https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113698
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=57275&action=edit testcase When OMP_PROC_BIND=true 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=72 ./a.out [main thread affinity right after main()]. tid:ffffae511020 aff:0000000000000000.0000000000000000.000000000000ffff.ffffffffffffffff duration: 402.949 msec $ OMP_PROC_BIND=true OMP_NUM_THREADS=72 ./a.out [main thread affinity right after main()]. tid:fffdded50020 aff:0000000000000000.0000000000000000.0000000000000000.0000000000000001 duration: 7879.59 msec $ OMP_PROC_BIND=true OMP_NUM_THREADS=72 ./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=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/aarch64-linux-gnu/11/lto-wrapper Target: aarch64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 11.3.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-11 --program-prefix=aarch64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --enable-fix-cortex-a53-843419 --disable-werror --enable-checking=release --build=aarch64-linux-gnu --host=aarch64-linux-gnu --target=aarch64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2 Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 11.3.0 (Ubuntu 11.3.0-1ubuntu1~22.04)