How Many Threads will be too Many? On the Scalability of OpenMP Implementations.

Exascale systems will exhibit much higher degrees of parallelism both in terms of the number of nodes and the number of cores per node. OpenMP is a widely used standard for exploiting parallelism on the level of individual nodes. Although successfully used on today's systems, it is unclear how well OpenMP implementations will scale to much higher numbers of threads. In this work, we apply automated performance modeling to examine the scalability of OpenMP constructs across different compilers and platforms. We ran tests on Intel Xeon multi-board, Intel Xeon Phi, and Blue Gene with compilers from GNU, IBM, Intel, and PGI. The resulting models reveal a number of scalability issues in implementations of OpenMP constructs and show unexpected differences between compilers.