lkml.org 
[lkml]   [2021]   [Apr]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Very slow clang kernel config ..
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'

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} -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 -'

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

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.

Cheers,
Nathan

\
 
 \ /
  Last update: 2021-04-30 02:52    [W:0.139 / U:0.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site