[Bug libgomp/113698] GNU OpenMP with OMP_PROC_BIND alters thread affinity in a way that negatively affects performance

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113698

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
You can use GOMP_CPU_AFFINITY or different setting of OMP_PROC_BIND together
with OMP_PLACES to bind the initial thread to different CPU.
While the OpenMP spec allows moving the initial thread from unspecified place
to the requested one only before the first active parallel region, doing that
there would add to the overhead of #pragma omp parallel and what exactly the
application wants is hard to guess, whether the allocations done in the initial
thread prior to the first parallel are meant for the initial thread or other
threads too.
And, if you just add an empty #pragma omp parallel at the start of main, then
OpenMP requires the thread to be bound there.
There are OpenMP allocators one can use and request specific thread binding for
the allocations, or perform the allocations from the threads.