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

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Date:	Wed, 15 Nov 2006 18:32:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <ak@...e.de>, Eric Dumazet <dada1@...mosbay.com>,
	akpm@...l.org, Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization


* Jeremy Fitzhardinge <jeremy@...p.org> 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.

i said this before: using segmentation tricks these days is /insane/. 
Segmentation is not for free, and it's not going to be cheap in the 
future. In fact, chances are that it will be /more/ expensive in the 
future, because sane OSs just make no use of them besides the trivial 
"they dont even exist" uses.

so /at a minimum/, as i suggested it before, the kernel's segment use 
should not overlap that of glibc's. I.e. the kernel should use %fs, not 
%gs.

	Ingo
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