Re: Copy prefetch optimizations and non-coherent caches

At 06:20 AM 9/6/2003, Paul Mackerras wrote:

quoted

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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