mike Travis wrote:
> Increase the limit of NR_CPUS to 4096 and introduce a boolean
> called "MAXSMP" which when set (e.g. "allyesconfig"), will set
> NR_CPUS = 4096 and NODES_SHIFT = 9 (512).


I'm not really getting the point of MAXSMP - people should simply pick 
their values, and when they want the maximum "(2-4096)" and "(1-15)" 
already provide this information (except that your patch hides the 
latter information from the user).

And with your patch, even with MAXSMP=y people could still set 
NR_CPUS=7 and NODES_SHIFT=15 or whatever else they want...

More interesting would be why you want it to set NODES_SHIFT to 
something less than the maximum value of 15. I'm getting the fact that
2^15 > 4096 and that 15 might be nonsensical high, but this sounds more 
like requiring a patch to limit the range to 9?


>...
> --- linux.trees.git.orig/arch/x86/Kconfig
> +++ linux.trees.git/arch/x86/Kconfig
> @@ -522,16 +522,24 @@ config SWIOTLB
>  	  access 32-bits of memory can be used on systems with more than
>  	  3 GB of memory. If unsure, say Y.
>  
> +config MAXSMP
> +	bool "Configure Maximum number of SMP Processors"
> +	depends on X86_64 && SMP
> +	default n
> +	help
> +	  Configure maximum number of CPUS for this architecture.
> +	  If unsure, say N.
>  
>  config NR_CPUS
> -	int "Maximum number of CPUs (2-255)"
> -	range 2 255
> +	int "Maximum number of CPUs (2-4096)"
> +	range 2 4096
>  	depends on SMP
> +	default "4096" if MAXSMP
>  	default "32" if X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000
>  	default "8"
>  	help
>  	  This allows you to specify the maximum number of CPUs which this
> -	  kernel will support.  The maximum supported value is 255 and the
> +	  kernel will support.  The maximum supported value is 4096 and the
>  	  minimum value which makes sense is 2.
>  
>  	  This is purely to save memory - each supported CPU adds
> @@ -918,12 +926,16 @@ config NUMA_EMU
>  	  number of nodes. This is only useful for debugging.
>  
>  config NODES_SHIFT
> -	int "Max num nodes shift(1-15)"
> +	int "Maximum NUMA Nodes (as a power of 2)"
>  	range 1 15  if X86_64
> +	default "9" if MAXSMP
>  	default "6" if X86_64
>  	default "4" if X86_NUMAQ
>  	default "3"
>  	depends on NEED_MULTIPLE_NODES
> +	help
> +	  Specify the maximum number of NUMA Nodes available on the target
> +	  system.  Increases memory reserved to accomodate various tables.
>  
>  config HAVE_ARCH_BOOTMEM_NODE
>  	def_bool y
> 

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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