linux-kernel - Re: [RFC] CPUMASK: proposal for replacing cpumask

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Date:	Fri, 12 Sep 2008 15:58:14 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mike Travis <travis@....com>
CC:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	davej@...emonkey.org.uk, David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jack Steiner <steiner@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jes Sorensen <jes@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC] CPUMASK: proposal for replacing cpumask_t

Mike Travis wrote:
> 
> Here's the thread:
> 
> 	http://marc.info/?l=linux-kernel&m=121976977901223&w=4
> 
> It doesn't seem worthwhile to force all systems to deal with large cpumask's
> if they don't need to.  Passing the value on the stack (actually usually in a
> reg) if it fits into a long makes a lot of sense.
> 
> And I don't think it's that abstract, but I'm willing to hear other opinions.
> 
> Btw, most likely only distros that distribute an "Enterprise" edition of
> Linux will ever set NR_CPUS so high, so the actual number of systems making
> use of this will be a very small percentage (big $$-wise though... ;-) 
> 
> I even think that perhaps BITS_PER_LONG might be too low a threshold to kick
> in this extra code.  A Larabee chip will have 128 cpus so maybe 128 or 256 is
> a better metric...?
> 
> As soon as I get a working kernel with the proposed changes, I will definitely
> be doing perf testing.
> 

If the performance difference isn't significant, then there is a major 
advantage to getting rid of a configuration option.  At that point we 
can basically scale to an arbitrary number of processors in a stock 
configuration.

	-hpa

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