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In computer
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
s, memory paging is a
memory management Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...
scheme by which a computer stores and retrieves data from secondary storage for use in main memory. In this scheme, the operating system retrieves data from secondary storage in same-size blocks called ''
pages Page most commonly refers to: * Page (paper), one side of a leaf of paper, as in a book Page, PAGE, pages, or paging may also refer to: Roles * Page (assistance occupation), a professional occupation * Page (servant), traditionally a young mal ...
''. Paging is an important part of
virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
implementations in modern operating systems, using secondary storage to let programs exceed the size of available physical memory. For simplicity, main memory is called "RAM" (an acronym of
random-access memory Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the ...
) and secondary storage is called "disk" (a shorthand for
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magne ...
,
drum memory Drum memory was a magnetic data storage device invented by Gustav Tauschek in 1932 in Austria. Drums were widely used in the 1950s and into the 1960s as computer memory. For many early computers, drum memory formed the main working memory of ...
or
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is a ...
, etc.), but as with many aspects of computing, the concepts are independent of the technology used. Depending on the memory model, paged memory functionality is usually hardwired into a CPU/MCU by using a
Memory Management Unit A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit having all memory references passed through itself, primarily performing the translation of virtual memory addresses to physical a ...
(MMU) or
Memory Protection Unit A memory protection unit (MPU), is a computer hardware unit that provides memory protection. It is usually implemented as part of the central processing unit (CPU). MPU is a trimmed down version of memory management unit (MMU) providing only memory ...
(MPU) and separately enabled by privileged system code in the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
's
kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...
. In CPUs implementing the
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
instruction set architecture (ISA) for instance, the memory paging is enabled via the CR0
control register A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, ...
.


History

In the 1960s, swapping was an early virtual memory technique. An entire program or entire segment would be "swapped out" (or "rolled out") from RAM to disk or drum, and another one would be ''swapped in'' (or ''rolled in''). A swapped-out program would be current but its execution would be suspended while its RAM was in use by another program; a program with a swapped-out segment could continue running until it needed that segment, at which point it would be suspended until the segment was swapped in. A program might include multiple overlays that occupy the same memory at different times. Overlays are not a method of paging RAM to disk but merely of minimizing the program's RAM use. Subsequent architectures used
memory segmentation Memory segmentation is an operating system memory management technique of division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifi ...
, and individual program segments became the units exchanged between disk and RAM. A segment was the program's entire code segment or data segment, or sometimes other large data structures. These segments had to be
contiguous Contiguity or contiguous may refer to: *Contiguous data storage, in computer science *Contiguity (probability theory) *Contiguity (psychology) *Contiguous distribution of species, in biogeography *Geographic contiguity of territorial land *Contigu ...
when resident in RAM, requiring additional computation and movement to remedy fragmentation.
Ferranti Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. The company was once a constituent of the FTSE 100 Index. The firm was known ...
's
Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
, and the
Atlas Supervisor The Atlas Supervisor was the program which managed the allocation of processing resources of Manchester University's Atlas Computer so that the machine was able to act on many tasks and user programs concurrently. Its various functions includ ...
developed at the
University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST (as university college; university 1994) 1904 – Victoria University of Manchester 1880 – Victoria Univ ...
, (1962), was the first system to implement memory paging. Subsequent early machines, and their operating systems, supporting paging include the
IBM M44/44X The IBM M44/44X was an experimental computer system from the mid-1960s, designed and operated at IBM's Thomas J. Watson Research Center at Yorktown Heights, New York. It was based on an IBM 7044 (the 'M44'), and simulated multiple 7044 virtual mach ...
and its MOS operating system (1964),, the SDS 940 and the
Berkeley Timesharing System The Berkeley Timesharing System was a pioneering time-sharing operating system implemented between 1964 and 1967 at the University of California, Berkeley. It was designed as part of Project Genie and marketed by Scientific Data Systems for the ...
(1966), a modified
IBM System/360 Model 40 The IBM System/360 Model 40 was a mid-range member of the IBM System/360 family. It was announced on April 7, 1964, shipped in 1965, and withdrawn on October 7, 1977. History On April 7, 1964, IBM announced the IBM System/360, to be availa ...
and the
CP-40 CP-40 was a research precursor to CP-67, which in turn was part of IBM's then-revolutionary CP/CMS, CP 67CMS – a virtual machine/virtual memory time-sharing operating system for the IBM System/360 Model 67, and the parent of IBM's VM (operati ...
operating system (1967), the
IBM System/360 Model 67 IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. Current mainframe computers in IBM's line of business computers are developments of the basic design of th ...
and operating systems such as
TSS/360 The IBM Time Sharing System TSS/360 is a discontinued early time-sharing operating system designed exclusively for a special model of the System/360 line of mainframes, the Model 67. Made available on a trial basis to a limited set of custom ...
and
CP/CMS CP/CMS (Control Program/Cambridge Monitor System) is a discontinued time-sharing operating system of the late 1960s and early 1970s, known for its excellent performance and advanced features. It had three distinct versions: * CP-40/CMS, an im ...
(1967), the RCA 70/46 and the
Time Sharing Operating System Time Sharing Operating System, or TSOS, is a discontinued operating system for RCA mainframe computers of the Spectra 70 series. TSOS was originally designed in 1968 for the Spectra 70/46, a modified version of the 70/45. TSOS quickly evolved in ...
(1967), the
GE 645 The GE 645 mainframe computer was a development of the GE 635 for use in the Multics project. This was the first computer that implemented a configurable hardware protected memory system. The original CTSS was implemented on a modified IBM 7094 wi ...
and
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of ...
(1969), and the
PDP-10 Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, espec ...
with added BBN-designed paging hardware and the TENEX operating system (1969). Those machines, and subsequent machines supporting memory paging, use either a set of
page address register A page address register (PAR) contains the physical addresses of pages currently held in the main memory of a computer system. PARs are used in order to avoid excessive use of an address table in some operating systems. A PAR may check a page's n ...
s or an in-memory
page table A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing proces ...
to allow the processor to operate on arbitrary pages anywhere in RAM as a seemingly contiguous
logical address In computing, a logical address is the address at which an item ( memory cell, storage element, network host) appears to reside from the perspective of an executing application program. A logical address may be different from the physical addr ...
space. These pages became the units exchanged between disk and RAM.


Page faults

When a process tries to reference a page not currently present in RAM, the processor treats this invalid memory reference as a
page fault In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added t ...
and transfers control from the program to the operating system. The operating system must: # Determine the location of the data on disk. # Obtain an empty
page frame A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table. It is the smallest unit of data for memory management in a virtual memory operating system. Similarly, a ...
in RAM to use as a container for the data. # Load the requested data into the available page frame. # Update the
page table A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing proces ...
to refer to the new page frame. # Return control to the program, transparently retrying the instruction that caused the page fault. When all page frames are in use, the operating system must select a page frame to reuse for the page the program now needs. If the evicted page frame was dynamically allocated by a program to hold data, or if a program modified it since it was read into RAM (in other words, if it has become "dirty"), it must be written out to disk before being freed. If a program later references the evicted page, another page fault occurs and the page must be read back into RAM. The method the operating system uses to select the page frame to reuse, which is its
page replacement algorithm In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out, sometimes called swap out, or write to disk, when a page of memory needs to be allocated. Page repl ...
, is important to efficiency. The operating system predicts the page frame least likely to be needed soon, often through the least recently used (LRU) algorithm or an algorithm based on the program's
working set Working set is a concept in computer science which defines the amount of memory that a process requires in a given time interval. Definition Peter Denning (1968) defines "the working set of information W(t, \tau) of a process at time t to be the ...
. To further increase responsiveness, paging systems may predict which pages will be needed soon, preemptively loading them into RAM before a program references them.


Page replacement techniques

; Demand paging : When pure demand paging is used, pages are loaded only when they are referenced. A program from a memory mapped file begins execution with none of its pages in RAM. As the program commits page faults, the operating system copies the needed pages from a file, e.g.,
memory-mapped file A memory-mapped file is a segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also b ...
, paging file, or a swap partition containing the page data into RAM. ; Anticipatory paging : This technique, sometimes also called ''swap prefetch,'' predicts which pages will be referenced soon, to minimize future page faults. For example, after reading a page to service a page fault, the operating system may also read the next few pages even though they are not yet needed (a prediction using
locality of reference In computer science, locality of reference, also known as the principle of locality, is the tendency of a processor to access the same set of memory locations repetitively over a short period of time. There are two basic types of reference localit ...
). If a program ends, the operating system may delay freeing its pages, in case the user runs the same program again. ; Free page queue, stealing, and reclamation : The free page queue is a list of page frames that are available for assignment. Preventing this queue from being empty minimizes the computing necessary to service a page fault. Some operating systems periodically look for pages that have not been recently referenced and then free the page frame and add it to the free page queue, a process known as "page stealing". Some operating systems support ''page reclamation''; if a program commits a page fault by referencing a page that was stolen, the operating system detects this and restores the page frame without having to read the contents back into RAM. ; Pre-cleaning : The operating system may periodically pre-clean dirty pages: write modified pages back to disk even though they might be further modified. This minimizes the amount of cleaning needed to obtain new page frames at the moment a new program starts or a new data file is opened, and improves responsiveness. (Unix operating systems periodically use sync to pre-clean all dirty pages; Windows operating systems use "modified page writer" threads.)


Thrashing

After completing initialization, most programs operate on a small number of code and data pages compared to the total memory the program requires. The pages most frequently accessed are called the
working set Working set is a concept in computer science which defines the amount of memory that a process requires in a given time interval. Definition Peter Denning (1968) defines "the working set of information W(t, \tau) of a process at time t to be the ...
. When the working set is a small percentage of the system's total number of pages, virtual memory systems work most efficiently and an insignificant amount of computing is spent resolving page faults. As the working set grows, resolving page faults remains manageable until the growth reaches a critical point. Then faults go up dramatically and the time spent resolving them overwhelms time spent on the computing the program was written to do. This condition is referred to as thrashing. Thrashing occurs on a program that works with huge data structures, as its large working set causes continual page faults that drastically slow down the system. Satisfying page faults may require freeing pages that will soon have to be re-read from disk. "Thrashing" is also used in contexts other than virtual memory systems; for example, to describe
cache Cache, caching, or caché may refer to: Places United States * Cache, Idaho, an unincorporated community * Cache, Illinois, an unincorporated community * Cache, Oklahoma, a city in Comanche County * Cache, Utah, Cache County, Utah * Cache County ...
issues in computing or silly window syndrome in networking. A worst case might occur on
VAX VAX (an acronym for Virtual Address eXtension) is a series of computers featuring a 32-bit instruction set architecture (ISA) and virtual memory that was developed and sold by Digital Equipment Corporation (DEC) in the late 20th century. The V ...
processors. A single MOVL crossing a page boundary could have a source operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and a destination operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and the source and destination could both cross page boundaries. This single instruction references ten pages; if not all are in RAM, each will cause a page fault. As each fault occurs the operating system needs to go through the extensive memory management routines perhaps causing multiple I/Os which might include writing other process pages to disk and reading pages of the active process from disk. If the operating system could not allocate ten pages to this program, then remedying the page fault would discard another page the instruction needs, and any restart of the instruction would fault again. To decrease excessive paging and resolve thrashing problems, a user can increase the number of pages available per program, either by running fewer programs concurrently or increasing the amount of RAM in the computer.


Sharing

In multi-programming or in a
multi-user Multi-user software is computer software that allows access by multiple users of a computer. Time-sharing systems are multi-user systems. Most batch processing systems for mainframe computers may also be considered "multi-user", to avoid leaving t ...
environment, many users may execute the same program, written so that its code and data are in separate pages. To minimize RAM use, all users share a single copy of the program. Each process's
page table A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing proces ...
is set up so that the pages that address code point to the single shared copy, while the pages that address data point to different physical pages for each process. Different programs might also use the same libraries. To save space, only one copy of the shared library is loaded into physical memory. Programs which use the same library have virtual addresses that map to the same pages (which contain the library's code and data). When programs want to modify the library's code, they use
copy-on-write Copy-on-write (COW), sometimes referred to as implicit sharing or shadowing, is a resource-management technique used in computer programming to efficiently implement a "duplicate" or "copy" operation on modifiable resources. If a resource is dupl ...
, so memory is only allocated when needed. Shared memory is an efficient way of communication between programs. Programs can share pages in memory, and then write and read to exchange data.


Implementations


Ferranti Atlas

The first computer to support paging was the supercomputer
Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
, jointly developed by
Ferranti Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. The company was once a constituent of the FTSE 100 Index. The firm was known ...
, the
University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST (as university college; university 1994) 1904 – Victoria University of Manchester 1880 – Victoria Univ ...
and
Plessey The Plessey Company plc was a British electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after World War II by acquisition of companies and formed overseas compani ...
in 1963. The machine had an associative ( content-addressable) memory with one entry for each 512 word page. The Supervisor handled non-equivalence interruptions and managed the transfer of pages between core and drum in order to provide a one-level store to programs.


Microsoft Windows


Windows 3.x and Windows 9x

Paging has been a feature of Microsoft Windows since
Windows 3.0 Windows 3.0 is the third major release of Microsoft Windows, launched in 1990. It features a new graphical user interface (GUI) where applications are represented as clickable icons, as opposed to the list of file names seen in its predeces ...
in 1990. Windows 3.x creates a
hidden file In computing, a hidden folder (sometimes hidden directory) or hidden file is a folder or file which filesystem utilities do not display by default when showing a directory listing. They are commonly used for storing user preferences or preservi ...
named 386SPART.PAR or WIN386.SWP for use as a swap file. It is generally found in the root directory, but it may appear elsewhere (typically in the WINDOWS directory). Its size depends on how much swap space the system has (a setting selected by the user under Control Panel → Enhanced under "Virtual Memory"). If the user moves or deletes this file, a blue screen will appear the next time Windows is started, with the
error message An error message is information displayed when an unforeseen occurs, usually on a computer or other device. On modern operating systems with graphical user interfaces, error messages are often displayed using dialog boxes. Error messages are use ...
"The permanent swap file is corrupt". The user will be prompted to choose whether or not to delete the file (whether or not it exists).
Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturi ...
,
Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released to ...
and
Windows Me Windows Millennium Edition, or Windows Me (marketed with the pronunciation of the pronoun "me"), is an operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It is the successor to Windo ...
use a similar file, and the settings for it are located under Control Panel → System → Performance tab → Virtual Memory. Windows automatically sets the size of the page file to start at 1.5× the size of physical memory, and expand up to 3× physical memory if necessary. If a user runs memory-intensive applications on a system with low physical memory, it is preferable to manually set these sizes to a value higher than default.


Windows NT

The file used for paging in the
Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...
family is pagefile.sys. The default location of the page file is in the root directory of the partition where Windows is installed. Windows can be configured to use free space on any available drives for page files. It is required, however, for the boot partition (i.e., the drive containing the Windows directory) to have a page file on it if the system is configured to write either kernel or full memory dumps after a Blue Screen of Death. Windows uses the paging file as temporary storage for the memory dump. When the system is rebooted, Windows copies the memory dump from the page file to a separate file and frees the space that was used in the page file.


Fragmentation

In the default configuration of Windows, the page file 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" page file size so that Windows will not expand it. However, the page file only expands when it has been filled, which, in its default configuration, is 150% of the total amount of physical memory. Thus the total demand for page file-backed virtual memory must exceed 250% of the computer's physical memory before the page file will expand. The fragmentation of the page file 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 page file is back to its original state. Locking a page file size can be problematic if a Windows application requests more memory than the total size of physical memory and the page file, leading to failed requests to allocate memory that may cause applications and system processes to fail. Also, the page file is rarely read or written in sequential order, so the performance advantage of having a completely sequential page file is minimal. However, a large page file generally allows the use of memory-heavy applications, with no penalties besides using more disk space. While a fragmented page file may not be an issue by itself, fragmentation of a variable size page file will over time create several fragmented blocks on the drive, causing other files to become fragmented. For this reason, a fixed-size contiguous page file is better, providing that the size allocated is large enough to accommodate the needs of all applications. The required disk space may be easily allocated on systems with more recent specifications (i.e. a system with 3  GB of memory having a 6  GB fixed-size page file on a 750  GB disk drive, or a system with 6  GB of memory and a 16  GB fixed-size page file and 2  TB of disk space). In both examples, the system uses about 0.8% of the disk space with the page file pre-extended to its maximum.
Defragment In the maintenance of file systems, defragmentation is a process that reduces the degree of fragmentation. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguo ...
ing 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 page file does not become fragmented over time. In general, performance concerns related to page file access are much more effectively dealt with by adding more physical memory.


Unix and Unix-like systems

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...
systems, and other
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems, use the term "swap" to describe the act of substituting disk space for RAM when physical RAM is full. In some of those systems, it is common to dedicate an entire partition of a hard disk to swapping. These partitions are called ''swap partitions''. Many systems have an entire hard drive dedicated to swapping, separate from the data drive(s), containing only a swap partition. A hard drive dedicated to swapping is called a "swap drive" or a "scratch drive" or a " scratch disk". Some of those systems only support swapping to a swap partition; others also support swapping to files.


Linux

The Linux kernel supports a virtually unlimited number of swap backends (devices or files), and also supports assignment of backend priorities. When the kernel swaps pages out of physical memory, it uses the highest-priority backend with available free space. If multiple swap backends are assigned the same priority, they are used in a round-robin fashion (which is somewhat similar to
RAID 0 In computer storage, the standard RAID levels comprise a basic set of RAID ("redundant array of independent disks" or "redundant array of inexpensive disks") configurations that employ the techniques of striping, mirroring, or parity to create lar ...
storage layouts), providing improved performance as long as the underlying devices can be efficiently accessed in parallel.


= Swap files and partitions

= From the end-user perspective, swap files in versions 2.6.x and later of the Linux kernel are virtually as fast as swap partitions; the limitation is that swap files should be contiguously allocated on their underlying file systems. To increase performance of swap files, the kernel keeps a map of where they are placed on underlying devices and accesses them directly, thus bypassing the cache and avoiding filesystem overhead. When residing on HDDs, which are rotational magnetic media devices, one benefit of using swap partitions is the ability to place them on contiguous HDD areas that provide higher data throughput or faster seek time. However, the administrative flexibility of swap files can outweigh certain advantages of swap partitions. For example, a swap file can be placed on any mounted file system, can be set to any desired size, and can be added or changed as needed. Swap partitions are not as flexible; they cannot be enlarged without using partitioning or
volume management In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate, ...
tools, which introduce various complexities and potential downtimes.


= Swappiness

= ''Swappiness'' is a Linux kernel parameter that controls the relative weight given to swapping out of runtime memory, as opposed to dropping
pages Page most commonly refers to: * Page (paper), one side of a leaf of paper, as in a book Page, PAGE, pages, or paging may also refer to: Roles * Page (assistance occupation), a professional occupation * Page (servant), traditionally a young mal ...
from the system
page cache In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache ...
, whenever a memory allocation request cannot be met from free memory. Swappiness can be set to a value from 0 to 200. A low value causes the kernel to prefer to evict pages from the page cache while a higher value causes the kernel to prefer to swap out "cold" memory pages. The default value is 60; setting it higher can cause high latency if cold pages need to be swapped back in (when interacting with a program that had been idle for example), while setting it lower (even 0) may cause high latency when files that had been evicted from the cache need to be read again, but will make interactive programs more responsive as they will be less likely to need to swap back cold pages. Swapping can also slow down HDDs further because it involves a lot of random writes, while
SSDs A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is a ...
do not have this problem. Certainly the default values work well in most workloads, but desktops and interactive systems for any expected task may want to lower the setting while batch processing and less interactive systems may want to increase it.


= Swap death

= When the system memory is highly insufficient for the current tasks and a large portion of memory activity goes through a slow swap, the system can become practically unable to execute any task, even if the CPU is idle. When every process is waiting on the swap, the system is considered to be in ''swap death''. Swap death can happen due to incorrectly configured
memory overcommitment Memory overcommitment is a concept in computing that covers the assignment of more memory to virtual computing devices (or processes) than the physical machine they are hosted, or running on, actually has. This is possible because virtual machines ( ...
. The original description of the "swapping to death" problem relates to the X server. If code or data used by the X server to respond to a keystroke is not in main memory, then if the user enters a keystroke, the server will take one or more page faults, requiring those pages to read from swap before the keystroke can be processed, slowing the response to it. If those pages don't remain in memory, they will have to be faulted in again to handle the next keystroke, making the system practically unresponsive even if it's actually executing other tasks normally.


macOS

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
uses multiple swap files. The default (and Apple-recommended) installation places them on the root partition, though it is possible to place them instead on a separate partition or device.


AmigaOS 4

AmigaOS 4.0 introduced a new system for allocating RAM and defragmenting physical memory. It still uses flat shared address space that cannot be defragmented. It is based on slab allocation method and paging memory that allows swapping. Paging was implemented in AmigaOS 4.1 but may lock up system if all physical memory is used up. Swap memory could be activated and deactivated any moment allowing the user to choose to use only physical RAM.


Performance

The backing store for a virtual memory operating system is typically many
orders of magnitude An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
slower than
RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...
. Additionally, using mechanical storage devices introduces delay, several milliseconds for a hard disk. Therefore, it is desirable to reduce or eliminate swapping, where practical. Some operating systems offer settings to influence the kernel's decisions. * Linux offers the /proc/sys/vm/
swappiness In computer operating systems, memory paging is a memory management scheme by which a computer stores and retrieves data from secondary storage for use in main memory. In this scheme, the operating system retrieves data from secondary storage ...
parameter, which changes the balance between swapping out runtime memory, as opposed to dropping pages from the system
page cache In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache ...
. * Windows 2000, XP, and Vista offer the DisablePagingExecutive registry setting, which controls whether kernel-mode code and data can be eligible for paging out. * Mainframe computers frequently used head-per-track disk drives or drums for page and swap storage to eliminate seek time, and several technologies to have multiple concurrent requests to the same device in order to reduce
rotational latency Higher performance in hard disk drives comes from devices which have better performance characteristics. These performance characteristics can be grouped into two categories: access time and data transfer time (or rate). Access time The ''access ...
. * Flash memory has a finite number of erase-write cycles (see limitations of flash memory), and the smallest amount of data that can be erased at once might be very large (128 KiB for an Intel X25-M SSD ), seldom coinciding with pagesize. Therefore, flash memory may wear out quickly if used as swap space under tight memory conditions. On the attractive side, flash memory is practically delayless compared to hard disks, and not volatile as RAM chips. Schemes like
ReadyBoost ReadyBoost (codenamed EMD) is a disk caching software component developed by Microsoft for Windows Vista and included in later versions of Windows. ReadyBoost enables NAND memory mass storage CompactFlash, SD card, and USB flash drive devices t ...
and
Intel Turbo Memory Intel Turbo Memory is a technology introduced by Intel Corporation that uses NAND flash memory modules to reduce the time it takes for a computer to power up, access programs, and write data to the hard drive. During development, the technology wa ...
are made to exploit these characteristics. Many
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems (for example
AIX Aix or AIX may refer to: Computing * AIX, a line of IBM computer operating systems *An Alternate Index, for a Virtual Storage Access Method Key Sequenced Data Set * Athens Internet Exchange, a European Internet exchange point Places Belgi ...
,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
, and Solaris) allow using multiple storage devices for swap space in parallel, to increase performance.


Swap space size

In some older virtual memory operating systems, space in swap backing store is reserved when programs allocate memory for runtime data. Operating system vendors typically issue guidelines about how much swap space should be allocated.


Addressing limits on 32-bit hardware

Paging is one way of allowing the size of the addresses used by a process, which is the process's "virtual address space" or "logical address space", to be different from the amount of main memory actually installed on a particular computer, which is the physical address space.


Main memory smaller than virtual memory

In most systems, the size of a process's virtual address space is much larger than the available main memory. For example: * The
address bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
that connects the CPU to main memory may be limited. The i386SX CPU's 32-bit internal addresses can address 4 GB, but it has only 24 pins connected to the address bus, limiting installed physical memory to 16 MB. There may be other hardware restrictions on the maximum amount of RAM that can be installed. * The maximum memory might not be installed because of cost, because the model's standard configuration omits it, or because the buyer did not believe it would be advantageous. * Sometimes not all internal addresses can be used for memory anyway, because the hardware architecture may reserve large regions for I/O or other features.


Main memory the same size as virtual memory

A computer with true ''n''-bit addressing may have 2 addressable units of RAM installed. An example is a 32-bit
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
processor with 4  GB and without
Physical Address Extension In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension, is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon proces ...
(PAE). In this case, the processor is able to address all the RAM installed and no more. However, even in this case, paging can be used to create a virtual memory of over 4 GB. For instance, many programs may be running concurrently. Together, they may require more than 4 GB, but not all of it will have to be in RAM at once. A paging system makes efficient decisions on which memory to relegate to secondary storage, leading to the best use of the installed RAM. Although the processor in this example cannot address RAM beyond 4 GB, the operating system may provide services to programs that envision a larger memory, such as files that can grow beyond the limit of installed RAM. The operating system lets a program manipulate data in the file arbitrarily, using paging to bring parts of the file into RAM when necessary.


Main memory larger than virtual address space

A few computers have a main memory larger than the virtual address space of a process, such as the Magic-1, some PDP-11 machines, and some systems using 32-bit
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
processors with
Physical Address Extension In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension, is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon proces ...
. This nullifies a significant advantage of paging, since a single process cannot use more main memory than the amount of its virtual address space. Such systems often use paging techniques to obtain secondary benefits: * The "extra memory" can be used in the
page cache In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache ...
to cache frequently used files and metadata, such as directory information, from secondary storage. * If the processor and operating system support multiple virtual address spaces, the "extra memory" can be used to run more processes. Paging allows the cumulative total of virtual address spaces to exceed physical main memory. * A process can store data in
memory-mapped file A memory-mapped file is a segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also b ...
s on memory-backed file systems, such as the
tmpfs tmpfs (short for Temporary File System) is a temporary file storage paradigm implemented in many Unix-like operating systems. It is intended to appear as a mounted file system, but data is stored in volatile memory instead of a persistent storage ...
file system or file systems on a
RAM drive Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...
, and map files into and out of the address space as needed. * A set of processes may still depend upon the enhanced security features page-based isolation may bring to a multitasking environment. The size of the cumulative total of virtual address spaces is still limited by the amount of secondary storage available.


See also

* Bélády's anomaly *
Demand paging 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 mad ...
, a "lazy" paging scheme *
Expanded memory In DOS memory management, expanded memory is a system of bank switching that provided additional memory to DOS programs beyond the limit of conventional memory (640 KiB). ''Expanded memory'' is an umbrella term for several incompatible tec ...
*
Memory management Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...
*
Memory segmentation Memory segmentation is an operating system memory management technique of division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifi ...
* Page (computer memory) *
Page cache In computing, a page cache, sometimes also called disk cache, is a transparent cache for the pages originating from a secondary storage device such as a hard disk drive (HDD) or a solid-state drive (SSD). The operating system keeps a page cache ...
, a disk cache that utilizes virtual memory mechanism *
Page replacement algorithm In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out, sometimes called swap out, or write to disk, when a page of memory needs to be allocated. Page repl ...
*
Page table A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing proces ...
*
Physical memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a compute ...
, a subject of paging *
Virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...
, an abstraction that paging may create


Notes


References


External links


Swap Facts and questions
by Ubuntu Documentation
Windows Server - Moving Pagefile to another partition or disk
by David Nudelman

from HowStuffWorks.com (in fact explains only swapping concept, and not virtual memory concept)

(outdated, as the author admits)
Guide On Optimizing Virtual Memory Speed
(outdated, and contradicts section 1.4 of this wiki page, and (at least) references 8, 9, and 11.)
Virtual Memory Page Replacement Algorithms

Windows XP: How to manually change the size of the virtual memory paging file

Windows XP: Factors that may deplete the supply of paged pool memory

SwapFs
driver that can be used to save the paging file of Windows on a swap partition of Linux {{Operating systems Memory management Virtual memory