Re: [PATCH] mm/mmap.c: refine data locality of find_vma_prev

[Wei Yang]

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
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