Computer multitasking
Encyclopedia
In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU
. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for that task.
Multitasking solves the problem by scheduling
which task may be the one running at any given time, and when another waiting task gets a turn. The act of reassigning a CPU from one task to another one is called a context switch
.
When context switches occur frequently enough the illusion of parallelism
is achieved.
Even on computers with more than one CPU (called multiprocessor
machines), multitasking allows many more tasks to be run than there are CPUs.
Operating systems may adopt one of many different scheduling strategies
, which generally fall into the following categories:
The term time-sharing is no longer commonly used, having been replaced by simply multitasking, and by the advent of personal computers and workstations rather than shared interactive systems.
was expensive, and peripheral
s were very slow. When the computer ran a program that needed access to a peripheral, the Central processing unit (CPU) would have to stop executing program instructions while the peripheral processed the data. This was deemed very inefficient.
The first computer using a multiprogramming system was the British Leo III owned by J. Lyons and Co.
. Several different programs in batch were loaded in the computer memory, and the first one began to run. When the first program reached an instruction waiting for a peripheral, the context of this program was stored away, and the second program in memory was given a chance to run. The process continued until all programs finished running.
Multiprogramming doesn't give any guarantee that a program will run in a timely manner. Indeed, the very first program may very well run for hours without needing access to a peripheral. As there were no users waiting at an interactive terminal, this was no problem: users handed on a deck of punched cards to an operator, and came back a few hours later for printed results. Multiprogramming greatly reduced wait times when multiple batches were being processed.
user wanted to see his program running as if it were the only program in the computer. The use of time sharing made this possible, with the qualification that the computer would not seem as fast to any one user as it really would be if it were running only that user's program.
Early multitasking systems used applications that voluntarily ceded time to one another. This approach, which was eventually supported by many computer operating system
s, is known today as cooperative multitasking. Although it is now rarely used in larger systems, cooperative multitasking was once the scheduling scheme employed by Microsoft Windows
(prior to Windows 95
and Windows NT
) and Mac OS
(prior to Mac OS X
) in order to enable multiple applications to be run simultaneously. Windows 9x
also used cooperative multitasking, but only for 16-bit legacy applications, much the same way as pre-Leopard
PowerPC
versions of Mac OS X used it for Classic
applications. The network operating system NetWare used cooperative multitasking up to NetWare 6.5. Cooperative multitasking is still used today on RISC OS
systems.
Because a cooperatively multitasked system relies on each process regularly giving up time to other processes on the system, one poorly designed program can consume all of the CPU time for itself or cause the whole system to hang
. In a server environment, this is a hazard that makes the entire network brittle and fragile. All software must be evaluated and cleared for use in a test environment before being installed on the main server, or a misbehaving program on the server slows down or freezes the entire network.
Despite the difficulty of designing and implementing cooperatively multitasked systems, time-constrained, real-time embedded systems (such as spacecraft) are often implemented using this paradigm. This allows highly reliable, deterministic control of complex real time sequences, for instance, the firing of thrusters for deep space course corrections.
Operating systems were developed to take advantage of these hardware capabilities and run multiple processes preemptively. For example, preemptive multitasking was implemented in the earliest version of Unix
in 1969, and is standard in Unix and Unix-like
operating systems, including Linux
, Solaris and BSD
with its derivatives
.
At any specific time, processes can be grouped into two categories: those that are waiting for input or output (called "I/O bound"), and those that are fully utilizing the CPU ("CPU bound
"). In primitive systems, the software would often "poll
", or "busywait
" while waiting for requested input (such as disk, keyboard or network input). During this time, the system was not performing useful work. With the advent of interrupts and preemptive multitasking, I/O bound processes could be "blocked", or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution.
The earliest preemptive multitasking OS available to home users was Sinclair QDOS
on the Sinclair QL
, released in 1984, but very few people bought the machine. Commodore's powerful Amiga
, released the following year, was the first commercially-successful home computer to use the technology, and its multimedia abilities make it a clear ancestor of contemporary multitasking personal computers. Microsoft
made preemptive multitasking a core feature of their flagship operating system in the early 1990s when developing Windows NT 3.1
and then Windows 95
. It was later adopted on the Apple Macintosh by Mac OS
9.x as an additional API, i.e. the application could be programmed to use the preemptive or cooperative model, and all legacy applications were multitasked cooperatively within a single process. Mac OS X
, being a Unix-like
system, uses preemptive multitasking for all native applications, although Classic
applications are multitasked cooperatively in a Mac OS 9 environment that itself is running as an OS X process (and is subject to preemption like any other OS X process).
A similar model is used in Windows 9x
and the Windows NT family
, where native 32-bit applications are multitasked preemptively, and legacy 16-bit Windows 3.x
programs are multitasked cooperatively within a single process, although in the NT family it is possible to force a 16-bit application to run as a separate preemptively multitasked process. 64-bit editions of Windows, both for the x86-64
and Itanium
architectures, no longer provide support for legacy 16-bit applications, and thus provide preemptive multitasking for all supported applications.
systems, where there are a number of possibly unrelated external activities needed to be controlled by a single processor system. In such systems a hierarchical interrupt system is coupled with process prioritization to ensure that key activities were given a greater share of available process time.
applications as sets of cooperating processes (e.g. one process gathering input data, one process
processing input data, one process writing out results on disk). This, however, required some tools to allow processes to efficiently exchange data.
Threads
were born from the idea that the most efficient way for cooperating processes to exchange
data would be to share their entire memory space. Thus, threads are basically processes that run
in the same memory context. Threads are described as lightweight because switching between threads does not involve changing the memory context.
While threads are scheduled preemptively, some operating systems provide a variant to threads, named fiber
s, that are scheduled cooperatively. On operating systems that do not provide fibers, an application may implement its own fibers using repeated calls to worker functions. Fibers are even more lightweight than threads, and somewhat easier to program with, although they tend to lose some or all of the benefits of threads on machines with multiple processors
.
Some systems directly support multithreading in hardware
.
The operating system therefore restricts the memory accessible to the running program. A program trying to access memory outside its allowed range is immediately stopped before it can change memory belonging to another process.
Another key innovation was the idea of privilege levels. Low privilege tasks are not allowed some kinds of memory access and are not allowed to perform certain instructions. When a task tries to perform a privileged operation a trap
occurs and a supervisory program running at a higher level is allowed to decide how to respond.
or swap partition is a way for the operating system to provide more memory than is physically available by keeping portions of the primary memory in secondary storage. While multitasking and memory swapping are two completely unrelated techniques, they are very often used together, as swapping memory allows more tasks to be loaded at the same time. Typically, a multitasking system allows another process to run when the running process hits a point where it has to wait for some portion of memory to be reloaded from secondary storage.
Various concurrent computing
techniques are used to avoid potential problems caused by multiple tasks attempting to access the same resource.
Bigger systems were sometimes built with a central processor(s) and some number of I/O processors
, a kind of asymmetric multi-processing.
Over the years, multitasking systems have been refined. Modern operating systems generally include detailed mechanisms for prioritizing processes, while symmetric multiprocessing
has introduced new complexities and capabilities.
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for that task.
Multitasking solves the problem by scheduling
Scheduling (computing)
In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...
which task may be the one running at any given time, and when another waiting task gets a turn. The act of reassigning a CPU from one task to another one is called a context switch
Context switch
A context switch is the computing process of storing and restoring the state of a CPU so that execution can be resumed from the same point at a later time. This enables multiple processes to share a single CPU. The context switch is an essential feature of a multitasking operating system...
.
When context switches occur frequently enough the illusion of parallelism
Parallel computing
Parallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
is achieved.
Even on computers with more than one CPU (called multiprocessor
Multiprocessor
Computer system having two or more processing units each sharing main memory and peripherals, in order to simultaneously process programs.Sometimes the term Multiprocessor is confused with the term Multiprocessing....
machines), multitasking allows many more tasks to be run than there are CPUs.
Operating systems may adopt one of many different scheduling strategies
Scheduling (computing)
In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...
, which generally fall into the following categories:
- In multiprogrammingMultiprogrammingComputer multiprogramming is the allocation of a computer system and its resources to more than one concurrent application, job or user ....
systems, the running task keeps running until it performs an operation that requires waiting for an external event (e.g. reading from a tape) or until the computer's scheduler forcibly swaps the running task out of the CPU. Multiprogramming systems are designed to maximize CPU usage. - In time-sharingTime-sharingTime-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...
systems, the running task is required to relinquish the CPU, either voluntarily or by an external event such as a hardware interrupt. Time sharing systems are designed to allow several programs to execute apparently simultaneously. The expression 'time sharing' was usually used to designate computers shared by interactive users at terminals, such as IBM's TSOTime Sharing OptionIn computing, Time Sharing Option is an interactive time-sharing environment for IBM mainframe operating systems, including OS/360 MVT, OS/VS2 , MVS, OS/390, and z/OS.- Overview :TSO fulfills a similar purpose to Unix login sessions...
, and VM/CMSCP/CMSCP/CMS was a time-sharing operating system of the late 60s and early 70s, known for its excellent performance and advanced features... - In real-timeReal-time computingIn computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
systems, some waiting tasks are guaranteed to be given the CPU when an external event occurs. Real time systems are designed to control mechanical devices such as industrial robots, which require timely processing.
The term time-sharing is no longer commonly used, having been replaced by simply multitasking, and by the advent of personal computers and workstations rather than shared interactive systems.
Multiprogramming
In the early days of computing, CPU timeCPU time
CPU time is the amount of time for which a central processing unit was used for processing instructions of a computer program, as opposed to, for example, waiting for input/output operations. The CPU time is often measured in clock ticks or as a percentage of the CPU's capacity...
was expensive, and peripheral
Peripheral
A peripheral is a device attached to a host computer, but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture....
s were very slow. When the computer ran a program that needed access to a peripheral, the Central processing unit (CPU) would have to stop executing program instructions while the peripheral processed the data. This was deemed very inefficient.
The first computer using a multiprogramming system was the British Leo III owned by J. Lyons and Co.
J. Lyons and Co.
J. Lyons & Co. was a market-dominant British restaurant-chain, food-manufacturing, and hotel conglomerate founded in 1887 as a spin-off from the Salmon & Gluckstein tobacco company....
. Several different programs in batch were loaded in the computer memory, and the first one began to run. When the first program reached an instruction waiting for a peripheral, the context of this program was stored away, and the second program in memory was given a chance to run. The process continued until all programs finished running.
Multiprogramming doesn't give any guarantee that a program will run in a timely manner. Indeed, the very first program may very well run for hours without needing access to a peripheral. As there were no users waiting at an interactive terminal, this was no problem: users handed on a deck of punched cards to an operator, and came back a few hours later for printed results. Multiprogramming greatly reduced wait times when multiple batches were being processed.
Cooperative multitasking/time-sharing
When computer usage evolved from batch mode to interactive mode, multiprogramming was no longer a suitable approach. Eachuser wanted to see his program running as if it were the only program in the computer. The use of time sharing made this possible, with the qualification that the computer would not seem as fast to any one user as it really would be if it were running only that user's program.
Early multitasking systems used applications that voluntarily ceded time to one another. This approach, which was eventually supported by many computer 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...
s, is known today as cooperative multitasking. Although it is now rarely used in larger systems, cooperative multitasking was once the scheduling scheme employed by 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...
(prior to Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
and Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
) and Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
(prior to 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...
) in order to enable multiple applications to be run simultaneously. Windows 9x
Windows 9x
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...
also used cooperative multitasking, but only for 16-bit legacy applications, much the same way as pre-Leopard
Mac OS X v10.5
Mac OS X Leopard is the sixth major release of Mac OS X, Apple's desktop and server operating system for Macintosh computers. Leopard was released on 26 October 2007 as the successor of Tiger , and is available in two variants: a desktop version suitable for personal computers, and a...
PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...
versions of Mac OS X used it for Classic
Classic (Mac OS X)
Classic, or Classic Environment, was a hardware and software abstraction layer in Mac OS X that allowed applications compatible with Mac OS 9 to run on the Mac OS X operating system...
applications. The network operating system NetWare used cooperative multitasking up to NetWare 6.5. Cooperative multitasking is still used today on RISC OS
RISC OS
RISC OS is a computer operating system originally developed by Acorn Computers Ltd in Cambridge, England for their range of desktop computers, based on their own ARM architecture. First released in 1987, under the name Arthur, the subsequent iteration was renamed as in 1988...
systems.
Because a cooperatively multitasked system relies on each process regularly giving up time to other processes on the system, one poorly designed program can consume all of the CPU time for itself or cause the whole system to hang
Hang (computing)
In computing, a hang or freeze occurs when either a single computer program, or the whole system ceases to respond to inputs. In the most commonly encountered scenario, a workstation with a graphical user interface, all windows belonging to the frozen program become static, and though the mouse...
. In a server environment, this is a hazard that makes the entire network brittle and fragile. All software must be evaluated and cleared for use in a test environment before being installed on the main server, or a misbehaving program on the server slows down or freezes the entire network.
Despite the difficulty of designing and implementing cooperatively multitasked systems, time-constrained, real-time embedded systems (such as spacecraft) are often implemented using this paradigm. This allows highly reliable, deterministic control of complex real time sequences, for instance, the firing of thrusters for deep space course corrections.
Preemptive multitasking/time-sharing
Preemptive multitasking allows the computer system to more reliably guarantee each process a regular "slice" of operating time. It also allows the system to rapidly deal with important external events like incoming data, which might require the immediate attention of one or another process.Operating systems were developed to take advantage of these hardware capabilities and run multiple processes preemptively. For example, preemptive multitasking was implemented in the earliest version of 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...
in 1969, and is standard in Unix 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, including 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...
, Solaris and 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...
with its derivatives
Comparison of BSD operating systems
There are a number of Unix-like operating systems based on or descended from the Berkeley Software Distribution series of Unix variants. The three most notable descendants in current use are FreeBSD, OpenBSD, and NetBSD, which are all derived from 386BSD and 4.4BSD-Lite, by various routes...
.
At any specific time, processes can be grouped into two categories: those that are waiting for input or output (called "I/O bound"), and those that are fully utilizing the CPU ("CPU bound
CPU bound
In computer science, CPU bound is when the time for a computer to complete a task is determined principally by the speed of the central processor: processor utilization is high, perhaps at 100% usage for many seconds or minutes...
"). In primitive systems, the software would often "poll
Polling (computer science)
Polling, or polled operation, in computer science, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output , and is also referred to as polled or software driven .Polling is sometimes used...
", or "busywait
Busy waiting
In software engineering, busy-waiting or spinning is a technique in which a process repeatedly checks to see if a condition is true, such as whether keyboard input is available, or if a lock is available. Spinning can also be used to generate an arbitrary time delay, a technique that was necessary...
" while waiting for requested input (such as disk, keyboard or network input). During this time, the system was not performing useful work. With the advent of interrupts and preemptive multitasking, I/O bound processes could be "blocked", or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution.
The earliest preemptive multitasking OS available to home users was Sinclair QDOS
Sinclair QDOS
QDOS is the multitasking operating system found on the Sinclair QL personal computer and its clones...
on the Sinclair QL
Sinclair QL
The Sinclair QL , was a personal computer launched by Sinclair Research in 1984, as the successor to the Sinclair ZX Spectrum...
, released in 1984, but very few people bought the machine. Commodore's powerful Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
, released the following year, was the first commercially-successful home computer to use the technology, and its multimedia abilities make it a clear ancestor of contemporary multitasking personal computers. Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
made preemptive multitasking a core feature of their flagship operating system in the early 1990s when developing Windows NT 3.1
Windows NT 3.1
Windows NT 3.1 is the first release of Microsoft's Windows NT line of server and business desktop operating systems, and was released to manufacturing on 27 July 1993. The version number was chosen to match the one of Windows 3.1, the then-latest operating environment from Microsoft, on account of...
and then Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
. It was later adopted on the Apple Macintosh by Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
9.x as an additional API, i.e. the application could be programmed to use the preemptive or cooperative model, and all legacy applications were multitasked cooperatively within a single process. 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...
, being a 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....
system, uses preemptive multitasking for all native applications, although Classic
Classic (Mac OS X)
Classic, or Classic Environment, was a hardware and software abstraction layer in Mac OS X that allowed applications compatible with Mac OS 9 to run on the Mac OS X operating system...
applications are multitasked cooperatively in a Mac OS 9 environment that itself is running as an OS X process (and is subject to preemption like any other OS X process).
A similar model is used in Windows 9x
Windows 9x
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...
and the Windows NT family
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
, where native 32-bit applications are multitasked preemptively, and legacy 16-bit Windows 3.x
Windows 3.1x
Windows 3.1x is a series of 16-bit operating systems produced by Microsoft for use on personal computers. The series began with Windows 3.1, which was first sold during March 1992 as a successor to Windows 3.0...
programs are multitasked cooperatively within a single process, although in the NT family it is possible to force a 16-bit application to run as a separate preemptively multitasked process. 64-bit editions of Windows, both for the 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...
and Itanium
Itanium
Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...
architectures, no longer provide support for legacy 16-bit applications, and thus provide preemptive multitasking for all supported applications.
Real time
Another reason for multitasking was in the design of real-time computingReal-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
systems, where there are a number of possibly unrelated external activities needed to be controlled by a single processor system. In such systems a hierarchical interrupt system is coupled with process prioritization to ensure that key activities were given a greater share of available process time.
Multithreading
As multitasking greatly improved the throughput of computers, programmers started to implementapplications as sets of cooperating processes (e.g. one process gathering input data, one process
processing input data, one process writing out results on disk). This, however, required some tools to allow processes to efficiently exchange data.
Threads
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
were born from the idea that the most efficient way for cooperating processes to exchange
data would be to share their entire memory space. Thus, threads are basically processes that run
in the same memory context. Threads are described as lightweight because switching between threads does not involve changing the memory context.
While threads are scheduled preemptively, some operating systems provide a variant to threads, named fiber
Fiber (computer science)
In computer science, a fiber is a particularly lightweight thread of execution.Like threads, fibers share address space. However, fibers use co-operative multitasking while threads use pre-emptive multitasking. Threads often depend on the kernel's thread scheduler to preempt a busy thread and...
s, that are scheduled cooperatively. On operating systems that do not provide fibers, an application may implement its own fibers using repeated calls to worker functions. Fibers are even more lightweight than threads, and somewhat easier to program with, although they tend to lose some or all of the benefits of threads on machines with multiple processors
Multiprocessing
Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...
.
Some systems directly support multithreading in hardware
Multithreading (computer hardware)
Multithreading computers have hardware support to efficiently execute multiple threads. These are distinguished from multiprocessing systems in that the threads have to share the resources of a single core: the computing units, the CPU caches and the translation lookaside buffer...
.
Memory protection
When multiple programs are present in memory, an ill-behaved program may (inadvertently or deliberately) overwrite memory belonging to another program, or even to the operating system itself.The operating system therefore restricts the memory accessible to the running program. A program trying to access memory outside its allowed range is immediately stopped before it can change memory belonging to another process.
Another key innovation was the idea of privilege levels. Low privilege tasks are not allowed some kinds of memory access and are not allowed to perform certain instructions. When a task tries to perform a privileged operation a trap
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...
occurs and a supervisory program running at a higher level is allowed to decide how to respond.
Memory swapping
Use of a swap fileVirtual 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...
or swap partition is a way for the operating system to provide more memory than is physically available by keeping portions of the primary memory in secondary storage. While multitasking and memory swapping are two completely unrelated techniques, they are very often used together, as swapping memory allows more tasks to be loaded at the same time. Typically, a multitasking system allows another process to run when the running process hits a point where it has to wait for some portion of memory to be reloaded from secondary storage.
Programming in a multitasking environment
Processes that are entirely independent are not much trouble to program. Most of the complexity in multitasking systems comes from the need to share computer resources between tasks and to synchronize the operation of co-operating tasks.Various concurrent computing
Concurrent computing
Concurrent computing is a form of computing in which programs are designed as collections of interacting computational processes that may be executed in parallel...
techniques are used to avoid potential problems caused by multiple tasks attempting to access the same resource.
Bigger systems were sometimes built with a central processor(s) and some number of I/O processors
Channel I/O
In computer science, channel I/O is a generic term that refers to a high-performance input/output architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers...
, a kind of asymmetric multi-processing.
Over the years, multitasking systems have been refined. Modern operating systems generally include detailed mechanisms for prioritizing processes, while symmetric multiprocessing
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
has introduced new complexities and capabilities.
See also
- ProcessProcess (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...
- Process statesProcess statesIn a multitasking computer system, processes may occupy a variety of states. These distinct states may not actually be recognized as such by the operating system kernel, however they are a useful abstraction for the understanding of processes....
- Process time
- TaskTask (computers)A task is an execution path through address space. In other words, a set of program instructions that are loaded in memory. The address registers have been loaded with the initial address of the program. At the next clock cycle, the CPU will start execution, in accord with the program. The sense is...
- MultiprocessingMultiprocessingMultiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...