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SubjectRe: [PATCH] i386-pda UP optimization

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> > Eric's test shows a 5% slowdown. That's far from cheap.
>
> It seems like an absurdly large difference. PDA references aren't all
> that common in the kernel; for the %gs prefix on PDA accesses to be
> causing a 5% overall difference in a test like this means that the
> prefixes would have to be costing hundreds or thousands of cycles,
> which seems absurd. Particularly since Eric's patch doesn't touch
> head.S, so the %gs save/restore is still being executed.

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.

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.

Ingo
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