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SubjectRe: Very slow clang kernel config ..
On Thu, Apr 29, 2021 at 5:52 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Thu, Apr 29, 2021 at 02:53:08PM -0700, Linus Torvalds wrote:
> > I haven't looked into why this is so slow with clang, but it really is
> > painfully slow:
> >
> > time make CC=clang allmodconfig
> > real 0m2.667s
> >
> > vs the gcc case:
> >
> > time make CC=gcc allmodconfig
> > real 0m0.903s
> >
> > Yeah, yeah, three seconds may sound like "not a lot of time, but
> > considering that the subsequent full build (which for me is often
> > empty) doesn't take all that much longer, that config time clang waste
> > is actually quite noticeable.
> >
> > I actually don't do allmodconfig builds with clang, but I do my
> > default kernel builds with it:
> >
> > time make oldconfig
> > real 0m2.748s
> >
> > time sh -c "make -j128 > ../makes"
> > real 0m3.546s
> >
> > so that "make oldconfig" really is almost as slow as the whole
> > "confirm build is done" thing. Its' quite noticeable in my workflow.
> >
> > The gcc config isn't super-fast either, but there's a big 3x
> > difference, so the clang case really is doing something extra wrong.
> >
> > I've not actually looked into _why_. Except I do see that "clang" gets
> > invoked with small (empty?) test files several times, probably to
> > check for command line flags being valid.
> >
> > Sending this to relevant parties in the hope that somebody goes "Yeah,
> > that's silly" and fixes it.
> >
> > This is on my F34 machine:
> >
> > clang version 12.0.0 (Fedora 12.0.0-0.3.rc1.fc34)
> >
> > in case it matters (but I don't see why it should).
> >
> > Many many moons ago the promise for clang was faster build speeds.
> > That didn't turn out to be true, but can we please at least try to
> > make them not painfully much slower?
>
> Hi Linus,
>
> I benchmarked this with your latest tree
> (8ca5297e7e38f2dc8c753d33a5092e7be181fff0) with my distribution versions
> of clang 11.1.0 and gcc 10.2.0 and I saw the same results, benchmarking
> with hyperfine.
>
> $ hyperfine -L comp_var "","CC=clang " -r 100 -S /bin/sh -w 5 'make {comp_var}allmodconfig'
> Benchmark #1: make allmodconfig
> Time (mean ± σ): 1.490 s ± 0.012 s [User: 1.153 s, System: 0.374 s]
> Range (min … max): 1.462 s … 1.522 s 100 runs
>
> Benchmark #2: make CC=clang allmodconfig
> Time (mean ± σ): 4.001 s ± 0.020 s [User: 2.761 s, System: 1.274 s]
> Range (min … max): 3.939 s … 4.038 s 100 runs
>
> Summary
> 'make allmodconfig' ran
> 2.69 ± 0.03 times faster than 'make CC=clang allmodconfig'

$ hyperfine -L comp_var "","CC=clang " -r 100 -S /bin/sh -w 5 'make
{comp_var}allmodconfig'
Benchmark #1: make allmodconfig
Time (mean ± σ): 2.095 s ± 0.025 s [User: 1.285 s, System: 0.880 s]
Range (min … max): 2.014 s … 2.168 s 100 runs

Benchmark #2: make CC=clang allmodconfig
Time (mean ± σ): 2.930 s ± 0.034 s [User: 1.522 s, System: 1.477 s]
Range (min … max): 2.849 s … 3.005 s 100 runs

Summary
'make allmodconfig' ran
1.40 ± 0.02 times faster than 'make CC=clang allmodconfig'

Swapping the order, I get pretty similar results to my initial run:

hyperfine -L comp_var "CC=clang ","" -r 100 -S /bin/sh -w 5 'make
{comp_var}allmodconfig'
Benchmark #1: make CC=clang allmodconfig
Time (mean ± σ): 2.915 s ± 0.031 s [User: 1.501 s, System: 1.482 s]
Range (min … max): 2.825 s … 3.004 s 100 runs

Benchmark #2: make allmodconfig
Time (mean ± σ): 2.093 s ± 0.022 s [User: 1.284 s, System: 0.879 s]
Range (min … max): 2.037 s … 2.136 s 100 runs

Summary
'make allmodconfig' ran
1.39 ± 0.02 times faster than 'make CC=clang allmodconfig'

So, yes, slower, but not quite as drastic as others have observed.

>
> It was also reproducible in a Fedora Docker image, which has newer
> versions of those tools than my distro does (GCC 11.1.0 and clang
> 12.0.0):
>
> $ hyperfine -L comp_var "","CC=clang " -r 100 -S /bin/sh -w 5 'make {comp_var}allmodconfig'
> Benchmark #1: make allmodconfig
> Time (mean ± σ): 989.9 ms ± 3.5 ms [User: 747.0 ms, System: 271.1 ms]
> Range (min … max): 983.0 ms … 998.2 ms 100 runs
>
> Benchmark #2: make CC=clang allmodconfig
> Time (mean ± σ): 3.328 s ± 0.005 s [User: 2.408 s, System: 0.948 s]
> Range (min … max): 3.316 s … 3.343 s 100 runs
>
> Summary
> 'make allmodconfig' ran
> 3.36 ± 0.01 times faster than 'make CC=clang allmodconfig'
>
> Unfortunately, I doubt there is much that can be done on the kernel side
> because this is reproducible just invoking the compilers without any
> source input.
>
> Clang 11.1.0 and GCC 10.2.0:
>
> $ hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo | {compiler} -x c -c -o /dev/null -'
> Benchmark #1: echo | gcc -x c -c -o /dev/null -
> Time (mean ± σ): 9.6 ms ± 1.0 ms [User: 6.5 ms, System: 3.4 ms]
> Range (min … max): 5.8 ms … 12.7 ms 5000 runs
>
> Benchmark #2: echo | clang -x c -c -o /dev/null -
> Time (mean ± σ): 33.0 ms ± 0.8 ms [User: 22.4 ms, System: 10.9 ms]
> Range (min … max): 30.3 ms … 36.0 ms 5000 runs
>
> Summary
> 'echo | gcc -x c -c -o /dev/null -' ran
> 3.45 ± 0.39 times faster than 'echo | clang -x c -c -o /dev/null -'

hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo |
{compiler} -x c -c -o /dev/null -'
Benchmark #1: echo | gcc -x c -c -o /dev/null -
Time (mean ± σ): 21.4 ms ± 2.4 ms [User: 11.6 ms, System: 10.8 ms]
Range (min … max): 12.8 ms … 27.3 ms 5000 runs

Benchmark #2: echo | clang -x c -c -o /dev/null -
Time (mean ± σ): 16.4 ms ± 2.3 ms [User: 8.6 ms, System: 8.8 ms]
Range (min … max): 10.4 ms … 25.4 ms 5000 runs

Summary
' echo | clang -x c -c -o /dev/null -' ran
1.31 ± 0.24 times faster than ' echo | gcc -x c -c -o /dev/null -'

>
> $ hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo | {compiler} -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -'
> Benchmark #1: echo | gcc -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -
> Time (mean ± σ): 11.9 ms ± 1.1 ms [User: 10.5 ms, System: 1.8 ms]
> Range (min … max): 8.2 ms … 15.1 ms 5000 runs
>
> Warning: Ignoring non-zero exit code.
>
> Benchmark #2: echo | clang -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -
> Time (mean ± σ): 31.0 ms ± 0.8 ms [User: 20.3 ms, System: 10.9 ms]
> Range (min … max): 27.9 ms … 33.8 ms 5000 runs
>
> Warning: Ignoring non-zero exit code.
>
> Summary
> 'echo | gcc -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -' ran
> 2.62 ± 0.26 times faster than 'echo | clang -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -'

hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo |
{compiler} -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -'
Benchmark #1: echo | gcc -Werror -Wflag-that-does-not-exit -x c -c -o
/dev/null -
Time (mean ± σ): 18.5 ms ± 2.4 ms [User: 17.0 ms, System: 2.7 ms]
Range (min … max): 12.2 ms … 24.6 ms 5000 runs

Warning: Ignoring non-zero exit code.

Benchmark #2: echo | clang -Werror -Wflag-that-does-not-exit -x c -c
-o /dev/null -
Time (mean ± σ): 15.4 ms ± 2.3 ms [User: 8.4 ms, System: 8.1 ms]
Range (min … max): 9.5 ms … 22.6 ms 5000 runs

Warning: Ignoring non-zero exit code.

Summary
' echo | clang -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -' ran
1.20 ± 0.23 times faster than ' echo | gcc -Werror
-Wflag-that-does-not-exit -x c -c -o /dev/null -'

>
> Clang 12.0.0 and GCC 11.1.0:
>
> $ hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo | {compiler} -x c -c -o /dev/null -'
> Benchmark #1: echo | gcc -x c -c -o /dev/null -
> Time (mean ± σ): 8.5 ms ± 0.3 ms [User: 5.6 ms, System: 3.3 ms]
> Range (min … max): 7.6 ms … 9.8 ms 5000 runs
>
> Benchmark #2: echo | clang -x c -c -o /dev/null -
> Time (mean ± σ): 27.4 ms ± 0.4 ms [User: 19.6 ms, System: 8.1 ms]
> Range (min … max): 26.4 ms … 29.1 ms 5000 runs
>
> Summary
> 'echo | gcc -x c -c -o /dev/null -' ran
> 3.22 ± 0.13 times faster than 'echo | clang -x c -c -o /dev/null -'
>
> $ hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo | {compiler} -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -'
> Benchmark #1: echo | gcc -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -
> Time (mean ± σ): 12.2 ms ± 0.3 ms [User: 11.5 ms, System: 1.0 ms]
> Range (min … max): 11.7 ms … 13.9 ms 5000 runs
>
> Warning: Ignoring non-zero exit code.
>
> Benchmark #2: echo | clang -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -
> Time (mean ± σ): 26.3 ms ± 0.5 ms [User: 19.1 ms, System: 7.5 ms]
> Range (min … max): 25.2 ms … 28.1 ms 5000 runs
>
> Warning: Ignoring non-zero exit code.
>
> Summary
> 'echo | gcc -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -' ran
> 2.16 ± 0.06 times faster than 'echo | clang -Werror -Wflag-that-does-not-exit -x c -c -o /dev/null -'
>
> Seems that GCC is faster to complete when it does not have to parse
> warning flags while clang shows no major variance. Thinking more about,
> cc-option gives clang an empty file so it should not have to actually
> parse anything so I do not think '-fsyntax-only' will gain us a whole
> ton because we should not be dipping into the backend at all.
>
> Tangentially, my version of clang built with Profile Guided Optimization
> gets me closed to GCC. I am surprised to see this level of gain.
>
> $ hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo | {compiler} -x c -c -o /dev/null -'
> Benchmark #1: echo | gcc -x c -c -o /dev/null -
> Time (mean ± σ): 9.6 ms ± 1.0 ms [User: 6.4 ms, System: 3.5 ms]
> Range (min … max): 5.6 ms … 12.9 ms 5000 runs

hyperfine -i -L compiler gcc,clang -r 5000 -S /bin/sh -w 5 'echo |
{compiler} -x c -c -o /dev/null -'
Benchmark #1: echo | gcc -x c -c -o /dev/null -
Time (mean ± σ): 21.3 ms ± 2.4 ms [User: 11.7 ms, System: 10.6 ms]
Range (min … max): 12.2 ms … 27.4 ms 5000 runs

Benchmark #2: echo | clang -x c -c -o /dev/null -
Time (mean ± σ): 16.3 ms ± 2.3 ms [User: 8.5 ms, System: 8.8 ms]
Range (min … max): 10.1 ms … 25.2 ms 5000 runs

Summary
' echo | clang -x c -c -o /dev/null -' ran
1.31 ± 0.24 times faster than ' echo | gcc -x c -c -o /dev/null -'

So now clang is faster? Am I holding it wrong?

>
> Benchmark #2: echo | clang -x c -c -o /dev/null -
> Time (mean ± σ): 8.7 ms ± 1.3 ms [User: 4.3 ms, System: 4.9 ms]
> Range (min … max): 4.9 ms … 12.1 ms 5000 runs
>
> Warning: Command took less than 5 ms to complete. Results might be inaccurate.
>
> Summary
> 'echo | clang -x c -c -o /dev/null -' ran
> 1.10 ± 0.20 times faster than 'echo | gcc -x c -c -o /dev/null -'
>
> $ hyperfine -L comp_var "","CC=clang " -r 100 -S /bin/sh -w 5 'make {comp_var}allmodconfig'
> Benchmark #1: make allmodconfig
> Time (mean ± σ): 1.531 s ± 0.011 s [User: 1.180 s, System: 0.388 s]
> Range (min … max): 1.501 s … 1.561 s 100 runs
>
> Benchmark #2: make CC=clang allmodconfig
> Time (mean ± σ): 1.828 s ± 0.015 s [User: 1.209 s, System: 0.760 s]
> Range (min … max): 1.802 s … 1.872 s 100 runs
>
> Summary
> 'make allmodconfig' ran
> 1.19 ± 0.01 times faster than 'make CC=clang allmodconfig'
>
> I think that we should definitely see what we can do to speed up the front end.

Numbers between machines probably aren't directly comparable, but I
would be curious if which toolchain was used to build clang makes a
difference; whether debug builds are significantly slower, and whether
distro toolchains vs local builds from the same branch (but all from
the same machine) are noticeably different.


--
Thanks,
~Nick Desaulniers

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