Re: [PATCH v8 00/16] Add support for Clang LTO

[Sami Tolvanen]

On Tue, Dec 8, 2020 at 2:20 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> On Tue, Dec 8, 2020 at 10:10 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
> > > <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> > > > >
> > > > > - one build seems to take even longer to link. It's currently at 35GB RAM
> > > > >   usage and 40 minutes into the final link, but I'm worried it might
> > > > > not complete
> > > > >   before it runs out of memory.  I only have 128GB installed, and google-chrome
> > > > >   uses another 30GB of that, and I'm also doing some other builds in parallel.
> > > > >   Is there a minimum recommended amount of memory for doing LTO builds?
> > > >
> > > > When building arm64 defconfig, the maximum memory usage I measured
> > > > with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
> > > > larger configurations, but I believe LLD can easily consume 3-4x that
> > > > much with full LTO allyesconfig.
> > >
> > > Ok, that's not too bad then. Is there actually a reason to still
> > > support full-lto
> > > in your series? As I understand it, full LTO was the initial approach and
> > > used to work better, but thin LTO is actually what we want to use in the
> > > long run. Perhaps dropping the full LTO option from your series now
> > > that thin LTO works well enough and uses less resources would help
> > > avoid some of the problems.
> >
> > While all developers agree that ThinLTO is a much more palatable
> > experience than full LTO; our product teams prefer the excessive build
> > time and memory high water mark (at build time) costs in exchange for
> > slightly better performance than ThinLTO in <benchmarks that I've been
> > told are important>.  Keeping support for full LTO in tree would help
> > our product teams reduce the amount of out of tree code they have.  As
> > long as <benchmarks that I've been told are important> help
> > sell/differentiate phones, I suspect our product teams will continue
> > to ship full LTO in production.
>
> Ok, fair enough. How about marking FULL_LTO as 'depends on
> !COMPILE_TEST' then? I'll do that locally for my randconfig tests,
> but it would help the other build bots that also force-enable
> COMPILE_TEST.

Sure, that sounds reasonable to me. I'll add it in v9.

Sami