Re: [GIT] kbuild/lto changes for 3.15-rc1

[Ingo Molnar <mingo@kernel.org>]

* Markus Trippelsdorf <markus@trippelsdorf.de> wrote:

> On 2014.04.09 at 08:01 +0200, Ingo Molnar wrote:
> > 
> > * Andi Kleen <ak@linux.intel.com> wrote:
> > 
> > > On Tue, Apr 08, 2014 at 03:44:25PM -0700, Linus Torvalds wrote:
> > > > On Tue, Apr 8, 2014 at 1:49 PM,  <josh@joshtriplett.org> wrote:
> > > > >
> > > > > In addition to making the kernel smaller and such (I'll leave the
> > > > > specific stats there to Andi), here's the key awesomeness of LTO that
> > > > > you, personally, should find useful and compelling: LTO will eliminate
> > > > > the need to add many lower-level Kconfig symbols to compile out bits of
> > > > > the kernel.
> > > > 
> > > > Actually that, to me, is a negative right now.
> > > > 
> > > > Since there's no way we'll make LTO the default in the foreseeable
> > > > future, people starting to use it like that is just a bad bad thing.
> > > > 
> > > > So really, the main advantage of LTO would be any actual 
> > > > optimizations it can do. And call me anal, but I want *numbers* 
> > > > for that before I merge it. Not handwaving. I'm not actually aware 
> > > > of how well - if at all - code generation actually improves.
> > > 
> > > Well it looks very different if you look at the generated code. gcc 
> > > becomes a lot more aggressive.
> > > 
> > > But as I said there's currently no significant performance 
> > > improvement known, so if your only goal is better performance this 
> > > patch (as currently) known is not a big winner.  My suspicion is 
> > > that's mostly because the standard benchmarks we run are not too 
> > > compiler sensitive.
> > > 
> > > However the users seem to care about the other benefits, like code 
> > > size.
> > > 
> > > And there may well be loads that are compiler sensitive. As Honza 
> > > posted, for non kernel workloads LTO is known to have large 
> > > benefits.
> > > 
> > > Besides at this point it's pretty much just some additions to the 
> > > Makefiles.
> > 
> > So the reason I've been mostly ignoring the LTO patches myself (I only 
> > took LTO related changes that had other justifications such as 
> > cleanups) is that I've actually implemented full LTO in a userspace 
> > project myself, and my experience was:
> > 
> >   1) There was very little if any measurable LTO runtime speedup, 
> >      despite agressive GCC options and despite user-space generally 
> >      offering more optimizations opportunities than kernel space.
> > 
> >   2) LTO with current build tools meant a 1.5x-3x build speed
> >      slowdown (on a very fast box with tons of CPUs and RAM),
> >      which made LTO essentially a non-starter for development
> >      work. (And that was with the Gold linker.)
> > 
> > and looking at your characterisation of LTO you only conceded
> > #1 much after you started pushing LTO and you are clearly trying
> > to avoid talking about #2 while it's very much relevant...
> > 
> > I'm willing to be convinced by actual numbers, and LTO tooling might 
> > eventually improve, etc., but right now LTO is much ado about very 
> > little, being pushed in a somewhat dishonest way.
> 
> I did some measurements on Andi's lto-3.14 branch:
> 
> options    size     build time
> ------------------------------
> -O2        4408880  1:56.98
> -flto -O2  4213072  2:36.22
> -Os        3833248  1:45.13         
> -flto -Os  3651504  2:34.51
> 
> This was measured on my AMD 4 core machine with a monolithic .config
> where "CONFIG_MODULES is not set". The compiler is gcc trunk (4.9).
> So on x86_86 you get 5% size reduction for 25-30% build time slowdown.

Note that the build slowdowns you measured are more like 30-45%:

   156.22/116.98 == 33.5% slowdown
   154.51/105.13 == 46.9% slowdown

not 25-30%.

But yeah, that sounds about right and is obviously relevant data, and 
goes beyond 'a bit slower' .

Also note that the 5% size reduction due to LTO consists of two big 
parts:

  - removal of unused facilities in that .config
  - true optimizations

it would be important to know the proportion of true optimizations, 
because those are that matter most. Unused facilities will take up a 
bit of RAM, and perhaps fragment the CPU cache a tiny bit, but aren't 
nearly as relevant as true optimizations.

So the '[LTO is] not a big winner' characterisation given by Andi is a 
euphemism AFAICS, a more accurate description would be something like: 
'LTO does not help kernel performance measurably and it slows down 
kernel development'.

Thanks,

	Ingo