Discussion
Demand paging means that a page is brought into memory
Answer: A
Correct answer: only when it is referenced
Demand paging loads a page only when it is actually referenced. A process begins with none of its pages resident and faults them in as it runs, which is the lazy swapper principle: never bring in a page that may not be needed. The benefits are substantial. Program startup is faster, since only the first few pages are read; less input and output is performed overall, because parts of a program never executed are never loaded; and more processes fit in memory at once, raising CPU utilisation. The alternative, prepaging, brings in a group of pages in anticipation. It reduces the number of faults if the guesses are right and wastes input and output if they are wrong, so it is used selectively - typically when a swapped-out process resumes and its working set is already known. Demand paging works only because of locality of reference: programs concentrate their accesses in small regions, so the fault rate settles low after an initial burst.
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