On Tue, Dec 8, 2020 at 2:20 PM Arnd Bergmann [off-list ref] wrote:

On Tue, Dec 8, 2020 at 10:10 PM 'Nick Desaulniers' via Clang Built
Linux [off-list ref] wrote:

quoted

On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann [off-list ref] wrote:

quoted

On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
[off-list ref] wrote:

quoted

On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann [off-list ref] wrote:

quoted

- 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