Evaluating parallel algorithms in Extender, a stellerator field evaluator code

The parallel code ``Extender'', originally developed by Michael Drevlak at IPP-Garching, is used to evaluate the magnetic field inside a stellerator at positions between the surface of the plasma and the first wall. This calculation has a number of applications in the design of NCSX. These include positioning the diagnostics and generating a database of information for different possible plasma equilibria. Two parallel algorithms were examined: a master-slave algorithm and an equal-distribution algorithm. The master-slave algorithm adds inter-process communication overhead, but under certain conditions, it uses the available processors more efficiently. The equal-distribution algorithm minimizes communication, but its efficiency relies on calculations taking a consistent amount of time to complete. In ``Extender'', some calculations take longer than others to finish. When these calculations take significant time compared to the time expended in one MPI communication call, as they generally do in ``Extender'', the master-slave algorithm performs reasonably well. Even in these cases, the equal-distribution approach does run somewhat faster.