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

[Vlastimil Babka]

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@xxxxxxxxx> 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 Â  9%     +17.2%    1315380 Â  4%  proc-vmstat.numa_hit
>>     214722 Â  5%     +21.6%     261076 Â  3%  proc-vmstat.numa_huge_pte_updates
>>    1108142 Â  9%     +17.4%    1300857 Â  4%  proc-vmstat.numa_local
>>     145368 Â 48%     +63.1%     237050 Â 17%  proc-vmstat.numa_miss
>>     159615 Â 44%     +57.6%     251573 Â 16%  proc-vmstat.numa_other
>>     185.50 Â 81%   +8278.6%      15542 Â 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
>