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Date: Sat, 30 Dec 2000 14:24:06 +0100 (CET)
From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Linus Torvalds <torvalds@transmeta.com>
cc: Andi Kleen <ak@suse.de>, Marcelo Tosatti <marcelo@conectiva.com.br>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: test13-pre5
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On Thu, 28 Dec 2000, Linus Torvalds wrote:
> On Thu, 28 Dec 2000, Andi Kleen wrote:
> > - Instead of having a zone pointer mask use a 8 or 16 byte index into a 
> > zone table. On a modern CPU it is much cheaper to do the and/shifts than
> > to do even a single cache miss during page aging. On a lot of systems 
> > that zone index could be hardcoded to 0 anyways, giving better code.
> > - Instead of using 4/8 bytes for the age use only 16bit (FreeBSD which
> > has the same swapping algorithm even only uses 8bit) 
> 
> This would be good, but can be hard.
> 
> FreeBSD doesn't try to be portable any more, but Linux does, and there are
> architectures where 8- and 16-bit accesses aren't atomic but have to be
> done with read-modify-write cycles.
> 
> And even for fields like "age", where we don't care whether the age itself
> is 100% accurate, we _do_ care that the fields close-by don't get strange
> effects from updating "age". We used to have exactly this problem on alpha
> back in the 2.1.x timeframe.
> 
> This is why a lot of fields are 32-bit, even though we wouldn't need more
> than 8 or 16 bits of them.

What about defining new types for this? Like e.g. `x8', being `u8' on platforms
were that's OK, and `u32' on platforms where that's more efficient?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds

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