Page fault
Encyclopedia
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 in physical memory or kills the program in the case of an illegal access. The hardware that detects a page fault is the memory management unit
in a processor. The exception handling
software that handles the page fault is generally part of the operating system
.
Contrary to what the name 'page fault' might suggest, page faults are not errors and are common and necessary to increase the amount of memory available to programs in any operating system that utilizes virtual memory
, including Microsoft Windows
, Unix-like
systems (including Mac OS X
, Linux
, *BSD
, Solaris, AIX, and HP-UX
), and z/OS
. Microsoft uses the term hard fault in more recent versions of the Resource Monitor (e.g., Windows Vista) to mean 'page fault'.
as being loaded in memory, then it is called a minor or soft page fault. The page fault handler in the operating system
merely needs to make the entry for that page in the memory management unit point to the page in memory and indicate that the page is loaded in memory; it does not need to read the page into memory. This could happen if the memory is shared
by different programs and the page is already brought into memory for other programs. The page could also have been removed from a process's Working Set, but not yet written to disk or erased, such as in operating systems that use Secondary Page Caching. For example, HP OpenVMS may remove a page that does not need to be written to disk (if it has remained unchanged since it was last read from disk, for example) and place it on a Free Page List if the working set is deemed too large. However, the page contents are not overwritten until the page is assigned elsewhere, meaning it is still available if it is referenced by the original process before being allocated. Since these faults do not involve disk latency, they are faster and less expensive than major page faults.
, so that there can't be a page in memory corresponding to it, then it is called an invalid page fault. The page fault handler in the operating system then needs to terminate the code that made the reference, or deliver an indication to that code that the reference was invalid. A null pointer is usually represented as a pointer to address 0 in the address space; many operating systems set up the memory management unit to indicate that the page that contains that address is not in memory, and do not include that page in the virtual address space, so that attempts to read or write the memory referenced by a null pointer get an invalid page fault.
depending on the semantics of the operating system environment. Frequently these problems are the manifestation of software bugs, but the hardware memory errors frequently caused by overclocking
may corrupt pointers causing even correct software to fail.
Operating systems such as Windows
and UNIX
(and other UNIX-like
systems) provide differing mechanisms for reporting errors caused by page faults. Windows uses structured exception handling to report page fault-based invalid accesses as access violation exceptions, and UNIX (and UNIX-like) systems typically use signals
, such as SIGSEGV
, to report these error conditions to programs.
If the program that received the error does not handle it, then the operating system typically performs some sort of default action, typically involving the termination of the running process
that caused the error condition, and a notification to the user that the program has malfunctioned. In this vein, recent versions of Windows often report such problems with less technical error messages simply stating something along the lines of "this program must close" (an experienced user or programmer
with access to a debugger
can still retrieve detailed information, if necessary). Additionally, recent Windows versions also write a minidump (similar in principle to a core dump
) describing the state of the crashed process for later analysis alongside such less-technical error messages. UNIX
and UNIX-like
operating systems typically report these conditions to the user with error messages such as "segmentation violation", or "bus error", and may also produce a core dump.
or operating system and in the degenerate case can cause thrashing. Optimizations
to programs and the operating system that reduce the number of page faults that occur improve the performance of the program or even the entire system. The two primary focuses of the optimization effort are reducing overall memory usage and improving memory locality. Generally, making more physical memory available also reduces page faults. Many page replacement algorithm
s have been proposed, such as implementing heuristic-based algorithms to reduce the incidence of page faults.
An average hard disk has an average rotational latency of 3ms, a seek-time of 5ms, and a transfer-time of 0.05ms/page. So the total time for paging comes in near 8ms (8 000 000 ns). If the memory access time is 200ns, then the page fault would make the operation about 40,000 times slower. To reduce the page faults in the system, programmers must make use of an appropriate page replacement algorithm that suits the current requirements and maximizes the page hits.
Trap (computing)
In computing and operating systems, a trap, also known as an exception or a fault, is typicallyThere is a wide variation in the nomenclature...
to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
, 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 in physical memory or kills the program in the case of an illegal access. The hardware that detects a page fault is the memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
in a processor. The exception handling
Exception handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....
software that handles the page fault is generally part of the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
.
Contrary to what the name 'page fault' might suggest, page faults are not errors and are common and necessary to increase the amount of memory available to programs in any operating system that utilizes 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...
, including 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...
, Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems (including Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, 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...
, *BSD
Berkeley Software Distribution
Berkeley Software Distribution is a Unix operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995...
, Solaris, AIX, and HP-UX
HP-UX
HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...
), and z/OS
Z/OS
z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...
. Microsoft uses the term hard fault in more recent versions of the Resource Monitor (e.g., Windows Vista) to mean 'page fault'.
Minor page fault
If the page is loaded in memory at the time the fault is generated, but is not marked in the memory management unitMemory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
as being loaded in memory, then it is called a minor or soft page fault. The page fault handler in the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
merely needs to make the entry for that page in the memory management unit point to the page in memory and indicate that the page is loaded in memory; it does not need to read the page into memory. This could happen if the memory is shared
Shared memory
In computing, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors...
by different programs and the page is already brought into memory for other programs. The page could also have been removed from a process's Working Set, but not yet written to disk or erased, such as in operating systems that use Secondary Page Caching. For example, HP OpenVMS may remove a page that does not need to be written to disk (if it has remained unchanged since it was last read from disk, for example) and place it on a Free Page List if the working set is deemed too large. However, the page contents are not overwritten until the page is assigned elsewhere, meaning it is still available if it is referenced by the original process before being allocated. Since these faults do not involve disk latency, they are faster and less expensive than major page faults.
Major page fault
If the page is not loaded in memory at the time the fault is generated, then it is called a major or hard page fault. The page fault handler in the operating system needs to find a free page in memory, or choose a page in memory to be used for this page's data, write out the data in that page if it hasn't already been written out since it was last modified, mark that page as not being loaded into memory, read the data for that page into the page, and then make the entry for that page in the memory management unit point to the page in memory and indicate that the page is loaded in memory. Major faults are more expensive than minor page faults and add disk latency to the interrupted program's execution. This is the mechanism used by an operating system to increase the amount of program memory available on demand. The operating system delays loading parts of the program from disk until the program attempts to use it and the page fault is generatedInvalid page fault
If a page fault occurs for a reference to an address that's not part of the virtual address spaceAddress space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
, so that there can't be a page in memory corresponding to it, then it is called an invalid page fault. The page fault handler in the operating system then needs to terminate the code that made the reference, or deliver an indication to that code that the reference was invalid. A null pointer is usually represented as a pointer to address 0 in the address space; many operating systems set up the memory management unit to indicate that the page that contains that address is not in memory, and do not include that page in the virtual address space, so that attempts to read or write the memory referenced by a null pointer get an invalid page fault.
Handling illegal accesses and invalid page faults
Illegal accesses and invalid page faults can result in a program crash, segmentation error, bus error or core dumpCore dump
In computing, a core dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally...
depending on the semantics of the operating system environment. Frequently these problems are the manifestation of software bugs, but the hardware memory errors frequently caused by overclocking
Overclocking
Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...
may corrupt pointers causing even correct software to fail.
Operating systems such as 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...
and UNIX
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 other UNIX-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems) provide differing mechanisms for reporting errors caused by page faults. Windows uses structured exception handling to report page fault-based invalid accesses as access violation exceptions, and UNIX (and UNIX-like) systems typically use signals
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
, such as SIGSEGV
SIGSEGV
On POSIX-compliant platforms, SIGSEGV is the signal sent to a process when it makes an invalid memory reference, or segmentation fault. The symbolic constant for SIGSEGV is defined in the header file signal.h...
, to report these error conditions to programs.
If the program that received the error does not handle it, then the operating system typically performs some sort of default action, typically involving the termination of the running process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
that caused the error condition, and a notification to the user that the program has malfunctioned. In this vein, recent versions of Windows often report such problems with less technical error messages simply stating something along the lines of "this program must close" (an experienced user or programmer
Programmer
A programmer, computer programmer or coder is someone who writes computer software. The term computer programmer can refer to a specialist in one area of computer programming or to a generalist who writes code for many kinds of software. One who practices or professes a formal approach to...
with access to a debugger
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...
can still retrieve detailed information, if necessary). Additionally, recent Windows versions also write a minidump (similar in principle to a core dump
Core dump
In computing, a core dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally...
) describing the state of the crashed process for later analysis alongside such less-technical error messages. UNIX
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 UNIX-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
operating systems typically report these conditions to the user with error messages such as "segmentation violation", or "bus error", and may also produce a core dump.
Performance
Page faults, by their very nature, degrade the performance of a programComputer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
or operating system and in the degenerate case can cause thrashing. Optimizations
Optimization (computer science)
In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
to programs and the operating system that reduce the number of page faults that occur improve the performance of the program or even the entire system. The two primary focuses of the optimization effort are reducing overall memory usage and improving memory locality. Generally, making more physical memory available also reduces page faults. Many page replacement algorithm
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 when a page of memory needs to be allocated...
s have been proposed, such as implementing heuristic-based algorithms to reduce the incidence of page faults.
An average hard disk has an average rotational latency of 3ms, a seek-time of 5ms, and a transfer-time of 0.05ms/page. So the total time for paging comes in near 8ms (8 000 000 ns). If the memory access time is 200ns, then the page fault would make the operation about 40,000 times slower. To reduce the page faults in the system, programmers must make use of an appropriate page replacement algorithm that suits the current requirements and maximizes the page hits.
External links
- "So What Is A Page Fault?" from OSR Online (a Windows-specific explanation)
- "Virtual Memory Details" from the Red Hat website.
- "UnhandledExceptionFilter (Windows)" from MSDN Online.