6.2 Multilevel Memories 333 It should be apparent that the above example is just one of a vast range of possibili- ties open to the multilevel memory designer. 6.2.5 locality of reference and Working Sets It is not obvious that an automatically managed multilevel memory system should perform well. The basic requirement for acceptable performance is that all informa- tion items stored in the memory must not have equal frequency of use. If every item is used with equal frequency, then a multilevel memory cannot have good performance, since the overall memory will operate at approximately the speed of the slowest memory component. To illustrate this effect, consider a two-level memory system. The average latency of a two-level memory is: AverageLatency 5 R hit 3 Latency primary 1 R miss 3 Latency secondary (6.2) The term R hit (known as the hit ratio) is the frequency with which items are found in the primary device, and R miss is (1 2 R hit ). This formula is a direct application of Equation 6.1, (in Section 6.1) which gives the average performance of a system with