[OpenMP] 40% slow down when setting OMP_PLACES="sockets"

nu quaquaraqua nuquaquaraqua@gmail.com
Mon Jan 4 21:28:14 GMT 2021

Hey there,
I have a multi-threaded experiment that runs with 60+ logical threads on a
dual-socket 40-cores-in total machine. When setting the env. var.
OMP_PLACES="sockets", the experiment is 40% slower. While the program is
indeed linked with -fopenmp, it does not use it any opemp clause in the
experiment nor before. Also this seems specific to GCC, as exporting the
env. var. does not cause any slow down when compiling the program with
Clang, despite both being linked to the same libgomp.

Can someone explain what it is occurring here?

Linux 5.0.17, Fedora 30, gcc v10.2


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