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.orgIn 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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