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. ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
