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Date:   Sun, 27 Nov 2022 23:40:56 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Rik van Riel <riel@surriel.com>
Cc:     "Huang, Ying" <ying.huang@intel.com>,
        kernel test robot <yujie.liu@intel.com>, lkp@lists.01.org,
        lkp@intel.com, Andrew Morton <akpm@linux-foundation.org>,
        Yang Shi <shy828301@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        feng.tang@intel.com, zhengjun.xing@linux.intel.com,
        fengwei.yin@intel.com
Subject: Re: [mm] f35b5d7d67: will-it-scale.per_process_ops -95.5% regression
Message-ID: <Y4RX+JZLddDHrLuQ@dev-arch.thelio-3990X>
References: <202210181535.7144dd15-yujie.liu@intel.com>
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Hi Rik,

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@kernel.org> 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