Memory Management — Basic Principles
Colin J. Theaker, Graham R. Brookes · 1993
The allocation of memory to the processes in a time-sharing system poses one of the most major problems to designers of operating systems. If the system is supporting a large number of user processes, say N , in general it is impractical to keep all of them in memory, as on average only 1/ N of the store will be in use at any given instant. Apart from the process that is currently running, some processes will be waiting for a time slice and some (usually the overwhelming majority) will be waiting for a response from the user. This latter category is the most problematic, as the typical response that can be expected from the user might be of the order of a few seconds (but might even be hours). Clearly, the system should not allow such a valuable commodity as its main memory to be under-utilised to such an extent.