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

[Ingo Molnar]

* Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote:

> On 2014.04.14 at 12:32 +0200, Ingo Molnar wrote:
> > 
> > * Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote:
> > 
> > > 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.
> > 
> > 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%.
> 
> It's a matter of definition. I've computed the slowdown by taking the
> difference: (a - b) * 100 / a

That definition is IMHO deficient to the level of being broken - for 
example "it got slower by 300%" would be impossible to achieve, as 
slowdowns get larger it just converges to 100% from below, which is 
not very intuitive.

So the typical definition for "X got slower by Y%" is:

    Z = X*1.Y

For small deltas it does not matter, for larger ones it does. Anyway, 
we seem to agree :)

Thanks,

	Ingo
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