linux-kernel - Re: [PATCH] thp: reduce usage of huge zero page's atomic counter

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Message-Id: <20160829203916.6a2b45845e8fb0c356cac17d@linux-foundation.org>
Date:   Mon, 29 Aug 2016 20:39:16 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        "'Kirill A. Shutemov'" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jerome Marchand <jmarchan@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Ebru Akagunduz <ebru.akagunduz@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thp: reduce usage of huge zero page's atomic counter

On Tue, 30 Aug 2016 11:09:15 +0800 Aaron Lu <aaron.lu@...el.com> wrote:

> >> Case used for test on Haswell EP:
> >> usemem -n 72 --readonly -j 0x200000 100G
> >> Which spawns 72 processes and each will mmap 100G anonymous space and
> >> then do read only access to that space sequentially with a step of 2MB.
> >>
> >> perf report for base commit:
> >>     54.03%  usemem   [kernel.kallsyms]   [k] get_huge_zero_page
> >> perf report for this commit:
> >>      0.11%  usemem   [kernel.kallsyms]   [k] mm_get_huge_zero_page
> > 
> > Does this mean that overall usemem runtime halved?
> 
> Sorry for the confusion, the above line is extracted from perf report.
> It shows the percent of CPU cycles executed in a specific function.
> 
> The above two perf lines are used to show get_huge_zero_page doesn't
> consume that much CPU cycles after applying the patch.
> 
> > 
> > Do we have any numbers for something which is more real-wordly?
> 
> Unfortunately, no real world numbers.
> 
> We think the global atomic counter could be an issue for performance
> so I'm trying to solve the problem.

So, umm, we don't actually know if the patch is useful to anyone?

Some more measurements would help things along, please.