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 page frame is the smallest fixed-length contiguous block of physical memory into which memory pages are mapped by the operating system.
Contents
- Page size trade off
- Determining the page size in a program
- Unix and POSIX based operating systems
- Windows based operating systems
- Huge pages
- References
A transfer of pages between main memory and an auxiliary store, such as a hard disk drive, is referred to as paging 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 bytes. 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.
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 and POSIX-based systems may use the system function sysconf()
, as illustrated in the following example written in the C programming language.
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 and Windows NT families, may use the system function GetSystemInfo()
from kernel32.dll
.
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.
Some instruction set architectures can support multiple page sizes, including pages significantly larger than the standard page size. Starting with the Pentium Pro, 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'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. This support for huge pages (known as superpages in FreeBSD, and large pages in Microsoft Windows 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 computers, are not in common use except in large servers and computational clusters. 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.
Linux 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 (SP1 and newer), Windows Vista and Windows Server 2008 support huge pages under the name of large pages. Windows 2000 and Windows XP support large pages internally, but do not expose them to applications. Solaris beginning with version 9 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.