context switch - TarisMajor/5143-OpSystems GitHub Wiki

Definition

A context switch is the process of saving the state of a currently running process or thread and restoring the state of a previously paused process or thread. This mechanism enables multitasking in operating systems, where multiple processes or threads can share a single CPU or core. A context switch occurs when the operating system switches from one process or thread to another, allowing the CPU to execute different tasks seemingly simultaneously, despite having only one processor.

The "context" refers to the state information of a process or thread, including the CPU registers, program counter, and other essential data that allow the process to continue from where it left off. During a context switch, the operating system saves the state of the current task (process or thread) to its Process Control Block (PCB) or Thread Control Block (TCB), then loads the state of the next task to be executed.

Key Elements of a Context Switch: Saving the State: The operating system saves the current state (registers, program counter, etc.) of the process that is being interrupted to its PCB or TCB. Selecting the Next Process: The operating system selects the next process or thread to be scheduled based on the scheduling algorithm (e.g., round-robin, priority-based scheduling). Restoring the State: The state of the next process (from its PCB or TCB) is restored, and the CPU begins executing it from the point it was last interrupted. Context switches can occur as part of preemptive multitasking, where the OS forcibly takes control of the CPU from one process and gives it to another, or cooperative multitasking, where processes voluntarily yield control.

Inventor and Year of Invention The concept of context switching has been around since the development of time-sharing systems in the 1960s. Multics (1965) and Unix (1971) were among the first operating systems to implement efficient multitasking and context switching. While there is no specific inventor, the development of context switching is often credited to the research teams behind these early operating systems, particularly at institutions like MIT and AT&T Bell Labs.

The idea of multitasking (and the need for context switching) became popular as time-sharing systems were introduced, where multiple users could interact with a computer at the same time, creating the need for efficient task switching to ensure each user received fair processing time.

Uses

Context switching is essential for:

Multitasking: It allows an operating system to manage multiple processes or threads by switching between them rapidly, giving users the illusion of simultaneous execution on a single CPU. Preemptive Scheduling: The operating system can interrupt a running process to execute a higher-priority process. For example, if a process is waiting for user input, the OS may switch to another process that is ready to execute. Efficient Resource Utilization: Context switching allows the CPU to remain active by rapidly switching between processes, ensuring that CPU time is allocated efficiently to the tasks that need it. Time-sharing Systems: In a time-sharing system, context switches allow the OS to allocate CPU time to each user or task for small time slices, improving system responsiveness. Examples of Where It Is Used Today Modern Desktop Operating Systems (Windows, macOS, Linux):

In these operating systems, context switches are frequently used as part of their preemptive multitasking model. When the system has multiple processes or threads running, the OS ensures that each gets a share of the CPU time. Context switches occur when the OS allocates CPU time between these processes, based on priorities and scheduling algorithms. For example, Windows performs a context switch every time it switches between applications or threads, allowing the user to seamlessly switch between tasks. Unix/Linux:

The Linux kernel uses context switches to manage multiple processes and threads. Each time the kernel preempts a running process (or a process voluntarily yields), a context switch occurs. The task_struct structure in Linux contains information about a process, and context switches are integral to its scheduling mechanism. Linux is a preemptive multitasking operating system, meaning it frequently uses context switches to allocate CPU time fairly to different tasks. Mobile Operating Systems (Android/iOS):

Both Android and iOS use context switching to manage multiple running applications. When a user switches between apps or when an app goes into the background, the OS performs a context switch to pause the app and resume it later from where it left off. For example, if you switch from an email app to a web browser, the system performs a context switch to save the state of the email app and load the state of the browser app. Real-Time Operating Systems (RTOS):

In embedded systems and real-time operating systems (such as FreeRTOS, VxWorks, or RTEMS), context switching ensures that high-priority tasks, such as real-time control loops or sensor data processing, are executed on time. The system performs context switches between tasks that have specific timing requirements. In a real-time system, context switching is crucial for ensuring tasks meet their deadlines and that the most critical tasks get CPU time when needed. Cloud Computing:

In virtualized environments (such as those using VMware, Hyper-V, or KVM), context switching occurs at multiple levels. The virtual machine monitor (VMM) performs context switches between virtual machines, while the operating system inside the virtual machine switches between processes running within it. For example, cloud servers that handle multiple requests from users or processes in parallel depend on efficient context switching to maximize resource utilization and meet service level agreements (SLAs). Performance Considerations Overhead: Context switches introduce overhead because the operating system must save and load the state of the processes involved, which consumes CPU time. Frequent context switches can degrade performance. Latency: While context switching allows multitasking, it also introduces latency. Each switch requires time to save and restore the state of processes, which can be significant in performance-critical applications. Efficiency: Operating systems often try to minimize context switch overhead by using algorithms like processor affinity, where processes are scheduled to run on the same CPU core to reduce the cost of switching contexts.

Sources

Tanenbaum, A. S., & Woodhull, D. J. (2009). Operating Systems: Design and Implementation (3rd ed.). Prentice Hall. Silberschatz, A., Galvin, P. B., & Gagne, G. (2018). Operating System Concepts (9th ed.). Wiley. Stallings, W. (2017). Operating Systems: Internals and Design Principles (9th ed.). Pearson Education. Bovet, D. P., & Cesati, M. (2005). Understanding the Linux Kernel (3rd ed.). O'Reilly Media. McKusick, M. K., & Neville-Neil, G. V. (2004). The Design and Implementation of the FreeBSD Operating System. Addison-Wesley.