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

[Markus Trippelsdorf]

On 2014.04.09 at 08:01 +0200, Ingo Molnar wrote:
> 
> * Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> 
> > On Tue, Apr 08, 2014 at 03:44:25PM -0700, Linus Torvalds wrote:
> > > On Tue, Apr 8, 2014 at 1:49 PM,  <josh@xxxxxxxxxxxxxxxx> 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
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