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 very ...
, 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
An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
. Similarly, a page frame is the smallest fixed-length contiguous block of
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 comput ...
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 platters coated with magn ...
, is referred to as
paging or swapping.
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 unit ...
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 2
32 virtual address space is mapped to 4
KiB (2
12 bytes) pages, the number of virtual pages is 2
20 = (2
32 / 2
12). However, if the page size is increased to 32 KiB (2
15 bytes), only 2
17 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
A translation lookaside buffer (TLB) is a memory cache that stores the recent translations of virtual memory to physical memory. It is used to reduce the time taken to access a user memory location. It can be called an address-translation cache. ...
(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 KiB. If a process allocates 1025 KiB, two pages must be used, resulting in 1023 KiB of unused space (where one page fully consumes 1024 KiB and the other only 1 KiB).
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, 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, 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 both the system- and user-level application programming inte ...
-based systems may use the system function
sysconf()
, as illustrated in the following example written in
the C programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
.
#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 series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in sub ...
and
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 Wi ...
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), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
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 ( 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, which i ...
, "superpages" in
FreeBSD
FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
, and "large pages" in
Microsoft Windows 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
Athlon is the brand name applied to a series of x86-compatible microprocessors designed and manufactured by Advanced Micro Devices (AMD). The original Athlon (now called Athlon Classic) was the first seventh-generation x86 processor and the fi ...
,
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
x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...
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, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
's
Westmere and later
Xeon
Xeon ( ) is a brand of x86 microprocessors designed, manufactured, and marketed by Intel, targeted at the non-consumer workstation, server, and embedded system markets. It was introduced in June 1998. Xeon processors are based on the same a ...
processors can use 1 GiB pages in
long mode
In the x86-64 computer architecture, long mode is the mode where a 64-bit operating system can access 64-bit instructions and registers. 64-bit programs are run in a sub-mode called 64-bit mode, while 32-bit programs and 16-bit protected mode p ...
.
IA-64
IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by Intel in col ...
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 (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tech ...
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 ( 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, which i ...
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 is the sixth version of Windows Server operating system produced by Microsoft. It is part of the Windows NT family of operating systems and was released to manufacturing on March 28, 2003 and generally available on April 24, ...
(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, which was released five years before, at the time being the longest time span between successive releases of ...
and
Windows Server 2008
Windows Server 2008 is the fourth release of the Windows Server operating system produced by Microsoft as part of the Windows NT family of the operating systems. It was released to manufacturing on February 4, 2008, and generally to retail on F ...
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 and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was released to manufacturing on December 15, 1999, and was official ...
and
Windows XP
Windows XP is a major release of Microsoft's Windows NT operating system. It was release to manufacturing, released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Wind ...
support large pages internally, but do not expose them to applications. Beginning with version 9,
Solaris supports large pages on
SPARC
SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed ...
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
*
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 very ...
*
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