Messages in this thread | | | Date | Wed, 15 Nov 2006 09:59:24 -0800 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH] i386-pda UP optimization |
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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?
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