Cooperative multitasking, also known as non-preemptive multitasking, is a style of
computer multitasking
In computing, multitasking is the concurrent execution of multiple tasks (also known as processes) over a certain period of time. New tasks can interrupt already started ones before they finish, instead of waiting for them to end. As a result ...
in which the
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
never initiates a
context switch from a running
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
to another process. Instead, in order to run multiple applications concurrently, processes voluntarily
yield control periodically or when idle or logically
blocked. This type of multitasking is called ''cooperative'' because all programs must cooperate for the scheduling scheme to work.
In this scheme, the
process scheduler of an operating system is known as a cooperative scheduler whose role is limited to starting the processes and letting them return control back to it voluntarily.
Usage
Although it is rarely used as the primary scheduling mechanism in modern operating systems, it is widely used in memory-constrained
embedded systems
An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' ...
and also, in specific applications such as
CICS
IBM CICS (Customer Information Control System) is a family of mixed-language application servers that provide online transaction management and connectivity for applications on IBM mainframe systems under z/OS and z/VSE.
CICS family products ...
or the
JES2 subsystem. Cooperative multitasking was the primary scheduling scheme for 16-bit applications employed by
Microsoft Windows before
Windows 95
Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturi ...
and
Windows NT, and by the
classic Mac OS.
Windows 9x used non-
preemptive multitasking
In computing, preemption is the act of temporarily interrupting an executing task, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task. This preemp ...
for 16-bit legacy applications, and the
PowerPC Versions of Mac OS X prior to
Leopard used it for
classic
A classic is an outstanding example of a particular style; something of lasting worth or with a timeless quality; of the first or highest quality, class, or rank – something that exemplifies its class. The word can be an adjective (a ''c ...
applications.
, which is a network-oriented operating system, used cooperative multitasking up to NetWare 6.5. Cooperative multitasking is still used on
RISC OS systems.
Cooperative multitasking is used with
await
In computer programming, the async/await pattern is a syntactic feature of many programming languages that allows an asynchronous, non-blocking function to be structured in a way similar to an ordinary synchronous function. It is semantically rel ...
in languages, such as
JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
or
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
, that feature a single-threaded event-loop in their runtime.
Problems
As 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, either by performing extensive calculations or by
busy wait
In computer science and software engineering, busy-waiting, busy-looping or spinning is a technique in which a process repeatedly checks to see if a condition is true, such as whether keyboard input or a lock is available. Spinning can also be used ...
ing; both would cause the whole system to
hang
Hang or Hanging may refer to:
People
* Choe Hang (disambiguation), various people
* Luciano Hang (born 1962/1963), Brazilian billionaire businessman
* Ren Hang (disambiguation), various people
Law
* Hanging, a form of capital punishment
Arts, e ...
. In a
server
Server may refer to:
Computing
*Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients
Role
* Waiting staff, those who work at a restaurant or a bar attending customers and su ...
environment, this is a hazard that makes the entire environment unacceptably fragile.
In contrast,
preemptive multitasking interrupts applications and gives control to other processes outside the application's control.
The potential for system hang can be alleviated by using a
watchdog timer
A watchdog timer (sometimes called a ''computer operating properly'' or ''COP'' timer, or simply a ''watchdog'') is an electronic or software timer that is used to detect and recover from computer malfunctions. Watchdog timers are widely used in ...
, often implemented in hardware; this typically invokes a
hardware reset
A hardware reset or hard reset of a computer system is a hardware operation that re-initializes the core hardware components of the system, thus ending all current software operations in the system. This is typically, but not always, followed by ...
.
Cooperative multitasking allows much simpler implementation of applications because their execution is never unexpectedly interrupted by the process scheduler; for example, various
functions inside the application do not need to be
reentrant.
See also
*
Preemptive multitasking
In computing, preemption is the act of temporarily interrupting an executing task, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task. This preemp ...
References
{{DEFAULTSORT:Nonpreemptive Multitasking
Concurrent computing
de:Multitasking#Kooperatives Multitasking