Messages in this thread | | | From | Eric Dumazet <> | Subject | Re: [PATCH] i386-pda UP optimization | Date | Wed, 15 Nov 2006 19:05:04 +0100 |
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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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