Preemptive Scheduling vs Non Preemptive Scheduling
Difference between Preemptive and Non preemptive scheduling
Preemptive Scheduling vs Non Preemptive Scheduling is an important topic in operating systems that we will explore on this page. We will learn what these two types of CPU scheduling mean and understand the difference between them.
In operating systems, process scheduling is a method used to manage how processes use the CPU. Preemptive scheduling allows the system to interrupt a running process if a higher priority task needs the CPU. In contrast, non-preemptive scheduling lets a process run until it finishes or switches by itself.
By understanding these concepts, we can see how different systems manage tasks and ensure better performance through effective CPU usage.
Preemptive Scheduling vs Non Preemptive Scheduling
The process manager in an operating system is responsible for process scheduling. It is an operating system activity in which a running process is removed from the CPU and is replaced by another high priority process following certain set of rules. Process scheduling is an important part of a multi programming system where multiple processes need execution. The process of scheduling can be categorised into two, Pre-emptive and non-pre-emptive scheduling.
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Preemptive Scheduling
Scheduling in which a process is switched from the running state to the ready state and from the waiting state to the ready state. In this concept, the CPU is allocated to the process for a limited time and is again taken back when its time is over. The current state of the program is saved and is resumed again when the process is assigned to the CPU.
Definition:
Preemptive scheduling is a CPU scheduling technique in which the operating system forcibly removes a process from the CPU if its allocated time slice (quantum) expires or if a higher priority process becomes ready to execute.
Key Characteristics:
- A running process can be interrupted and moved to the ready state.
- A waiting process (e.g., waiting for I/O) can also be moved to the ready state once the event it was waiting for is completed.
- The state of the interrupted process is saved so it can be resumed later.
- This allows for better responsiveness in multi-tasking environments.
Example Algorithms:
- Round Robin
- Shortest Remaining Time First (SRTF)
- Preemptive Priority Scheduling
Non Pre-emptive Scheduling
In this case, once the process is allocated the CPU, it is only released when the processing is over. The states into which a process switches are the running state to the waiting state. It is an approach in which a process is not interrupted when it is under process by the CPU.
Definition:
Non-preemptive scheduling is a CPU scheduling method in which, once a process is assigned the CPU, it keeps the CPU until it either completes execution or enters the waiting state (e.g., for I/O). The operating system does not interrupt the process mid-execution.
Key Characteristics:
- A running process cannot be forcibly removed from the CPU.
- The CPU is only reallocated when the current process completes or voluntarily yields it (e.g., due to I/O).
- This results in less overhead, as there’s no need to save and restore process states frequently.
- However, it may cause longer wait times for shorter or more urgent processes.
Process State Transition:
- The process moves from running to waiting (e.g., for I/O), or
- From running to terminated when execution completes.
Example Algorithms:
- First-Come, First-Served (FCFS)
- Non-Preemptive Shortest Job First (SJF)
- Non-Preemptive Priority Scheduling
| Feature | Preemptive Scheduling | Non-Preemptive Scheduling |
|---|---|---|
| CPU Control | CPU can be taken away from a running process | CPU is not taken away until the process finishes or waits |
| Interruption | Process can be interrupted at any time | Process runs to completion without interruption |
| Process Priority Handling | Higher priority processes can preempt lower priority ones | Higher priority processes must wait until the CPU is free |
| Context Switching | More frequent due to interruptions | Less frequent, only when a process finishes or waits |
| Responsiveness | More responsive to real-time or urgent tasks | Less responsive; can cause delays for short processes |
| Overhead | Higher overhead due to frequent context switching | Lower overhead |
| Complexity | More complex to implement | Simpler to implement |
| Fairness | More fair processes are time-shared | Can lead to longer wait times for short or low priority processes |
Wrapping Up
understanding the difference between preemptive and non-preemptive scheduling is key to grasping how operating systems manage CPU resources efficiently. Preemptive scheduling offers better responsiveness and fairness but comes with higher overhead and complexity. Non-preemptive scheduling, on the other hand, is simpler and incurs less overhead but may lead to longer wait times for some processes. Each approach has its own strengths and is suited for different system requirements. Choosing the right scheduling method depends on the goals of performance, simplicity, and responsiveness.
FAQs
Yes, modern operating systems often use a hybrid approach, combining both to optimize performance and responsiveness.
Preemptive scheduling is generally preferred for real-time systems due to its ability to quickly respond to high-priority tasks.
No, non-preemptive scheduling does not rely on timer interrupts since processes run to completion unless they yield voluntarily.
Preemptive systems feel more responsive to users, while non-preemptive systems may appear slower, especially under heavy loads.
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