The physical memory acts as a cache backed by the disk. When the physical memory is full, and we want to read in another page from disk, we have to evict a page from physical memory. How do we choose which page to evict? This is determined by the cache replacement policy. Note that in a theoretical algorithms course, you are generally interested in the worst-case asymptotic performance of an algorithm. In building systems, worst-case performance can be important (for instance, in systemswith real-time requirements), but oftenwe canmost about the algorithm's performance for the most common case. If a particular algorithm has good worst-case performance, but bad common-case performance, it is a bad algorithm.