Hybrid MPI/OpenMP showing poor performance when using all cores on a socket vs. N-1 cores per socket

Hi,

I'm running a hydrid MPI/OpenMP application on Intel® E5-2600v3 (Haswell) Series cores and I notice a drop in performance by 40% when using all cores (N) on a socket vs. N-1 cores on a socket. This behavior is pronounced with higher core counts >= 160 cores. The cluster is built using 2 CPUs per node. As a test case I tried a similar run on Intel® E5-2600 Series (Sandy-Bridge) and I don't see this behavior and the performance is comparable.

I'm using Intel MPI 5.0. Both the clusters use the same IB hardware. Profiling revealed MPI time is what is causing the performance drop. The application only performs MPI communication outside OpenMP regions. Any help will be appreciated.

Thanks,

GK