An Operating Systems Vade Mecum

Perspective 49Figure 2.13.3.2 Evaluating policiesGiven a policy, how can we evaluate its performance? Three common approaches areanalysis, simulation, and experimentation.Analysis involves a mathematical formulation of the policy and a derivation of itsbehavior. Such a formulation is often described in the language of queueing networks,which are pictures like the one shown in Figure 2.14. This picture shows a simplifiedmodel of the transitions through which a process might pass. Lines indicate transitions,rectangular objects represent queues, and circles represent servers. The model in thisfigure shows a multiprocessor with two cpu’s that share a single queue. A process(called a ‘‘customer’’ in the jargon of queueing theory) enters from the right and isqueued for execution. It can be served by either cpu after waiting in the cpu queue.After the cpu burst, the process moves to one of the three transput-wait queues, dependingon which one it needs.To complete the model, one describes the probability of taking any branch (forexample, how likely it is for an average process to need disk, as opposed to printer, service),and the queuing parameters for each service station. In particular, one needs toknow the arrival distribution for all the arrival arcs in the network, the service distributionfor all servers (how long does it take to service a disk request?), and the schedulingpolicy at each queue.no instrinsicinformationintrinsicinformationRRFBSRRPSPNpreemptiveFCFSSPNHPRNnon preemptiveFigure 2.13 Classification of scheduling policies