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SubjectRe: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell"
On Tue, Jan 04, 2022 at 11:47:30AM +0100, Ingo Molnar wrote:
> > > With the fast-headers kernel that's down to ~36,000 lines of code,
> > > almost a factor of 3 reduction:
> > >
> > > # fast-headers-v1:
> > > kepler:~/mingo.tip.git> wc -l kernel/pid.i
> > > 35941 kernel/pid.i
> >
> > Coming from someone who often has to reduce a preprocessed kernel source
> > file with creduce/cvise to report compiler bugs, this will be a very
> > welcomed change, as those tools will have to do less work, and I can get
> > my reports done faster.
>
> That's nice, didn't think of that side effect.
>
> Could you perhaps measure this too, to see how much of a benefit it is?

As it turns out, I got an opportunity to measure this sooner rather than
later [1]. Using cvise [2] with an identical set of toolchains and
interestingness test [3], reducing net/core/skbuff.c took significantly
less time with the version from the fast-headers tree.

v5.16-rc8:

$ wc -l skbuff.i
105135 skbuff.i

$ time cvise test.fish skbuff.i
...
________________________________________________________
Executed in 114.02 mins fish external
usr time 1180.43 mins 69.29 millis 1180.43 mins
sys time 229.80 mins 248.11 millis 229.79 mins

fast-headers:

$ wc -l skbuff.i
78765 skbuff.i

$ time cvise test.fish skbuff.i
...
________________________________________________________
Executed in 47.38 mins fish external
usr time 620.17 mins 32.78 millis 620.17 mins
sys time 123.70 mins 122.38 millis 123.70 mins

I was not expecting that much of a difference but it somewhat makes
sense, as the tool spends less time eliminated unused code and the
compiler invocations will be incrementally quicker as the input becomes
smaller.

[1]: https://github.com/ClangBuiltLinux/linux/issues/1563
[2]: https://github.com/marxin/cvise
[3]: https://github.com/nathanchance/creduce-files/tree/61056fd763ae3bfb53ff0ae4c1d95550c7c0a5b7/cbl-1563

Cheers,
Nathan

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