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Date:   Thu, 8 Aug 2019 08:02:10 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        kirill.shutemov@linux.intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/mmap.c: refine data locality of find_vma_prev
Message-ID: <20190808060210.GE11812@dhcp22.suse.cz>
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 <20190807003109.GB24750@richard>
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 <20190808032638.GA28138@richard>
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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?
-- 
Michal Hocko
SUSE Labs