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 :

Demand paging, as opposed to loading all pages immediately:

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 of initial disk overhead because of fewer page reads.

Does not need extra hardware support than what paging needs, since protection fault can be used to get page fault.

Advantages over non-virtual memory systems:

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 easier for a programmer than an old manual overlays.

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.

Programs running on low-cost, low-power embedded systems may not have a memory management unit that supports page replacement.

Memory management with page replacement algorithms becomes slightly more complex.

Possible security risks, including vulnerability to timing attacks.


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Paging Terminology.

Mode and media for Communication.

PAGING TERMINOLOGY

Historically, paging sometimes also referred simply to memory allocation scheme using fixed-length pages as opposed to variable-length segments, and without implicit suggestion that virtual memory technique is employed at all or that those pages are transferred to disk. Such usage is rare today.

Some modern systems use the term swapping along with paging. Historically, swapping referred to moving from/to secondary storage a whole program at a time, in a scheme known as roll-in/roll-out. In the 1960s, after the concept of virtual memory was introduced—in two variants, either using segments or pages—the term swapping was applied to moving, respectively, either segments or pages, between disk and memory. Today with the virtual memory mostly based on pages, not segments, swapping became a fairly close synonym of paging, although with one difference.

 

In many popular systems, there is a concept known as page cache, of using the same single mechanism for both virtual memory and disk caching. A page may be then transferred to or from any ordinary disk file, not necessarily a dedicated space.

Fragmentation

 

In Windows's default configuration the pagefile is allowed to expand beyond its initial allocation when necessary. If this happens gradually, it can become heavily fragmented which can potentially cause performance problems. The common advice given to avoid this is to set a single "locked" pagefile size so that Windows will not expand it. However, the pagefile only expands when it has been filled, which, in its default configuration, is 150% the total amount of physical memory.Thus the total demand for pagefile-backed virtual memory must exceed 250% of the computer's physical memory before the pagefile will expand.

 

The fragmentation of the pagefile that occurs when it expands is temporary. As soon as the expanded regions are no longer in use (at the next reboot, if not sooner) the additional disk space allocations are freed and the pagefile is back to its original state.

 

Locking a page file's size can be problematic in the case that a Windows application requests more memory than the total size of physical memory and the pagefile. In this case, requests to allocate memory fail, which may cause applications and system processes to fail. Supporters of this view will note that the pagefile is rarely read or written in sequential order, so the performance advantage of having a completely sequential pagefile is minimal. However, it is generally agreed that a large pagefile will allow use of memory-heavy applications, and there is no penalty except that more disk space is used.

Defragmenting the page file is also occasionally recommended to improve performance when a Windows system is chronically using much more memory than its total physical memory. This view ignores the fact that, aside from the temporary results of expansion, the pagefile does not become fragmented over time. In general, performance concerns related to pagefile access are much more effectively dealt with by adding more physical memory.