Preemptive and Non-Preemptive Scheduling

Prerequisite – CPU Scheduling
1. Preemptive Scheduling:
Preemptive scheduling is used when a process switches from running state to ready state or from waiting state to ready state. The resources (mainly CPU cycles) are allocated to the process for the limited amount of time and then is taken away, and the process is again placed back in the ready queue if that process still has CPU burst time remaining. That process stays in ready queue till it gets next chance to execute.

Algorithms based on preemptive scheduling are: Round Robin (RR),Shortest Remaining Time First (SRTF), Priority (preemptive version), etc.

2. Non-Preemptive Scheduling:
Non-preemptive Scheduling is used when a process terminates, or a process switches from running to waiting state. In this scheduling, once the resources (CPU cycles) is allocated to a process, the process holds the CPU till it gets terminated or it reaches a waiting state. In case of non-preemptive scheduling does not interrupt a process running CPU in middle of the execution. Instead, it waits till the process complete its CPU burst time and then it can allocate the CPU to another process.

Algorithms based on non-preemptive scheduling are: Shortest Job First (SJF basically non preemptive) and Priority (non preemptive version), etc.

Key Differences Between Preemptive and Non-Preemptive Scheduling:

  1. In preemptive scheduling the CPU is allocated to the processes for the limited time whereas in Non-preemptive scheduling, the CPU is allocated to the process till it terminates or switches to waiting state.
  2. The executing process in preemptive scheduling is interrupted in the middle of execution when higher priority one comes whereas, the executing process in non-preemptive scheduling is not interrupted in the middle of execution and wait till its execution.
  3. In Preemptive Scheduling, there is the overhead of switching the process from ready state to running state, vise-verse, and maintaining the ready queue. Whereas in case of non-preemptive scheduling has no overhead of switching the process from running state to ready state.
  4. In preemptive scheduling, if a high priority process frequently arrives in the ready queue then the process with low priority has to wait for a long, and it may have to starve. On the other hands, in the non-preemptive scheduling, if CPU is allocated to the process having larger burst time then the processes with small burst time may have to starve.
  5. Preemptive scheduling attain flexible by allowing the critical processes to access CPU as they arrive into the ready queue, no matter what process is executing currently. Non-preemptive scheduling is called rigid as even if a critical process enters the ready queue the process running CPU is not disturbed.
  6. The Preemptive Scheduling has to maintain the integrity of shared data that’s why it is cost associative as it which is not the case with Non-preemptive Scheduling.

Comparison Chart:

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 untill it terminates itself or its time is up.
Starvation If a process having high priority frequently arrives in the ready queue, low priority process may starve. If a process with 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.
Flexibility flexible rigid
Cost cost associated no cost associated
CPU Utilization In preemptive scheduling, CPU utilization is high. It is low in non preemptive scheduling.
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.

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