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From: "kugan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libgomp/113698] New: GNU OpenMP with OMP_PROC_BIND alters
 thread affinity in a way that negatively affects performance
Date: Thu, 01 Feb 2024 04:48:21 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libgomp
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113698

            Bug ID: 113698
           Summary: GNU OpenMP with OMP_PROC_BIND alters thread affinity
                    in a way that negatively affects performance
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kugan at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

Created attachment 57275
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D57275&action=3Dedit
testcase

When OMP_PROC_BIND=3Dtrue it seems gomp set the affinity even before main()
starts. In particular, the main thread gets affinity 0x1 (i.e. pinned to the
first core). For the attached, I get

$ OMP_NUM_THREADS=3D72 ./a.out
[main thread affinity right after main()]. tid:ffffae511020
aff:0000000000000000.0000000000000000.000000000000ffff.ffffffffffffffff
duration: 402.949 msec

$ OMP_PROC_BIND=3Dtrue OMP_NUM_THREADS=3D72 ./a.out
[main thread affinity right after main()]. tid:fffdded50020
aff:0000000000000000.0000000000000000.0000000000000000.0000000000000001
duration: 7879.59 msec

$ OMP_PROC_BIND=3Dtrue OMP_NUM_THREADS=3D72 ./a.out
[main thread affinity right after main()]. tid:ffffae54c020
aff:0000000000000000.0000000000000000.0000000000000000.0000000000000001
duration: 311219 msec

Compiler options used:
gcc -O0 -fopenmp repro.c

gcc -v:


Using built-in specs.
COLLECT_GCC=3Dgcc
COLLECT_LTO_WRAPPER=3D/usr/lib/gcc/aarch64-linux-gnu/11/lto-wrapper
Target: aarch64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion=3D'Ubuntu
11.3.0-1ubuntu1~22.04' --with-bugurl=3Dfile:///usr/share/doc/gcc-11/README.=
Bugs
--enable-languages=3Dc,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=3D/usr
--with-gcc-major-version-only --program-suffix=3D-11
--program-prefix=3Daarch64-linux-gnu- --enable-shared --enable-linker-build=
-id
--libexecdir=3D/usr/lib --without-included-gettext --enable-threads=3Dposix
--libdir=3D/usr/lib --enable-nls --enable-bootstrap --enable-clocale=3Dgnu
--enable-libstdcxx-debug --enable-libstdcxx-time=3Dyes
--with-default-libstdcxx-abi=3Dnew --enable-gnu-unique-object
--disable-libquadmath --disable-libquadmath-support --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=3Drelea=
se
--with-target-system-zlib=3Dauto --enable-objc-gc=3Dauto --enable-multiarch
--enable-fix-cortex-a53-843419 --disable-werror --enable-checking=3Drelease
--build=3Daarch64-linux-gnu --host=3Daarch64-linux-gnu --target=3Daarch64-l=
inux-gnu
--with-build-config=3Dbootstrap-lto-lean --enable-link-serialization=3D2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.3.0 (Ubuntu 11.3.0-1ubuntu1~22.04)=