Copy prefetch optimizations and non-coherent caches

[Eugene Surovegin]

At 06:20 AM 9/6/2003, Paul Mackerras wrote:
> > I think we should disable prefetch if CONFIG_NONCOHERENT_CACHE is defined.
> > Other more complex solutions are possible, e.g. we can still prefetch our
> > own buffer but don't touch anything outside (I'll try to do some
> > performance testing to determine whether it's worth the effort :).

I did some simple testing on Ebony, and it turns out that read prefetching
gives significant performance boost (~30%).

>The measurements I did on a ppc64 kernel indicated that most
>copy_tofrom_user calls were either for relatively small buffers
>(i.e. less than 256 bytes) or were page-sized and page-aligned.
>Therefore I did two routines, one optimized for small copies that
>didn't use any prefetching or dcbz's, and one optimized for page-sized
>copies.
>
>We could do something similar on ppc32 - we could do the small copy
>case with no prefetching (or maybe we could just prefetch on the first
>cache line), plus a page-copy case that does prefetching.  If you know
>you are doing exactly one page, it shouldn't be hard to set up the
>prefetching so you don't prefetch past the end of the source buffer.
>In fact it should be possible to code up a relatively simple optimized
>copy loop that avoids prefetching outside the source region if we just
>assume that the source and destination addresses are
>cacheline-aligned, and the size is a multiple of the cacheline size
>and is at least 8 (say) cache lines.

Frankly, I don't feel to be qualified to do such significant changes to
__copy_tofrom_user :).

So, I tried to modify copy_page and __copy_tofrom_user as little as
possible and make them non-coherent cache safe while still using read
prefetching. Idea is simple, we prefetch as before but only till the end of
the source buffer.

Updated patches:

  - against linuxppc-2.4: http://kernel.ebshome.net/read_prefetch-2.4-2.diff

  - against linuxppc-2.5 (not tested, but compiles):
http://kernel.ebshome.net/read_prefetch-2.6-2.diff

Eugene.


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