Hi Michael, On Fri, May 18, 2018 at 12:13:52AM +1000, Michael Ellerman wrote: > wei.guo.si...@gmail.com writes: > > From: Simon Guo <wei.guo.si...@gmail.com> > > > > This patch is based on the previous VMX patch on memcmp(). > > > > To optimize ppc64 memcmp() with VMX instruction, we need to think about > > the VMX penalty brought with: If kernel uses VMX instruction, it needs > > to save/restore current thread's VMX registers. There are 32 x 128 bits > > VMX registers in PPC, which means 32 x 16 = 512 bytes for load and store. > > > > The major concern regarding the memcmp() performance in kernel is KSM, > > who will use memcmp() frequently to merge identical pages. So it will > > make sense to take some measures/enhancement on KSM to see whether any > > improvement can be done here. Cyril Bur indicates that the memcmp() for > > KSM has a higher possibility to fail (unmatch) early in previous bytes > > in following mail. > > https://patchwork.ozlabs.org/patch/817322/#1773629 > > And I am taking a follow-up on this with this patch. > > > > Per some testing, it shows KSM memcmp() will fail early at previous 32 > > bytes. More specifically: > > - 76% cases will fail/unmatch before 16 bytes; > > - 83% cases will fail/unmatch before 32 bytes; > > - 84% cases will fail/unmatch before 64 bytes; > > So 32 bytes looks a better choice than other bytes for pre-checking. > > > > This patch adds a 32 bytes pre-checking firstly before jumping into VMX > > operations, to avoid the unnecessary VMX penalty. And the testing shows > > ~20% improvement on memcmp() average execution time with this patch. > > > > The detail data and analysis is at: > > https://github.com/justdoitqd/publicFiles/blob/master/memcmp/README.md > > > > Any suggestion is welcome. > > Thanks for digging into that, really great work. > > I'm inclined to make this not depend on KSM though. It seems like a good > optimisation to do in general. > > So can we just call it the 'pre-check' or something, and always do it? > Sound reasonable to me. I will expand the change to .Ldiffoffset_vmx_cmp case and test accordingly.
Thanks, - Simon
