📕 Introduction
In a multitasking operating system, multiple processes or threads can be running concurrently on a single CPU. The CPU has to switch between these processes/threads to give the illusion of parallelism. The mechanism of switching between processes/threads is called context switching.
📕 Context Switching
Context switching is the process of saving the context of one process/thread and restoring the context of another process/thread so that execution can be resumed from the point where it was left off. Context switching involves saving the contents of the CPU registers and memory of one process/thread and restoring the contents of the CPU registers and memory of another process/thread.
📕 Why do we need Context Switching?
We need context switching to provide the illusion of parallelism in a single CPU environment. In a multitasking operating system, multiple processes/threads need to be executed concurrently. The CPU time is shared between these processes/threads. Context switching allows the CPU to switch between processes/threads and execute them concurrently.
📕 How does Context Switching work?
Context switching is initiated by an interrupt. When an interrupt occurs, the CPU transfers control to the interrupt handler. The interrupt handler saves the context of the currently executing process/thread in its Process Control Block (PCB) and selects the next process/thread to execute. The interrupt handler then loads the context of the selected process/thread from its PCB and restores it to the CPU registers and memory. The CPU then resumes execution of the selected process/thread.
📕 Context Switching Example
Suppose we have two processes, Process A and Process B. Initially, Process A is executing. When an interrupt occurs, the CPU transfers control to the interrupt handler. The interrupt handler saves the context of Process A in its PCB and selects Process B to execute. The interrupt handler then loads the context of Process B from its PCB and restores it to the CPU registers and memory. The CPU then resumes execution of Process B. When Process B is completed, another interrupt occurs, and the same process is repeated in reverse. The interrupt handler saves the context of Process B in its PCB and selects Process A to execute. The interrupt handler then loads the context of Process A from its PCB and restores it to the CPU registers and memory. The CPU then resumes execution of Process A.
📕 Context Switching Overhead
Context switching involves overhead in terms of time and resources. When an interrupt occurs, the CPU has to save the context of the currently executing process/thread, select the next process/thread to execute, load its context, and restore it to the CPU registers and memory. This process takes time and resources. The more frequently the CPU has to switch between processes/threads, the more overhead is involved. Therefore, the overhead of context switching should be minimized.
📕 Trigger in Context switching
Context switching in an operating system can be triggered by various events, including:
Time slicing: When the operating system gives a fixed amount of time for each process to run on the CPU, after which it switches to another process. This is called time slicing, and it ensures that no single process hogs the CPU for too long.
I/O operations: When a process needs to wait for an input/output operation to complete, the operating system may switch to another process to keep the CPU busy. Once the I/O operation is complete, the operating system switches back to the original process.
Process termination: When a process completes its execution or is terminated due to an error, the operating system switches to another process.
Process creation: When a new process is created, the operating system must switch to the new process to initialize it and start its execution.
Interrupts: When an interrupt occurs, the operating system suspends the currently executing process and switches to the interrupt handler to service the interrupt. Once the interrupt is serviced, the operating system switches back to the original process.
In summary, context switching is triggered by various events that require the operating system to temporarily suspend the execution of one process and switch to another. This ensures that multiple processes can run concurrently on a single CPU, improving overall system performance.
✔ Advantages of Context Switching:
Helps to achieve multitasking by allowing multiple processes to run on a single processor.
Enables efficient use of CPU resources by allowing the system to switch between processes when one is waiting for an I/O operation or a system call.
Helps in time-sharing, where multiple users can access the system simultaneously without any interference with each other's tasks.
Allows for better system responsiveness by allowing the system to quickly switch to a high-priority process when necessary.
✔ Disadvantages of Context Switching:
Context switching requires overhead, which can cause a performance penalty.
The more processes running on a system, the more context switches will occur, which can further degrade performance.
Context switching requires saving and restoring process states, which consumes system resources.
Poorly designed systems may experience deadlocks or other synchronization issues that can be difficult to diagnose and fix.
📕 Conclusion
Context switching is a fundamental mechanism of multitasking operating systems that allows multiple processes/threads to execute concurrently on a single CPU. Context switching involves saving the context of one process/thread and restoring the context of another process/thread so that execution can be resumed from the point where it was left off. Context switching is initiated by an interrupt, and it involves overhead in terms of time and resources. The overhead of context switching should be minimized to ensure efficient execution of multiple processes/threads.
Thanks for reading, and happy coding!
CPU Context Switching in Operating System: Definition, Process, and Examples -> Introduction to CPU Scheduling in Operating Systems: Concepts and Algorithms