Cooperative multitasking, also known as non-preemptive multitasking, is a
computer multitasking
In computing, multitasking is the concurrent computing, concurrent execution of multiple tasks (also known as Process (computing), processes) over a certain period of time. New tasks can interrupt already started ones before they finish, instea ...
technique in which the
operating system
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ...
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 s ...
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.
This is related to the
asynchronous programming approach.
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 specialized 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 em ...
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 business transaction management, transaction management and connectivity for applications on IBM mainframe systems under z/OS ...
or the
JES2 subsystem. Cooperative multitasking was the primary scheduling scheme for 16-bit applications employed by
Microsoft Windows
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
before
Windows 95 and
Windows NT, and by the
classic Mac OS.
Windows 9x used non-
preemptive multitasking for 16-bit legacy applications, and the
PowerPC
PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...
Versions of Mac OS X prior to
Leopard
The leopard (''Panthera pardus'') is one of the five extant cat species in the genus ''Panthera''. It has a pale yellowish to dark golden fur with dark spots grouped in rosettes. Its body is slender and muscular reaching a length of with a ...
used it for
classic
A classic is an outstanding example of a particular style; something of Masterpiece, lasting worth or with a timeless quality; of the first or Literary merit, highest quality, class, or rank – something that Exemplification, exemplifies its ...
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 similar to
async/await in languages, such as
JavaScript
JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior.
Web browsers have ...
or
Python, that feature a single-threaded event-loop in their runtime. This contrasts with cooperative multitasking in that await cannot be invoked from a non-async function, but only an async function, which is a kind of
coroutine
Coroutines are computer program components that allow execution to be suspended and resumed, generalizing subroutines for cooperative multitasking. Coroutines are well-suited for implementing familiar program components such as cooperative task ...
.
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.
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 waiting; both would cause the whole system to
hang. In a
server environment, this is a hazard that is often considered to make the entire environment unacceptably fragile,
though, as noted above,
cooperative multitasking has been
used frequently in server environments including NetWare and CICS.
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 (WDT, or simply a ''watchdog''), sometimes called a ''computer operating properly timer'' (''COP timer''), is an electronic or software timer that is used to detect and recover from computer malfunctions. Watchdog timers are wide ...
, often implemented in hardware; this typically invokes a
hardware reset.
References
{{DEFAULTSORT:Nonpreemptive Multitasking
Concurrent computing
de:Multitasking#Kooperatives Multitasking