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

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Message-ID: <455B557C.7020602@goop.org>
Date:	Wed, 15 Nov 2006 09:59:24 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
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

Ingo Molnar wrote:
> 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.
>   

Many, many systems use %fs/%gs to implement some kind of thread-local
storage, and such usage is becoming more common; the PDA's use of it in
the kernel is no different.  I would agree that using all the obscure
corners of segmentation is just asking for trouble, but using %gs as an
address offset seems like something that's going to be efficient on x86
32/64 processors indefinitely.

> 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.

Last time you raised this I did a pretty comprehensive set of tests
which showed there was flat out zero difference between using %fs and
%gs.  There doesn't seem to be anything to the theory that reloading a
null segment selector is in any way cheaper than loading a real
selector.  Did you find a problem in my methodology?

    J
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