What is a context switch?

Context switching is a process in an operating system that switches CPU execution among multiple processes or threads to enable multitasking. Context switching typically occurs in multitasking operating systems, enabling the system to utilize CPU resources more efficiently and enhance system performance and user experience. Specifically, when a process or thread needs to pause execution due to certain reasons (such as waiting for I/O operations to complete or time slices expiring), the operating system saves the current process's state (i.e., the context) and transfers CPU control to another ready process. This process of saving and restoring the state is context switching. The context typically includes the program counter, register set, memory management information, and other processor states. This information is stored in the process control block (PCB) to ensure that the process can resume execution from where it was paused at a later time. For example, consider two processes, A and B. Process A is executing but needs to wait for a file read operation. In this case, the operating system saves the context of process A (such as the current register state and program counter) to its process control block, then selects another ready process, such as process B, based on scheduling policies, loads the context of process B into the CPU, and begins executing process B. When the file read operation completes, process A can be rescheduled, and its saved context is restored to continue execution. Context switching is a critical feature but also incurs performance overhead. Frequent context switching can cause the CPU to spend significant time on saving and restoring process states rather than executing actual work, which is known as context switching overhead. Therefore, operating system designers optimize scheduling algorithms to minimize unnecessary context switching and improve overall system performance.