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Context Switching in Operating Systems
Introduction to Context Switching
- Context switching is the process of storing and restoring the state (context) of a CPU so that multiple processes can share a single CPU resource.
- It enables multitasking by allowing the operating system to switch between processes, ensuring efficient CPU utilization.
- The state of a process includes the contents of its CPU registers, program counter, and memory allocation.
- Context switching is a critical function in modern operating systems, especially in time-sharing systems.
- While context switching allows for concurrent execution, it introduces overhead that can affect system performance.
Steps in Context Switching
Process of Context Switching
- Save the state of the currently running process.
- Store the state in the process control block (PCB) of the current process.
- Choose the next process to execute based on scheduling algorithms.
- Load the saved state from the PCB of the chosen process.
- Resume execution of the new process.
Factors Affecting Context Switching
Influencing Factors
- Process Priority: Higher priority processes may preempt lower priority ones.
- Time Quantum: In time-sharing systems, the duration for which a process runs before switching.
- System Load: More processes increase the frequency of context switches.
- Hardware Support: Some CPUs have features to reduce context switching overhead.
- Scheduling Algorithm: Determines the order and frequency of process execution.
Performance Implications
Impact on System Performance
- Increased Overhead: Frequent context switches consume CPU time and resources.
- Cache Misses: Switching processes can lead to cache invalidation and misses.
- Latency: Can introduce delays in process execution.
- Throughput: Excessive switching can reduce overall system throughput.
- Energy Consumption: More switches can lead to higher energy usage.
Example: Context Switching in a Preemptive Scheduler
Preemptive Scheduling Scenario
- Process A is executing and reaches its time quantum limit.
- The scheduler saves Process A's state and selects Process B to run next.
- The state of Process B is loaded, and it begins execution.
- Process A waits in the ready queue until it is selected again.
- This ensures fair CPU time distribution among processes.
Example: Context Switching in a Non-Preemptive Scheduler
Non-Preemptive Scheduling Scenario
- Process A runs until it voluntarily yields the CPU or completes execution.
- The scheduler then selects Process B to execute next.
- There is no forced interruption, reducing context switching overhead.
- Suitable for batch processing where response time is not critical.
- Ensures that each process completes its task without interference.
Example: Context Switching in Multithreading
Multithreading Context Switching
- Threads within the same process share memory space.
- Switching between threads is generally faster than between processes.
- Thread context includes stack, registers, and thread-specific data.
- Efficient for applications requiring concurrent operations.
- Reduces the overhead associated with process-level context switching.
Example: Context Switching in Real-Time Systems
Real-Time System Context Switching
- Real-time systems require deterministic context switching.
- Switching latency must be minimized to meet timing constraints.
- Priority-based scheduling often used to manage context switches.
- Critical tasks are given higher priority to ensure timely execution.
- Context switching algorithms are optimized for minimal delay.
