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

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Date:	Thu, 16 Nov 2006 16:24:53 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Arjan van de Ven <arjan@...radead.org>, 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

Ingo Molnar wrote:
> what point would there be in using it? It's not like the kernel could 
> make use of the thread keyword anytime soon (it would need /all/ 
> architectures to support it) ...

The plan was to implement the x86 arch-specific percpu stuff to use it,
since it allows gcc better optimisation opportunities.

>  and the kernel doesnt mind how the 
> current per_cpu() primitives are implemented, via assembly or via C. In 
> any case, it very much matters to see the precise cost of having the pda 
> selector value in %gs versus %fs.
>   

Hm, well, unfortunately for me, there is a small but distinct advantage
to using %fs rather than %gs (around 0-5ns per iteration).  The notable
exception being the "AMD-K6(tm) 3D+ Processor", where %gs is about 25%
(15ns) faster.

I'll revise the patches to use %fs and resubmit.

    J

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