linux-kernel - Re: [4.9-rc1] Build-time 2x slower

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Date:   Mon, 17 Oct 2016 09:46:09 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [4.9-rc1] Build-time 2x slower

On Mon, Oct 17, 2016 at 9:04 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> not sure whom to address on this issue.
>
> I have built Linux v4.9-rc1, v4.8.2 and v4.4.25 kernels (in this
> order) this morning.
>
> Building a Linux v4.8.2 under Linux v4.9-rc1 took two times longer.
>
> As usually I build with 2 parallel-make-jobs.
> This takes approx. 30mins.
> Under Linux v4.9-rc1 it took approx. an hour.

Hmm. Would you mind just bisecting it? I've not noticed the same thing
on my setup, but to be honest I generally don't time things very
closely because the variation for me tends to be much more along the
lines of "damn, that pull request touched <linux./fs.h> now it's
rebuilding everything" vs "40 seconds to build just the driver that
changed". Most of my builds are the "allmodconfig" builds I do in
between pulls..

You can even automate it, since it's going to take some time, using
"git bisect run" and writing a small script that looks at how long it
takes to build the kernel from scratch each time. I'd suggest trying
to write the script using a smaller repository (maybe git itself). The
script would just needs to do a clean rebuild, time it, and have
return success or error based on how long it took.

The script *might* look something like

  #!/bin/sh
  git clean -dqfx
  make oldconfig
  time -o time-file -f '%e' sh -c "make -j8 > ../output"
  # remove fractional seconds
  time=$(cat time-file | sed 's/\..*//')
  # less than 35 minutes is good?
  [ $time -lt $((35*60)) ]

but I didn't really test that very much. Anyway, you *should* be able
to do something like

  git bisect good v4.8
  git bisect bad v4.9-rc1
  git bisect run ../timing-script

(put the "timing-script" somewhere _else_ than the kernel sources so
that the "git clean" doesn't remove it - that's what my first silly
test did ;)

You may have to tweak that script a bit, but I think you get the idea..

               Linus