linux-kernel - Re: [PATCH 1/2] kbuild: Support split debug info v3

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Date:	Wed, 23 Jul 2014 20:05:01 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, Dirk Gouders <dirk@...ders.net>,
	mmarek@...e.cz
Subject: Re: [PATCH 1/2] kbuild: Support split debug info v3

On Sun, Jul 20, 2014 at 02:19:01PM -0700, Andi Kleen wrote:
> > Why are as only called with -Wa,-gdwarf-2 in the non-split case?
> > Is this a bug or intentional?
> > If it is intentional then it is not explained.
> 
> Probably was a merging mistake. Will fix.
> 
> > > +	help
> > > +	  Generate debug info into separate .dwo files. This can be
> > > +	  faster for building than including the debug information directly
> > Here the "faster" part is promoted.
> > But in your cover letter you actually highlight the "smaller" case
> > as more significant.
> > Maybe include info on both positive items.
> 
> I currently don't see it significantly faster in my setup, will
> mention size. It may be in others though. Feel free to test.

I tried to test this on my setup - it is a Intel Atom with 2 cores.
16 GB RAM, a 7200 RPM stat disk.
In other words a low-spec setup.

Base config was "make allnoconfig"

make KCFLAGS=-g
real    3m35.967s
user    12m19.270s
sys     1m15.082s

make KCFLAGS=-gsplit-dwarf
real    3m33.366s
user    12m11.156s
sys     1m17.772s

I did like this:
echo time make -j4 -s KCFLAGS=-gsplit-dwarf
time make -j4 -s KCFLAGS=-gsplit-dwarf
echo time make -j4 -s KCFLAGS=-g
time make -j4 -s KCFLAGS=-g
echo time make -j4 -s KCFLAGS=-gsplit-dwarf
time make -j4 -s KCFLAGS=-gsplit-dwarf
echo time make -j4 -s KCFLAGS=-g
time make -j4 -s KCFLAGS=-g
echo time make -j4 -s KCFLAGS=-gsplit-dwarf
time make -j4 -s KCFLAGS=-gsplit-dwarf
echo time make -j4 -s KCFLAGS=-g
time make -j4 -s KCFLAGS=-g

The figures are from last run - but the other runs had compareable times.
So in short I see a ~3 seconds speedup - out of 210 seconds.
Thats less than I anticipated.

But a speed-up never the less - which was consistent over all three runs.

	Sam
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