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

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Message-ID: <20080326165554.GD1789@cs181133002.pp.htv.fi>
Date:	Wed, 26 Mar 2008 18:55:54 +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 Wed, Mar 26, 2008 at 09:31:22AM -0700, Mike Travis wrote:
> Adrian Bunk wrote:
> > 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?
> 
> I guess the main effect is that "MAXSMP" represents what's really
> usable for an architecture based on other factors.  The limit of
> NODES_SHIFT = 15 is that it's represented in some places as a signed
> 16-bit value, so 15 is the hard limit without coding changes, not
> an architecture limit.


This is the x86-specific Kconfig file that presents the x86 specific 
limits to the users.

If NODES_SHIFT=15 is offered to the user although it's higher than the 
current architecture limit on x86 then this is simply a bug that should 
be fixed.


> Thanks,
> Mike

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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