* 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%. 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
