Early experiences with OpenMP on the Origin 2000

OpenMP has been marketed as THE emerging standard for shared memory parallelism (SMP). The rst compiler for OpenMP is now available on the Cray Origin 2000. In this paper we report on some early experiences with this compiler on a (quasi-)application code, an implementation of the NAS, BT benchmark. OpenMP includes, of course, the traditional do-loop parallelization. For programmers familiar with Power Fortran this is simply a question of adapting to a slight modiication of syntax. Not surprisingly the performance equals that of Power Fortran. This part is easy and straightforward. The interesting thing about OpenMP is that it goes beyond the simple do-loop parallelization and opens possibilities for a more course grain parallelism by splitting the programming in diierent independent tasks (Parallel sections) and/or creating parallel regions. Combining this with the more ne grained do-loop parallelism keeps the promise of higher scalability than one usually nd in SMP-programming. Allowing for nested parallelism, OpenMP gives us this opportunity. With appearance of SMP-system scaling beyond 128 processors this is very important new features. In this paper we focus on those features of OpenMP which extend the model for parallel computing already available with Power Fortran. To get a feel for how it works on real application we have applied the techniques to a BT-code for the NAS parallel benchmark and report results on short coming of this rst release of the compiler as well as speed-up and scalability results. Comparison with MPI will also be provided.