linux-kernel - Re: [PATCH] i386-pda UP optimization

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Date:	Wed, 15 Nov 2006 19:01:42 +0100
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	Eric Dumazet <dada1@...mosbay.com>, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization

On Wed, 2006-11-15 at 09:28 -0800, Jeremy Fitzhardinge wrote:
> Ingo Molnar wrote:
> > Eric's test shows a 5% slowdown. That's far from cheap.
> >   
> 
> It seems like an absurdly large difference.  PDA references aren't all
> that common in the kernel; for the %gs prefix on PDA accesses to be
> causing a 5% overall difference in a test like this means that the
> prefixes would have to be costing hundreds or thousands of cycles, which
> seems absurd.  Particularly since Eric's patch doesn't touch head.S, so
> the %gs save/restore is still being executed.


segment register accesses really are not cheap. 
Also really it'll be better to use the register userspace is not using,
but we had that discussion before; could you remind me why you picked 
%gs in the first place?


-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
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