Scheduling Strategies: Preemptive vs Non-Preemptive
In the realm of operating systems, CPU scheduling plays a pivotal role in determining the execution order of processes in the ready queue, aiming to enhance overall system performance and improve user experience. Two primary types of CPU scheduling can be distinguished: Preemptive and Non-Preemptive.
Preemptive scheduling allows the operating system to forcibly interrupt and suspend a currently running process to allocate the CPU to another process, usually based on priority or time slicing. This ensures that no process monopolizes the CPU, improving responsiveness and multitasking efficiency. Typical preemptive algorithms include Round Robin, Shortest Remaining Time First (SRTF), and priority scheduling with preemption.
On the other hand, non-preemptive scheduling means once a process starts using the CPU, it runs until it finishes or voluntarily yields control (e.g., waiting for I/O). The OS cannot forcibly take the CPU from the process. Examples include First Come First Serve (FCFS) and Shortest Job First (non-preemptive version). This approach is simpler and has less overhead but can lead to poor CPU utilization and system unresponsiveness if a process hogs the CPU.
In terms of usage in modern operating systems like Windows, Linux, and macOS, preemptive scheduling is predominantly employed because it supports better responsiveness, efficient CPU utilization, and multitasking:
- Windows since Windows 95 uses preemptive multitasking, allowing the OS to interrupt processes for a responsive user experience.
- Linux kernel incorporates preemptive scheduling with complex schedulers supporting multiple policies and priorities.
- macOS also uses a preemptive multitasking model derived from its Unix-based roots.
Non-preemptive scheduling was used in older OS versions like Windows 3.x and early macOS but is largely obsolete in modern desktop and server OS environments.
| Aspect | Preemptive Scheduling | Non-Preemptive Scheduling | |----------------------------|-----------------------------------------------|----------------------------------------------| | CPU Control | OS can forcibly take CPU away | Process runs until completes or blocks | | Responsiveness | High; avoids hogging CPU | Low; if process doesn't yield, system stalls | | Complexity | Higher; involves context switching overhead | Simpler; minimal overhead | | Common Algorithms | Round Robin, Priority (preemptive), SRTF | FCFS, SJF (non-preemptive), Priority (non-preemptive) | | Typical Usage in Modern OS | Windows, Linux, macOS | Older OS versions (Windows 3.x, macOS 8/9) |
Thus, preemptive scheduling is more commonly used in modern operating systems like Windows, Linux, and macOS due to its superior multitasking and responsiveness features. However, non-preemptive scheduling has its place in specific scenarios where simplicity and reduced overhead are prioritized over flexibility and potential responsiveness issues.
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