Messages in this thread | | | Date | Tue, 25 Jun 2002 12:47:20 -0700 | From | Andrew Morton <> | Subject | Re: efficient copy_to_user and copy_from_user routines in Linux Kernel |
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kuznet@ms2.inr.ac.ru wrote: > > Hello! > > > I changed tcp to use a different copy if either source or dest were > > not eight-byte aligned, and found that the resulting improvement > > across a mixed networking load was only 1%. Your numbers are higher, > > so perhaps there are different alignments in the mix... > > Did you look at sender or changed both of the functions?
I changed it to use csum_copy_from_user() instead of copy_from_user() if the source and dest weren't 8-byte aligned. No other changes in there.
> After that accident TCP was changed and it does not use copy_from_user more, > it does copy_and_csum even when no checksum is required. So, his results > on sender side (except for strange anomaly at msg size 8K) just confirm > nil effect of copy_from_user.
Yup.
> What's about copy_to_user, we forgot about this at all, > worrying mostly about sender side. :-)
We didn't really forget, but we were trying to get a 2.4 kernel out, so it became a "fix in 2.5" item. You're right, we should fix it in 2.4.
I wrote a little app to test this - it times a couple of copy algorithms at all possible alignments. It may be useful for someone... http://www.zip.com.au/~akpm/linux/cptimer.tar.gz I think it covers everything - uncached/cache source/dest, all possible transfer alignemnts.
The cost of getting it wrong is, iirc, 40% slowdown. In the kernel's single most expensive function.
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