On Tue, Dec 21, 2015 at 5:43 AM, Arnd Bergmann <[email protected]> wrote: > > On Monday 21 December 2015, Will Deacon wrote: >> On Sat, Dec 19, 2015 at 04:11:18PM -0800, Andrew Pinski wrote: >> > Adding a check for the cache line size is not much overhead. >> > Special case 128 byte cache line size. >> > This improves copy_page by 85% on ThunderX compared to the original >> > implementation. >> >> So this patch seems to: >> >> - Align the loop >> - Increase the prefetch size >> - Unroll the loop once >> >> Do you know where your 85% boost comes from between these? I'd really >> like to avoid having multiple versions of copy_page, if possible, but >> maybe we could end up with something that works well enough regardless >> of cacheline size. Understanding what your bottleneck is would help to >> lead us in the right direction.
I think it is the prefetching. ThunderX T88 pass 1 and pass 2 does not have a hardware prefetcher so prefetching a half of a cacheline ahead does not help at all. >> >> Also, how are you measuring the improvement? If you can share your >> test somewhere, I can see how it affects the other systems I have >> access to. You can find my benchmark at https://github.com/apinski-cavium/copy_page_benchmark . copy_page is my previous patch. copy_page128 is just the unrolled and only 128 byte prefetching copy_page64 is the original code copy_page64unroll is the new patch which I will be sending out soon. > > A related question would be how other CPU cores are affected by the change. > The test for the cache line size is going to take a few cycles, possibly a > lot on certain implementations, e.g. if we ever get one where 'mrs' is > microcoded or trapped by a hypervisor. > > Are there any possible downsides to using the ThunderX version on other > microarchitectures too and skip the check? Yes that is a good idea. I will send out a new patch in a little bit which just unrolls the loop with keeping of the two prefetch instructions in there. Thanks, Andrew Pinski > > Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
