linux-kernel - Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression

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Date:   Tue, 27 Nov 2018 20:05:33 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        rong.a.chen@...el.com
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>, s.priebe@...fihost.ag,
        alex.williamson@...hat.com, mgorman@...hsingularity.net,
        zi.yan@...rutgers.edu, rientjes@...gle.com, kirill@...temov.name,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        lkp@...org
Subject: Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3%
 regression

On 11/27/18 6:08 PM, Linus Torvalds wrote:
> On Mon, Nov 26, 2018 at 10:24 PM kernel test robot
> <rong.a.chen@...el.com> wrote:
>>
>> FYI, we noticed a -61.3% regression of vm-scalability.throughput due
>> to commit ac5b2c18911f ("mm: thp: relax __GFP_THISNODE for
>> MADV_HUGEPAGE mappings")
> 
> Well, that's certainly noticeable and not good.
> 
> Andrea, I suspect it might be causing fights with auto numa migration..
> 
> Lots more system time, but also look at this:
> 
>>    1122389 ±  9%     +17.2%    1315380 ±  4%  proc-vmstat.numa_hit
>>     214722 ±  5%     +21.6%     261076 ±  3%  proc-vmstat.numa_huge_pte_updates
>>    1108142 ±  9%     +17.4%    1300857 ±  4%  proc-vmstat.numa_local
>>     145368 ± 48%     +63.1%     237050 ± 17%  proc-vmstat.numa_miss
>>     159615 ± 44%     +57.6%     251573 ± 16%  proc-vmstat.numa_other
>>     185.50 ± 81%   +8278.6%      15542 ± 40%  proc-vmstat.numa_pages_migrated
> 
> Should the commit be reverted? Or perhaps at least modified?

This part of the test's config is important:

thp_defrag: always

While the commit targets MADV_HUGEPAGE mappings (such as Andrea's
kvm-qemu usecase), with defrag=always, all mappings behave almost as a
MADV_HUGEPAGE mapping. That's no longer a default for some years now and
I think nobody recommends it. In the default configuration nothing
changes for non-madvise mappings.

Vlastimil

>                      Linus
>