Editing Demand paging

{{Short description|Method of virtual memory management}}
{{No footnotes|date=June 2009}}
In [[computer]] [[operating system]]s, '''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 [[paging|page]] into physical memory only when 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 (computing)|process]] begins execution with none of its pages in physical memory, and triggers many page faults until most of its [[working set]] of pages are present in physical memory. This is an example of a [[lazy loading]] technique.

==Basic concept==
Demand paging only brings pages into memory when an executing process demands them. This is often referred to as [[lazy loading]], as only those pages demanded by the process are swapped from [[secondary storage]] to [[main memory]]. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory when the process starts up or resumes execution.

Commonly, to achieve this process a [[memory management unit]] is used. The memory management unit maps [[logical memory]] to [[physical memory]]. Entries in the memory management unit include a bit that indicates whether a page is valid or invalid. A valid page is one that currently resides in main memory. An invalid page is one that currently resides in secondary memory. When a process tries to access a page, the following steps are generally followed:
* Attempt to access page.
* If page is valid (in memory) then continue processing instruction as normal.
* If page is invalid then a '''page-fault trap''' occurs.
* Check if the memory reference is a valid reference to a location on secondary memory. If not, the process is terminated ('''illegal memory access'''). Otherwise, we have to '''page in''' the required page.
* Schedule disk operation to read the desired page into main memory.
* Restart the instruction that was interrupted by the operating system trap.

==Advantages==
Demand paging, as opposed to loading all pages immediately:
* Only loads pages that are demanded by the executing process.
* As there is more space in main memory, more processes can be loaded, reducing the [[context switch]]ing time, which utilizes large amounts of resources.
* Less loading latency occurs at program startup, as less information is accessed from secondary storage and less information is brought into main memory.
* As main memory is expensive compared to secondary memory, this technique helps significantly reduce the [[bill of material]] (BOM) cost in smart phones for example. Symbian OS had this feature.

==Disadvantages==
* Individual programs face extra latency when they access a page for the first time.
* Low-cost, low-power [[embedded system]]s may not have a [[memory management unit]] that supports page replacement.
* Memory management with [[page replacement algorithm]]s becomes slightly more complex.
* Possible security risks, including vulnerability to [[timing attack]]s; see {{cite conference |last=Percival |first=Colin |date=2005-05-13 |title=Cache missing for fun and profit |conference=BSDCan 2005 |url=http://css.csail.mit.edu/6.858/2014/readings/ht-cache.pdf}} (specifically the virtual memory attack in section 2).
*[[Thrashing (computer science)|Thrashing]] which may occur due to repeated page faults.

==See also==
{{Wikisource|The Paging Game}}
* [[Lazy evaluation]]
* [[Page cache]]
* [[Memory management]]
* [[Virtual memory]]

==References==
* Tanenbaum, Andrew S. ''Operating Systems: Design and Implementation (Second Edition)''. New Jersey: Prentice-Hall 1997.

{{Memory management}}
[[Category:Virtual memory]]