PSPN preempts the current process when another process is available with a total service requirement less than the remaining service time required by the current process. When a process has used a certain quanta in its queue, the scheduler moves it to another queue. Use Round Robin policy within each queue to select a process of the highest priority. Interactive processes have priority over longer ones, because long process migrate to lower priority queues. Give larger quanta to lower-priority.
Many additional policies exist.
Evaluating Policies Mathematical Analysis involves a mathematical formulation of the policy and a derivation of its behavior. The primary drawback of mathematical analysis is that the model is only an approximation to the actual system, and complex systems are often impossible to solve. Simulation involves tracking a large number of processes through a model and collecting statistics. Unfortunately, because the system tested must be built, experimentation is usually expensive. Always run real-time processes above normal.