A little tool to explore the thread placement and pinning of the OpenMP runtime.
I found it hard to understand, how OMP threads are assigned to the sockets and physical/logical cores, depending on the different OMP environment variables (OMP_PROC_BIND, OMP_PLACES, OMP_NUM_THREADS), so I wrote this little tool that prints the initial assignments and also if threads are allowed to move during the execution.
Maybe it helps someone (link with OMP when compiling). Feel free to edit / distribute.
It also lead to the following observation:
If you do not set OMP_PROC_BIND, but set OMP_PLACES=sockets, threads can move between different logical cores during the execution, but only on their initial socket -- they are pinned to a socket.
If you do not set OMP_PROC_BIND, but set OMP_PLACES=cores, threads can move between different logical cores during the execution, but only on their initial physical core -- they are pinned to a physical core.
If you do not set OMP_PROC_BIND, but set OMP_PLACES=threads, threads are pinned to logical cores and can not move at all -- they are pinned to a logical core (hw-thread).