The invention concerns scheduling an application comprised of precedence constrained parallel tasks on a high-performance computer system. The computer system has a plurality of processors each enabled to operate on different voltage supply levels. First, a priority order for the tasks based on the computation and communication costs of the tasks is determined. Next, the based on the priority order of the tasks, assigning each task both a processor and a voltage level that substantially minimizes energy consumption and completion time for performing that task when compared to energy consumption and completion time for performing that task on different combinations of processor and voltage level. It is an advantage of the invention that the scheduling takes account not only completion time (makespan), but also energy consumption. Aspects of the invention include a method, software, a scheduling module of a computer and a schedule.