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

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Date:   Tue, 18 Oct 2016 11:28:31 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [4.9-rc1] Build-time 2x slower

On Mon, Oct 17, 2016 at 6:46 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> 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..
>

OK, thanks for confirming the problem.
I was not sure if this is a "local" problem.

Your timing-script looks interesting and test... will report.

- Sedat -