An Operating Systems Vade Mecum

104 Space Management Chapter 3To illustrate the Law of Diminishing Returns, we will start with a poor method that iseasy to describe. When a page frame must be freed, use the following rule:Random replacementSelect any page frame at random.Figure 3.20 shows the warm-start fault-rate graph on our synthesized pagereferencestring for random replacement as well as some of the other methods we willstudy. The curves are in the same order as the table in the corner of the graph.The curve for random replacement shows the highest fault rate, with a characteristicnumber almost 115,000. It displays anomalies, in which increasing the size of mainstore occasionally makes the fault rate increase. The reason Random does so poorly isthat the page frame we evacuate often contains a page in the current region of locality.7.2 Min10000coldfaults90008000700060005000MethodRandomFIFOLRUMinArea1146941047399415960360400030002000100000 10 20 30 40 50 60 70page frames availableFigure 3.20 Faults for four page-replacement strategies80