Hi, For testing purposes I run some MPI+OpenMP benchmarks with `mpirun -np 1 ./a.out`, and I am using OpenMPI 3.1.3. As far as I understand, `mpirun` sets an affinity mask, and the OpenMP runtime (in my case the LLVM OpenMP RT) respects this mask and only sees 1 physical core. In my case, I am running in a POWER8 which has 8 logical cores per physical core. The OpenMP runtime in this case creates always the max number of available logical cores in the machines (160 in my machine), but because of the mask in this case will create 8 threads. All this threads runs in the same physical cores making the program slower than if it would run each thread in a different physical core.
So my question is, what's the right way to run a single MPI process such that the OpenMP threads can run in different physical cores independently from the mask set by mpirun? I know about the option `--bind-to none` and using that all the cores in the system become available and the OpenMP runtime uses all of them. Otherwise, doing some web search I read that a singleton MPI program should be executed with ` OMPI_MCA_ess_singleton_isolated=1 ./a.out` without `mpirun` at all, but I couldn't find a good explanation of it. Is there anyone that could clarify this? Thank you! Simone ----------------------------------------------------------------------------------- This email message is for the sole use of the intended recipient(s) and may contain confidential information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply email and destroy all copies of the original message. -----------------------------------------------------------------------------------
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