Lec / Virtual Memory / Operating System Software

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The design of the memory management portion of an operating system depends on three fundamental areas of choice: • Whether or not to use virtual memory techniques • The use of paging or segmentation or both • The algorithms employed for various aspects of memory management The choices made in the first two areas depend on the hardware platform available.Thus,earlier UNIX implementations did not provide virtual memory because the processors on which the system ran did not support paging or segmentation.Neither of these techniques is practical without hardware support for address translation and other basic functions. Two additional comments about the first two items in the preceding list:First, with the exception of operating systems for some of the older personal computers, such as MS-DOS,and specialized systems,all important operating systems provide virtual memory.Second,pure segmentation systems are becoming increasingly rare. When segmentation is combined with paging,most of the memory management issues confronting the operating system designer are in the area of paging.4 Thus, we can concentrate in this section on the issues associated with paging. The choices related to the third item are the domain of operating system software and are the subject of this section.Table 8.4 lists the key design elements that we examine.In each case,the key issue is one of performance:We would like to minimize the rate at which page faults occur, because page faults cause considerable software overhead.At a minimum, the overhead includes deciding which resident page or pages to replace,and the I/O of exchanging pages.Also,the operating system must schedule another process to run during the page I/O, causing a process switch.Accordingly,we would like to arrange matters so that,during the time that a process is executing,the probability of referencing a word on a missing page is minimized.In all of the areas referred to in Table 8.4,there is no definitive policy that works best.As we shall see, the task of memory management in a paging environment is fiendishly complex. Furthermore, the performance of any particular set of policies depends on main memory size, the relative speed of main and secondary memory,the size and number of processes competing for resources,and the execution behavior of individual programs.This latter characteristic depends on the nature of the application,the programming language and compiler employed,the style of the programmer who wrote it, and, for an interactive program, the dynamic behavior of the user.Thus,the reader must expect no final answers here or anywhere.