In
computer operating systems,
demand paging (as opposed to anticipatory paging) is a method of
virtual memory management. 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 and that page is not already in memory (
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 a
lazy loading technique.