Demand Paging

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Demand Paging

In computer operating systems, demand paging is an application of virtual memory. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it (i.e., if a page fault occurs). It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process's working set of pages is located in physical memory. This is an example of lazy loading techniques.


Advantages of Demand Paging

• Does not load the pages that are never accessed, so saves the memory for other programs and increases the degree of multiprogramming. 
• Less loading latency at the program startup. 
• Less disk overhead because of fewer page reads.
• Pages will be shared by multiple programs until they are modified by one of them, so a technique called copy on write will be used to save more resources.
• Ability to run large programs on the machine, even though it does not have sufficient memory to run the program. This method is better than an old technique called overlays.
• Does not need extra hardware support than what paging needs, since protection fault can be used to get page fault.

Disadvantages

• Individual programs face extra latency when they access a page for the first time. So prepaging, a method of remembering which pages a process used when it last executed and preloading a few of them, is used to improve performance.
• Memory management with page replacement algorithms becomes slightly more complex.