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SubjectRE: [NFS] Re: [PATCH] zerocopy NFS for 2.5.36
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dude, that's pretty cool.

if you were re-implementing XDR, you think a series of movl
instructions would be best? i'm not sure how practical that
is for an architecture-independent implementation.

> > > It was discussed long ago that csum_and_copy_from_user() performs
> > > better than plain copy_from_user() on x86. I do not remember all
> >
> > The better was a freak of PPro/PII scheduling I think
> >
> > > details, but I do know that using copy_from_user() is not a real
> > > improvement at least on x86 architecture.
> >
> > The same as bit is easy to explain. Its totally memory bandwidth
> > limited on current x86-32 processors. (Although I'd welcome
> > demonstrations to the contrary on newer toys)
>
> Nope. There are distinct alignment problems with movsl-based
> memcpy on PII and (at least) "Pentium III (Coppermine)",
> which is tested here:
>
> copy_32 uses movsl. copy_duff just uses a stream of "movl"s
>
> Time uncached-to-uncached memcpy, source and dest are 8-byte-aligned:
>
> akpm:/usr/src/cptimer> ./cptimer -d -s
> nbytes=10240 from_align=0, to_align=0
> copy_32: copied 19.1 Mbytes in 0.078 seconds at 243.9 Mbytes/sec
> __copy_duff: copied 19.1 Mbytes in 0.090 seconds at 211.1 Mbytes/sec
>
> OK, movsl wins. But now give the source address 8+1 alignment:
>
> akpm:/usr/src/cptimer> ./cptimer -d -s -f 1
> nbytes=10240 from_align=1, to_align=0
> copy_32: copied 19.1 Mbytes in 0.158 seconds at 120.8 Mbytes/sec
> __copy_duff: copied 19.1 Mbytes in 0.091 seconds at 210.3 Mbytes/sec
>
> The "movl"-based copy wins. By miles.
>
> Make the source 8+4 aligned:
>
> akpm:/usr/src/cptimer> ./cptimer -d -s -f 4
> nbytes=10240 from_align=4, to_align=0
> copy_32: copied 19.1 Mbytes in 0.134 seconds at 142.1 Mbytes/sec
> __copy_duff: copied 19.1 Mbytes in 0.089 seconds at 214.0 Mbytes/sec
>
> So movl still beats movsl, by lots.
>
> I have various scriptlets which generate the entire matrix.
>
> I think I ended up deciding that we should use movsl _only_
> when both src and dsc are 8-byte-aligned. And that when you
> multiply the gain from that by the frequency*size with which
> funny alignments are used by TCP the net gain was 2% or something.
>
> It needs redoing. These differences are really big, and this
> is the kernel's most expensive function.
>
> A little project for someone.
>
> The tools are at http://www.zip.com.au/~akpm/linux/cptimer.tar.gz
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