Software preparations Sample Clauses

Software preparations. Given the memory per core will most likely be substantially reduced when compared to today’s HPC platforms, the practice of assigning one MPI task per physical core will have to be substituted by using every 2nd or 4th core for each MPI task. This is known as under-populating nodes. At first glance, this appears to suggest that we cannot fully exploit the hardware, as we simply avoid using 50% or even 75% of the cores; however, these spare cores can be employed via mix-mode codes, where each MPI task runs threaded routines/loops to run on the remaining cores. Essentially, authors must expose as many levels of parallelism as possible within their code. Coding this can involve ensemble runs to coupling multiscale codes, multiprocessing (with interprocess communication) to multithreading, vector processing to accelerator-specific commands. This process can slow the code down on present day platforms but will future-proof the code. For instance, there are sets of serial algorithms, so-called Optimal Serial Algorithms, which are often difficult or simply impossible to parallelise, as these algorithms employ data from the previous steps or even the current step to make improvements at the current step. Such dependencies can prevent concurrent execution of threads in the program, for instance. The inelegant yet empowering solution is to replace the optimal serial algorithm with a sub-optimal serial algorithm which is, however, parallelisable. Whilst the serial performance may be worse, the parallel performance will soon outperform the serial version as the number of cores increases.