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A page, memory page, or virtual page is a fixed-length contiguous block 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 ver ...
, described by a single entry in a page table. It is the smallest unit of data for memory management in an operating system that uses virtual memory. Similarly, a page frame is the smallest fixed-length contiguous block of physical memory into which memory pages are mapped by the operating system. A transfer of pages between main memory and an auxiliary store, such as a
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 hard disk drive platter, pla ...
, is referred to as
paging In computer operating systems, memory paging is a memory management scheme that allows the physical Computer memory, memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compact ...
or swapping.


Explanation

Computer memory is divided into pages so that information can be found more quickly. The concept is named by analogy to the pages of a printed book. If a reader wanted to find, for example, the 5,000th word in the book, they could count from the first word. This would be time-consuming. It would be much faster if the reader had a listing of how many words are on each page. From this listing they could determine which page the 5,000th word appears on, and how many words to count on that page. This listing of the words per page of the book is analogous to a page table of a computer file system.


Page size


Page size trade-off

Page size is usually determined by the processor architecture. Traditionally, pages in a system had uniform size, such as 4,096
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
s. However, processor designs often allow two or more, sometimes simultaneous, page sizes due to its benefits. There are several points that can factor into choosing the best page size.


Page table size

A system with a smaller page size uses more pages, requiring a page table that occupies more space. For example, if a 232 virtual address space is mapped to 4  KiB (212 bytes) pages, the number of virtual pages is 220 = (232 / 212). However, if the page size is increased to 32 KiB (215 bytes), only 217 pages are required. A multi-level paging algorithm can decrease the memory cost of allocating a large page table for each process by further dividing the page table up into smaller tables, effectively paging the page table.


TLB usage

Since every access to memory must be mapped from virtual to physical address, reading the page table every time can be quite costly. Therefore, a very fast kind of cache, the translation lookaside buffer (TLB), is often used. The TLB is of limited size, and when it cannot satisfy a given request (a ''TLB miss'') the page tables must be searched manually (either in hardware or software, depending on the architecture) for the correct mapping. Larger page sizes mean that a TLB cache of the same size can keep track of larger amounts of memory, which avoids the costly TLB misses.


Internal fragmentation

Rarely do processes require the use of an exact number of pages. As a result, the last page will likely only be partially full, wasting some amount of memory. Larger page sizes lead to a large amount of wasted memory, as more potentially unused portions of memory are loaded into the main memory. Smaller page sizes ensure a closer match to the actual amount of memory required in an allocation. As an example, assume the page size is 1024 B. If a process allocates 1025 B, two pages must be used, resulting in 1023 B of unused space (where one page fully consumes 1024 B and the other only 1 B).


Disk access

When transferring from a rotational disk, much of the delay is caused by seek time, the time it takes to correctly position the read/write heads above the disk platters. Because of this, large sequential transfers are more efficient than several smaller transfers. Transferring the same amount of data from disk to memory often requires less time with larger pages than with smaller pages.


Getting page size programmatically

Most operating systems allow programs to discover the page size at runtime. This allows programs to use memory more efficiently by aligning allocations to this size and reducing overall internal fragmentation of pages.


Unix and POSIX-based operating systems

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user 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, a ...
and
POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...
-based systems may use the system function sysconf(), as illustrated in the following example written in the C programming language. #include #include /* sysconf(3) */ int main(void) In many Unix systems, the command-line utility getconf can be used. For example, getconf PAGESIZE will return the page size in bytes.


Windows-based operating systems

Win32-based operating systems, such as those in the
Windows 9x Windows 9x is a generic term referring to a line of discontinued Microsoft Windows operating systems released from 1995 to 2000 and supported until 2006, which were based on the kernel introduced in Windows 95 and modified in succeeding version ...
and
Windows NT Windows NT is a Proprietary software, proprietary Graphical user interface, graphical operating system produced by Microsoft as part of its Windows product line, the first version of which, Windows NT 3.1, was released on July 27, 1993. Original ...
families, may use the system function GetSystemInfo() from kernel32.dll. #include #include int main(void)


Multiple page sizes

Some
instruction set architecture In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, ...
s can support multiple page sizes, including pages significantly larger than the standard page size. The available page sizes depend on the instruction set architecture, processor type, and operating (addressing) mode. The operating system selects one or more sizes from the sizes supported by the architecture. Note that not all processors implement all defined larger page sizes. This support for larger pages (known as "huge pages" in
Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
, "superpages" in FreeBSD, and "large pages" in
Microsoft Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
and IBM AIX terminology) allows for "the best of both worlds", reducing the pressure on the TLB cache (sometimes increasing speed by as much as 15%) for large allocations while still keeping memory usage at a reasonable level for small allocations. Starting with the Pentium Pro, and the AMD Athlon, x86 processors support 4 MiB pages (called Page Size Extension) (2 MiB pages if using PAE) in addition to their standard 4 KiB pages; newer x86-64 processors, such as AMD's newer AMD64 processors and
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
's Westmere and later Xeon processors can use 1 GiB pages in long mode. IA-64 supports as many as eight different page sizes, from 4 KiB up to 256 MiB, and some other architectures have similar features. Larger pages, despite being available in the processors used in most contemporary
personal computer A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...
s, are not in common use except in large-scale applications, the applications typically found in large servers and in computational clusters, and in the operating system itself. Commonly, their use requires elevated privileges, cooperation from the application making the large allocation (usually setting a flag to ask the operating system for huge pages), or manual administrator configuration; operating systems commonly, sometimes by design, cannot page them out to disk. However, SGI IRIX has general-purpose support for multiple page sizes. Each individual process can provide hints and the operating system will automatically use the largest page size possible for a given region of address space. Later work proposed transparent operating system support for using a mix of page sizes for unmodified applications through preemptible reservations, opportunistic promotions, speculative demotions, and fragmentation control.
Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
has supported huge pages on several architectures since the 2.6 series via the hugetlbfs filesystem and without hugetlbfs since 2.6.38.
Windows Server 2003 Windows Server 2003, codenamed "Whistler Server", is the sixth major version of the Windows NT operating system produced by Microsoft and the first server version to be released under the Windows Server brand name. It is part of the Windows NT ...
(SP1 and newer),
Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, released five years earlier, which was then the longest time span between successive releases of Microsoft W ...
and Windows Server 2008 support huge pages under the name of large pages.
Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft, targeting the server and business markets. It is the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RT ...
and
Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct successor to Windows 2000 for high-end and business users a ...
support large pages internally, but do not expose them to applications. Reserving large pages under Windows requires a corresponding right that the system administrator must grant to the user because large pages cannot be swapped out under Windows. Beginning with version 9, Solaris supports large pages on SPARC and x86. FreeBSD 7.2-RELEASE features superpages. Note that until recently in Linux, applications needed to be modified in order to use huge pages. The 2.6.38 kernel introduced support for transparent use of huge pages. On Linux kernels supporting transparent huge pages, as well as FreeBSD and Solaris, applications take advantage of huge pages automatically, without the need for modification.


See also

* Grant table * Page fault * Page table * Memory 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 ver ...
* Zero page - a (often 256-bytes large) memory area at the very start of a processor's address room * Zero page (CP/M) - a 256-byte data structure at the start of a program


References


Further reading

* {{DEFAULTSORT:Page (Computer Memory) Articles with example C code Central processing unit Memory management Virtual memory