On Thu, Aug 08, 2019 at 08:02:10AM +0200, Michal Hocko wrote: >On Thu 08-08-19 11:26:38, Wei Yang wrote: >> On Wed, Aug 07, 2019 at 09:51:01AM +0200, Michal Hocko wrote: >> >On Wed 07-08-19 08:31:09, Wei Yang wrote: >> >> On Tue, Aug 06, 2019 at 11:29:52AM +0200, Vlastimil Babka wrote: >> >> >On 8/6/19 10:11 AM, Wei Yang wrote: >> >> >> When addr is out of the range of the whole rb_tree, pprev will points to >> >> >> the biggest node. find_vma_prev gets is by going through the right most >> >> > >> >> >s/biggest/last/ ? or right-most? >> >> > >> >> >> node of the tree. >> >> >> >> >> >> Since only the last node is the one it is looking for, it is not >> >> >> necessary to assign pprev to those middle stage nodes. By assigning >> >> >> pprev to the last node directly, it tries to improve the function >> >> >> locality a little. >> >> > >> >> >In the end, it will always write to the cacheline of pprev. The caller has most >> >> >likely have it on stack, so it's already hot, and there's no other CPU stealing >> >> >it. So I don't understand where the improved locality comes from. The compiler >> >> >can also optimize the patched code so the assembly is identical to the previous >> >> >code, or vice versa. Did you check for differences? >> >> >> >> Vlastimil >> >> >> >> Thanks for your comment. >> >> >> >> I believe you get a point. I may not use the word locality. This patch tries >> >> to reduce some unnecessary assignment of pprev. >> >> >> >> Original code would assign the value on each node during iteration, this is >> >> what I want to reduce. >> > >> >Is there any measurable difference (on micro benchmarks or regular >> >workloads)? >> >> I wrote a test case to compare these two methods, but not find visible >> difference in run time. > >What is the point in changing this code if it doesn't lead to any >measurable improvement? You are right. >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me
