Re: [mm] f35b5d7d67: will-it-scale.per_process_ops -95.5% regression

[Thorsten Leemhuis]

Hi, this is your Linux kernel regression tracker.

On 28.11.22 07:40, Nathan Chancellor wrote:
> Hi Rik,

I wonder what we should do about below performance regression. Is
reverting the culprit now and reapplying it later together with a fix a
viable option? Or was anything done/is anybody doing something already
to address the problem and I just missed it?

Yang Shi, Andrew, what's your option on this? I ask you directly,
because it looks like Rik hasn't posted anything to lists archived on
lore during the last few weeks. :-/

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.

> On Thu, Oct 20, 2022 at 10:16:20AM -0700, Nathan Chancellor wrote:
>> On Thu, Oct 20, 2022 at 11:28:16AM -0400, Rik van Riel wrote:
>>> On Thu, 2022-10-20 at 13:07 +0800, Huang, Ying wrote:
>>>> Nathan Chancellor <nathan@xxxxxxxxxx> writes:
>>>>>
>>>>> For what it's worth, I just bisected a massive and visible
>>>>> performance
>>>>> regression on my Threadripper 3990X workstation to commit
>>>>> f35b5d7d676e
>>>>> ("mm: align larger anonymous mappings on THP boundaries"), which
>>>>> seems
>>>>> directly related to this report/analysis. I initially noticed this
>>>>> because my full set of kernel builds against mainline went from 2
>>>>> hours
>>>>> and 20 minutes or so to over 3 hours. Zeroing in on x86_64
>>>>> allmodconfig,
>>>>> which I used for the bisect:
>>>>>
>>>>> @ 7b5a0b664ebe ("mm/page_ext: remove unused variable in
>>>>> offline_page_ext"):
>>>>>
>>>>> Benchmark 1: make -skj128 LLVM=1 allmodconfig all
>>>>>   Time (mean ± σ):     318.172 s ±  0.730 s    [User: 31750.902 s,
>>>>> System: 4564.246 s]
>>>>>   Range (min … max):   317.332 s … 318.662 s    3 runs
>>>>>
>>>>> @ f35b5d7d676e ("mm: align larger anonymous mappings on THP
>>>>> boundaries"):
>>>>>
>>>>> Benchmark 1: make -skj128 LLVM=1 allmodconfig all
>>>>>   Time (mean ± σ):     406.688 s ±  0.676 s    [User: 31819.526 s,
>>> System: 16327.022 s]
>>>>>   Range (min … max):   405.954 s … 407.284 s    3 run
>>>>
>>>> Have you tried to build with gcc?  Want to check whether is this
>>>> clang
>>>> specific issue or not.
>>>
>>> This may indeed be something LLVM specific. In previous tests,
>>> GCC has generally seen a benefit from increased THP usage.
>>> Many other applications also benefit from getting more THPs.
>>
>> Indeed, GCC builds actually appear to be slightly faster on my system now,
>> apologies for not trying that before reporting :/
>>
>> 7b5a0b664ebe:
>>
>> Benchmark 1: make -skj128 allmodconfig all
>>   Time (mean ± σ):     355.294 s ±  0.931 s    [User: 33620.469 s, System: 6390.064 s]
>>   Range (min … max):   354.571 s … 356.344 s    3 runs
>>
>> f35b5d7d676e:
>>
>> Benchmark 1: make -skj128 allmodconfig all
>>   Time (mean ± σ):     347.400 s ±  2.029 s    [User: 34389.724 s, System: 4603.175 s]
>>   Range (min … max):   345.815 s … 349.686 s    3 runs
>>
>>> LLVM showing 10% system time before this change, and a whopping
>>> 30% system time after that change, suggests that LLVM is behaving
>>> quite differently from GCC in some ways.
>>
>> The above tests were done with GCC 12.2.0 from Arch Linux. The previous LLVM
>> tests were done with a self-compiled version of LLVM from the main branch
>> (16.0.0), optimized with BOLT [1]. To eliminate that as a source of issues, I
>> used my distribution's version of clang (14.0.6) and saw similar results as
>> before:
>>
>> 7b5a0b664ebe:
>>
>> Benchmark 1: make -skj128 LLVM=/usr/bin/ allmodconfig all
>>   Time (mean ± σ):     462.517 s ±  1.214 s    [User: 48544.240 s, System: 5586.212 s]
>>   Range (min … max):   461.115 s … 463.245 s    3 runs
>>
>> f35b5d7d676e:
>>
>> Benchmark 1: make -skj128 LLVM=/usr/bin/ allmodconfig all
>>   Time (mean ± σ):     547.927 s ±  0.862 s    [User: 47913.709 s, System: 17682.514 s]
>>   Range (min … max):   547.429 s … 548.922 s    3 runs
>>
>>> If we can figure out what these differences are, maybe we can
>>> just fine tune the code to avoid this issue.
>>>
>>> I'll try to play around with LLVM compilation a little bit next
>>> week, to see if I can figure out what might be going on. I wonder
>>> if LLVM is doing lots of mremap calls or something...
>>
>> If there is any further information I can provide or patches I can test,
>> I am more than happy to do so.
>>
>> [1]: https://github.com/llvm/llvm-project/tree/96552e73900176d65ee6650facae8d669d6f9498/bolt
> 
> Was there ever a follow up to this report that I missed? I just
> noticed that I am still reverting f35b5d7d676e in my mainline kernel.
> 
> Cheers,
> Nathan
> 

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