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

Markus Trippelsdorf
2014-04-09T08:17:12Z

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.

The huge RAM requirements of LTO are solved with gcc-4.9.

As for tooling support, the next version of binutils will support
automatic loading of the liblto_plugin and that will allow one to get
rid of the gcc wrappers (gcc-ar, etc.).

It is unfortunate that a special version of binutils is still required
to build the kernel, but that will change as soon as all "ld -r"
invocations are eliminated (I think Sam Ravnborg has a patch for this).

--
Markus