Preemptive and Non-Preemptive Scheduling

Last Updated : 21 Sep, 2025

CPU scheduling in operating systems is the method of selecting which process in the ready queue will execute on the CPU next. It aims to utilize the processor efficiently while minimizing waiting and response times. By determining an optimal execution order, CPU scheduling enhances overall system performance, supports smooth multitasking, and improves the user experience.

Scheduling can be broadly classified into two types: Preemptive and Non-Preemptive.

Preemptive-Scheduling-vs--Non-Preemptive-Scheduling — Preemptive Scheduling vs Non-Preemptive Scheduling

Preemptive Scheduling

In preemptive scheduling, the operating system can interrupt a running process to allocate the CPU to another process usually due to priority rules or time-sharing policies.A process may be moved from Running → Ready state before it finishes.

In the following example P2 is preempted at time 1 due to arrival of a higher priority process.

Preemptive Scheduling — Preemptive Scheduling+

Examples:

Round Robin
Shortest Remaining Time First (SRTF)
Priority Scheduling (preemptive version)

Advantages of Preemptive Scheduling

Prevents one process from monopolizing CPU
Better average response time in multi-user systems
Widely used in modern OS (Windows, Linux, macOS)

Disadvantages of Preemptive Scheduling

More complex to implement
Higher overhead from context switching
Can cause starvation of low-priority processes
Risk of concurrency issues if preempted during shared resource access

Non-Preemptive Scheduling

In non-preemptive scheduling, once a process starts using the CPU, it runs until it finishes or moves to a waiting state. The OS cannot forcibly take away the CPU.

Below is the table and Gantt Chart according to the First Come First Serve (FCFS) Algorithm: We can notice that every process finishes execution once it gets CPU.

Non Preemptive Scheduling

Examples:

First Come First Serve (FCFS)
Shortest Job First (SJF)
Priority Scheduling (non-preemptive version)

Advantages of Non-Preemptive Scheduling

It is easy to implement in an operating system. It was used in Windows 3.11 and early macOS.
It has a minimal scheduling burden.
Less computational resources are used.

Disadvantages of Non-Preemptive Scheduling

It is open to denial of service attack. A malicious process can take CPU forever.
Since we cannot implement round robin, the average response time becomes less.

Differences Between Preemptive and Non-Preemptive Scheduling

Parameter	Preemptive Scheduling	Non-Preemptive Scheduling
Basic	In this resources(CPU Cycle) are allocated to a process for a limited time.	Once resources(CPU Cycle) are allocated to a process, the process holds it till it completes its burst time or switches to waiting state
Interrupt	Process can be interrupted in between.	Process can not be interrupted until it terminates itself or its time is up
Starvation	If a process having high priority frequently arrives in the ready queue, a low priority process may starve	If a process with a long burst time is running CPU, then later coming process with less CPU burst time may starve
Overhead	It has overheads of scheduling the processes	It does not have overheads
Response Time	Average process response time is less	Average process response time is high
Decision making	Decisions are made by the scheduler and are based on priority and time slice allocation	Decisions are made by the process itself and the OS just follows the process's instructions
Concurrency Overhead	More as a process might be preempted when it was accessing a shared resource.	Less as a process is never preempted.
Examples	Examples of preemptive scheduling are Round Robin and Shortest Remaining Time First	Examples of non-preemptive scheduling are First Come First Serve and Shortest Job First

Preemptive and Non Preemptive Scheduling in OS

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