Demand paging

<h2 id="basic-concept">Basic concept</h2>
<p>Demand paging only brings pages into memory when an executing process demands them. This is often referred to as <a href="/facts/Lazy_loading/BpH8qyWJ">lazy loading</a>, as only those pages demanded by the process are swapped from <a href="/facts/Secondary_storage/wPHKFpdx">secondary storage</a> to <a href="/facts/Main_memory/wPHKFpdx">main memory</a>. 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.
</p><p>Commonly, to achieve this process a <a href="/facts/Memory_management_unit/tJQtAQzI">memory management unit</a> is used. The memory management unit maps <a href="/facts/Logical_memory/kTaLBkzt">logical memory</a> to <a href="/facts/Physical_memory/wPHKFpdx">physical memory</a>. 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:
</p>
<ul><li>Attempt to access page.</li>
<li>If page is valid (in memory) then continue processing instruction as normal.</li>
<li>If page is invalid then a page-fault trap occurs.</li>
<li>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.</li>
<li>Schedule disk operation to read the desired page into main memory.</li>
<li>Restart the instruction that was interrupted by the operating system trap.</li></ul>
<h2 id="advantages">Advantages</h2>
<p>Demand paging, as opposed to loading all pages immediately:
</p>
<ul><li>Only loads pages that are demanded by the executing process.</li>
<li>As there is more space in main memory, more processes can be loaded, reducing the <a href="/facts/Context_switch/N0oJQ6CC">context switching</a> time, which utilizes large amounts of resources.</li>
<li>Less loading latency occurs at program startup, as less information is accessed from secondary storage and less information is brought into main memory.</li>
<li>As main memory is expensive compared to secondary memory, this technique helps significantly reduce the <a href="/facts/Bill_of_material/cFFW9fga">bill of material</a> (BOM) cost in smart phones for example. Symbian OS had this feature.</li></ul>
<h2 id="disadvantages">Disadvantages</h2>
<ul><li>Individual programs face extra latency when they access a page for the first time.</li>
<li>Low-cost, low-power <a href="/facts/Embedded_system/a4muX1wU">embedded systems</a> may not have a <a href="/facts/Memory_management_unit/tJQtAQzI">memory management unit</a> that supports page replacement.</li>
<li>Memory management with <a href="/facts/Page_replacement_algorithm/ApuSJLaq">page replacement algorithms</a> becomes slightly more complex.</li>
<li>Possible security risks, including vulnerability to <a href="/facts/Timing_attack/Aau4qNoh">timing attacks</a>; see Percival, Colin (2005-05-13). <a href="http://css.csail.mit.edu/6.858/2014/readings/ht-cache.pdf"><i>Cache missing for fun and profit</i></a> (PDF). BSDCan 2005. (specifically the virtual memory attack in section 2).</li>
<li><a href="/facts/Thrashing_(computer_science)/7Xvz6xEa">Thrashing</a> which may occur due to repeated page faults.</li></ul>
<h2 id="see-also">See also</h2>

<a href="/facts/Wikisource/TrMdylNf">Wikisource</a> has original text related to this article:
The Paging Game

<ul><li><a href="/facts/Lazy_evaluation/mD3DxDkc">Lazy evaluation</a></li>
<li><a href="/facts/Page_cache/rcKoMtMD">Page cache</a></li>
<li><a href="/facts/Memory_management/vthLkUpz">Memory management</a></li>
<li><a href="/facts/Virtual_memory/Lw2GemF5">Virtual memory</a></li></ul>

<ul><li>Tanenbaum, Andrew S. <i>Operating Systems: Design and Implementation (Second Edition)</i>. New Jersey: Prentice-Hall 1997.</li></ul>

Demand paging open-in-new

Demand paging