Why only half of the cores are used in OPENMP

[志强_赵_]

I have 20 cores per node. I run codes on the machine  using OPENMP.

Why only half of the number of the cores(10) are used, while the others are sleeping?

It puzzles me a lot. 

[jimdempseyatthecove]

Check your OpenMP environment variables relating to affinity. This would be KMP_AFFINITY, GOMP_CPU_AFFINITY, OMP_PLACES, OMP_PROC_BIND, KMP_PLACE_THREADS. And the non-affinity related settings: OMP_THREAD_LIMIT, KMP_ALL_THREADS.

The system administrator may also be able to restrict the number of (and which) logical processors are available to a process (e.g. taskset, cpuset, ...).

Jim Dempsey

[jimdempseyatthecove]

Are you sure you have 20 cores per node as opposed to 10 cores/20 logical processors per node? (Hyperthreading provides for 2 hardware threads per core). As to why this is important, the OpenMP environment variables can be set to limit the OpenMP thread pool to 1 thread per core (each thread can float between either of the 2 hardware threads of a core, no two thread sharing the same core).

Jim Dempsey

[志强_赵_]

The cpuinfo is attached. My program only use the 10 cores with physical id=0.

jimdempseyatthecove wrote:

Are you sure you have 20 cores per node as opposed to 10 cores/20 logical processors per node? (Hyperthreading provides for 2 hardware threads per core). As to why this is important, the OpenMP environment variables can be set to limit the OpenMP thread pool to 1 thread per core (each thread can float between either of the 2 hardware threads of a core, no two thread sharing the same core).

Jim Dempsey