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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 8 Dec 2020 13:09:54 -0800
Message-ID: <CAKwvOd=hL=Vt1ATYqky9jmv+tM5hpTnLRuZudG-7ki0EYoFGJQ@mail.gmail.com>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>
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On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
> <clang-built-linux@googlegroups.com> wrote:
> >
> > On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> 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.
-- 
Thanks,
~Nick Desaulniers