What is context switching?

Context Switching is the process of saving the state of one process (or thread) and restoring the state of another process (or thread) by the operating system. This allows multiple processes to share a single processor.

The algorithm includes the following steps:

1. Saving the current context: The processor registers, program counter (PC), stack state, and other information about the current process are saved in its descriptor (PCB - Process Control Block).
2. Selecting the next process: The operating system scheduler chooses the next process to execute.
3. Restoring the context: The saved state of the selected process is loaded from its PCB into the processor registers.

Overhead costs for context switching include:

• Time spent on saving and restoring state.
• Loss of efficiency due to cache misses, as the cache filled with data from one process becomes invalid for another.
• Costs associated with executing scheduler instructions.

Example:

Suppose we have two processes, A and B, running on a single CPU core.

// Initially, Process A is executing
// Process A uses CPU resources

// An event occurs requiring a switch, for example, the time quantum of Process A expires
// The state of Process A (registers, PC) is saved
// The OS selects Process B for execution
// The state of Process B is loaded
// Process B begins execution

Context switching is a fundamental mechanism of multitasking in operating systems, enabling the illusion of parallel execution of multiple processes on a single physical processor.