Page (computing) - AbsoluteAstronomy.com

"Page size" redirects to this article. For information on paper see Paper size

Paper size

Many paper size standards conventions have existed at different times and in different countries. Today there is one widespread international ISO standard and a localised standard used in North America . The paper sizes affect writing paper, stationery, cards, and some printed documents...

A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory

Virtual memory

In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...

that is the smallest unit of data for the following:

memory allocation performed by the operating system for a program; and
transfer between main memory and any other auxiliary store, such as a hard disk drive.

Virtual memory allows a page that does not currently reside in main memory to be addressed and used. If a program tries to access a location in such a page, an exception called a page fault

Page fault

A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space, but not loaded in physical memory. In the typical case the operating system tries to handle the page fault by making the required page accessible at a location...

is generated. The hardware or operating system is notified and loads the required page from the auxiliary store automatically. A program addressing the memory has no knowledge of a page fault or a process following it. Thus a program can address more (virtual) RAM than physically exists in the computer.

A transfer of pages between main memory and an auxiliary store, such as a hard disk drive, is referred to as paging

Paging

In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...

or swapping.

Page size trade-off

Page size is usually determined by processor architecture. Traditionally, pages in a system had uniform size, for example 4096 byte

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

s. However, processor designs often allow two or more, sometimes simultaneous, page sizes due to the benefits and penalties. There are several points that can factor into choosing the best page size.

Page size versus page table size

A system with a smaller page size uses more pages, requiring a page table

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 those unique to the accessing process...

that occupies more space. For example, if a 2³² virtual address space is mapped to 4KB (2¹² bytes) pages, the number of virtual pages is 2²⁰ =( 2³² / 2¹²). However, if the page size is increased to 32KB (2¹⁵ bytes), only 2¹⁷ pages are required.

Page size versus 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

Translation Lookaside Buffer

A translation lookaside buffer is a CPU cache that memory management hardware uses to improve virtual address translation speed. All current desktop and server processors use a TLB to map virtual and physical address spaces, and it is ubiquitous in any hardware which utilizes virtual memory.The...

(TLB), is often used. The TLB is typically 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 of pages

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 clearly increase the potential for wasted memory this way, as more potentially unused portions of memory are loaded into 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 1024KB. If a process allocates 1025KB, two pages must be used, resulting in 1023KB of unused space (where one page fully consumes 1024KB and the other only 1KB).

Page size versus disk access

When transferring from 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.

Determining the page size in a program

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 is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

and POSIX

POSIX

POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

-based systems may use the system function sysconf, as illustrated in the following example written in the C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

programming language.

include
include /* sysconf(3) */

int main(void) {
printf("The page size for this system is %ld bytes.\n",
sysconf(_SC_PAGESIZE)); /* _SC_PAGE_SIZE is OK too. */

return 0;
}

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 Windows 9x, and NT may use the system function GetSystemInfo from kernel32.dll.

include
include

int main(void) {
SYSTEM_INFO si;
GetSystemInfo(&si);

printf("The page size for this system is %u bytes.\n", si.dwPageSize);

return 0;
}

Huge pages

Huge page size depends on processor architecture, processor type, and operating (addressing) mode. The operating system selects one from the sizes supported by the architecture. Note that not all processors implement all defined Huge/Large page sizes.

Architecture	Page Size	Huge Page Size	Large Page Size
i386	4KB	4M (2M in PAE mode)	1GB
ia64	4KB	4K, 8K, 64K, 256K, 1M, 4M, 16M, 256M	-
ppc64	4KB	16M	-

Info from: http://wiki.debian.org/Hugepages
(todo: supplement information for processors manufactures documentations)

Some instruction set architectures can support multiple page sizes, including pages significantly larger than the standard page size. Starting with the Pentium Pro

Pentium Pro

The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel introduced in November 1, 1995 . It introduced the P6 microarchitecture and was originally intended to replace the original Pentium in a full range of applications...

, x86 processors support 4MB pages (called Page Size Extension

Page Size Extension

In computing, Page Size Extension refers to a feature of x86 processors that allows for pages larger than the traditional 4 KiB size. It was introduced in the original Pentium processor, but it was only publicly documented by Intel with the release of the Pentium Pro. The CPUID instruction can be...

) (2MB pages if using PAE

Physical Address Extension

In computing, Physical Address Extension is a feature to allow x86 processors to access a physical address space larger than 4 gigabytes....

) in addition to their standard 4kB pages; newer x86-64

X86-64

x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...

processors, such as AMD's newer AMD64 processors and Intel's Westmere, processors can use 1GB pages in long mode

Long mode

In the x86-64 computer architecture, long mode is the mode where a 64-bit application can access 64-bit instructions and registers...

. IA-64 supports as many as eight different page sizes, from 4kB up to 256MB, and some other architectures have similar features. This support for huge pages (known as superpages in FreeBSD

FreeBSD

FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...

, and large pages in Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

terminology) allows for "the best of both worlds", reducing the pressure on the TLB cache (sometimes increasing speed by as much as 15%, depending on the application and the allocation size) for large allocations while still keeping memory usage at a reasonable level for small allocations.

Huge pages, despite being available in the processors used in most contemporary personal computer

Personal computer

A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

s, are not in common use except in large servers and computational clusters

High-performance computing

High-performance computing uses supercomputers and computer clusters to solve advanced computation problems. Today, computer systems approaching the teraflops-region are counted as HPC-computers.-Overview:...

. 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.

Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

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

Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005...

(SP1 and newer), Windows Vista

Windows Vista

Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

and Windows Server 2008 support huge pages under the name of large pages. Windows 2000

Windows 2000

Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...

and Windows XP

Windows XP

Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...

support large pages internally, but do not expose them to applications. Solaris beginning with version 9 supports large pages on SPARC

SPARC

SPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....

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 FreeBSD and Solaris, applications take advantage of huge pages automatically, without the need for modification.