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

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Date:	Wed, 15 Nov 2006 19:05:04 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	akpm@...l.org, Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization

On Wednesday 15 November 2006 18:59, Jeremy Fitzhardinge wrote:
> 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?

I have the feeling (most probably wrong, but I prefer to speak than keeping 
this for myself) that the cost of segment load is delayed up to the first use 
of a segment selector. Sort of a lazy reload...

I had this crazy idea while looking at oprofile numbers

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