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 daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

never initiates a

context switch In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state. This allows multiple processe ...

from a running process 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 In computing, scheduling is the action of assigning ''resources'' to perform ''tasks''. The ''resources'' may be processors, network links or expansion cards. The ''tasks'' may be threads, processes or data flows. The scheduling activity is ...

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 Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...

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 Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Windo ...

, and by the classic Mac OS.

Windows 9x Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in su ...

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 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– IBM– ...

Versions of Mac OS X prior to

Leopard The leopard (''Panthera pardus'') is one of the five extant species in the genus '' Panthera'', a member of the cat family, Felidae. It occurs in a wide range in sub-Saharan Africa, in some parts of Western and Central Asia, Southern Russia, ...

used it for classic applications.

NetWare NetWare is a discontinued computer network operating system developed by Novell, Inc. It initially used cooperative multitasking to run various services on a personal computer, using the IPX network protocol. The original NetWare product in ...

, 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, 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 u ...

ing; both would cause the whole system to hang. 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 b ...

. 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 Reentrant or re-entrant can refer to: *Re-entrant (landform), the low ground formed between two hill spurs. *Reentrancy (computing) in computer programming * Reentrant mutex in computer science *Reentry (neural circuitry) in neuroscience * Salien ...

References

{{DEFAULTSORT:Nonpreemptive Multitasking Concurrent computing de:Multitasking#Kooperatives Multitasking

Usage

Problems

See also

References