When using a shared memory multiprocessor, the programmer faces the selection of the portable programming model which will deliver the best performance. Even if he restricts his choice to the standard programming environments (MPI and OpenMP), he has a choice of a broad range of programming approaches. To help the programmer in his selection, we compare MPI with three OpenMP programming styles (loop level, loop level with large parallel sections, SPMD) using a subset of the NAS benchmark (CG, MG, FT, LU), two dataset sizes (A and B) and two shared memory multiprocessors (IBM SP3 Night Hawk II, SGI Origin 3800). We also present a path from MPI to OpenMP SPMD guiding the programmers starting from an existing MPI code. We present the first SPMD OpenMP version of the NAS benchmark and compare it with other OpenMP versions from independent sources (PBN, SDSC and RWCP). Experimental results demonstrate that OpenMP provides competitive performance compared to MPI for a large set of experime...