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

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Message-ID: <455B5B55.20803@goop.org>
Date:	Wed, 15 Nov 2006 10:24:21 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Arjan van de Ven <arjan@...radead.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

Arjan van de Ven wrote:
> 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?
>   

To leave open the possibility of using the compiler's TLS support in the
kernel for percpu.  I also measured the cost of reloading %gs vs %fs,
and found no difference between reloading a null selector vs a non-null
selector.

    J

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