[Openmp-dev] Build is terribly slow on Arm

[Michael Kruse via Openmp-dev]

Ok, you are using a non-assert/release build of clang, therefore
Jonas' remark does not apply.

Since your original question was how to investigate further, we have
to find out where the time is spent.

1. Run clang -ftime-trace:
https://aras-p.info/blog/2019/01/16/time-trace-timeline-flame-chart-profiler-for-Clang/

2. Create a performance trace, e.g. https://github.com/brendangregg/FlameGraph
Note that for this, a non-optimized build with debug symbols of clang
would be better, but Linux perf may still get useful information
without.


Michael

Am Mi., 10. Juni 2020 um 18:45 Uhr schrieb Itaru Kitayama
<itaru.kitayama at gmail.com>:
>
> ```
>
> cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_LLD=ON
> -DCMAKE_INSTALL_PREFIX=$HOME/opt/clang/${now} -DCMAKE_C_COMPILER=clang
> -DCMAKE_CXX_COMPILER=clang++ -DLLVM_TARGETS_TO_BUILD=all
> -DLLVM_ENABLE_PROJECTS="openmp;clang;lld;libcxx;libcxxabi;libunwind"
> -DCLANG_OPENMP_NVPTX_DEFAULT_ARCH=sm_70
> -DLIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES=70 /tmp/llvm-project/llvm
> ```
>
> On Thu, Jun 11, 2020 at 8:43 AM Michael Kruse <llvm at meinersbur.de> wrote:
> >
> > How do you compile LLVM (your cmake ... command line)?
> >
> > What flags do you use to compile your application?
> >
> > Michael
> >
> > Am Mi., 10. Juni 2020 um 18:26 Uhr schrieb Itaru Kitayama
> > <itaru.kitayama at gmail.com>:
> > >
> > >  Michael, David,
> > >
> > > Sorry I wasn't clear; I am building LLVM on Arm with CUDA Toolkit 11 RC.
> > >
> > > On Thu, Jun 11, 2020 at 8:23 AM Michael Kruse <llvm at meinersbur.de> wrote:
> > > >
> > > > Itaru could mean compiling his application with debug symbols (-O0 -g)
> > > > using a release build of clang (it's always good to mention which
> > > > version of clang you are using) or an optimized version of his
> > > > application using an CMAKE_BUILD_TYPE=Debug build of clang. Maybe he
> > > > could clarify?
> > > >
> > > > If you are running linux, a performance trace (flame graph / Linux
> > > > perf record) would be useful, especially if you are using a debug
> > > > build of clang.
> > > >
> > > > Michael
> > > >
> > > > Am Mi., 10. Juni 2020 um 04:55 Uhr schrieb Jonas Hahnfeld via
> > > > Openmp-dev <openmp-dev at lists.llvm.org>:
> > > > >
> > > > > You wrote "They were both Debug build". Maybe I misunderstand?
> > > > >
> > > > > Am Mittwoch, den 10.06.2020, 18:52 +0900 schrieb Itaru Kitayama:
> > > > > > Why do you think I build in Debug? I try to
> > > > > > avoid that all time.
> > > > > >
> > > > > > On Wed, Jun 10, 2020 at 18:38 Jonas Hahnfeld <hahnjo at hahnjo.de> wrote:
> > > > > > > You should not use Debug build to compare the performance of compilers.
> > > > > > > As you're targeting different architectures, you might run into
> > > > > > > different checks in the two backends. I believe it's possible this is
> > > > > > > triggered by OpenMP code generation which might hit some corner cases.
> > > > > > > This could also happen in Release builds, but might be less probable.
> > > > > > >
> > > > > > > Also keep in mind that AArch64 (which you're probably talking about in
> > > > > > > the context of HPC) now uses GlobalISel in some configurations. I don't
> > > > > > > remember if it's with or without optimizations (or maybe even both
> > > > > > > nowadays?), but you can try to deactivate this on the command line to
> > > > > > > rule out this area.
> > > > > > >
> > > > > > > Regards
> > > > > > > Jonas
> > > > > > >
> > > > > > > Am Dienstag, den 09.06.2020, 08:12 +0900 schrieb Itaru Kitayama via
> > > > > > > Openmp-dev:
> > > > > > > > They were both Debug build, but it took 20-25 minutes to finish on Arm.
> > > > > > > >
> > > > > > > > On Tue, Jun 9, 2020 at 7:50 AM Itaru Kitayama <
> > > > > > > > itaru.kitayama at gmail.com
> > > > > > > > > wrote:
> > > > > > > > > My OpenMP offloading app builds within a minute or so on
> > > > > > > > > x86, but the same app takes longer than 5 minutes on Arm.
> > > > > > > > > How do I investigate this further, supplying -v and post
> > > > > > > > > the log here?
> > > > > >
> > > > > >
> > > > > _______________________________________________
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> > > > > Openmp-dev at lists.llvm.org
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