Question:
Which one of the following CPU scheduling algorithms cannot be preemptive?
Which one of the following CPU scheduling algorithms cannot be preemptive?
Show Hint
Preemption means interrupting a running process before it finishes. FCFS always runs a process to completion once started, so it has no preemptive form.
Updated On: Jul 22, 2026
- Shortest Remaining Time First (SRTF) Scheduling
- First Come First Serve (FCFS) Scheduling
- Round Robin Scheduling
- Priority Scheduling
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The Correct Option is B
Solution and Explanation
Step 1: Recall what "preemptive" means for a scheduling algorithm.
A scheduling algorithm is preemptive if the CPU can be taken away from a currently running process before it finishes or voluntarily gives it up, typically because a new process arrives that the rule judges more deserving of the CPU right now, or because a fixed time slice expires. A non-preemptive algorithm, once it has given the CPU to a process, lets that process run to completion, or until it blocks on I/O, before considering any other process.
Step 2: Check SRTF.
Shortest Remaining Time First always runs whichever ready process has the least remaining burst time. If a new process arrives with a remaining time shorter than the currently running process's remaining time, SRTF immediately switches the CPU to that new process. This ongoing comparison and switching is exactly preemption, so SRTF is a preemptive algorithm; it is the preemptive version of Shortest Job First.
Step 3: Check FCFS.
First Come First Serve simply runs processes strictly in the order they arrive in the ready queue. Once a process starts executing, FCFS has no rule that ever interrupts it for a later-arriving process, no matter how short or urgent that new process is; the running process keeps the CPU until it finishes or blocks by itself. Because there is no mechanism in FCFS to take the CPU away early, FCFS is defined to be non-preemptive, and it cannot be operated in a preemptive form; a version that reordered or interrupted arrivals would no longer be "first come first serve".
Step 4: Check Round Robin.
Round Robin gives every process a fixed time quantum and forcibly switches to the next process in the queue when that quantum expires, even if the running process has not finished. This forced switch at the end of the quantum is preemption by definition, so Round Robin is preemptive.
Step 5: Check Priority Scheduling.
Priority scheduling can be implemented in either a preemptive or a non-preemptive way. In its preemptive form, if a new process arrives with higher priority than the one currently running, the CPU is immediately taken from the running process and given to the higher priority arrival. So priority scheduling CAN be preemptive, though it does not have to be.
Step 6: Conclusion.
SRTF, Round Robin, and Priority scheduling can all run in a preemptive form. FCFS is the only one of the four that is inherently non-preemptive by its very definition and can never preempt a running process.
Final Answer:
First Come First Serve (FCFS) Scheduling cannot be preemptive. \[ \boxed{\text{Option (B)}} \]
A scheduling algorithm is preemptive if the CPU can be taken away from a currently running process before it finishes or voluntarily gives it up, typically because a new process arrives that the rule judges more deserving of the CPU right now, or because a fixed time slice expires. A non-preemptive algorithm, once it has given the CPU to a process, lets that process run to completion, or until it blocks on I/O, before considering any other process.
Step 2: Check SRTF.
Shortest Remaining Time First always runs whichever ready process has the least remaining burst time. If a new process arrives with a remaining time shorter than the currently running process's remaining time, SRTF immediately switches the CPU to that new process. This ongoing comparison and switching is exactly preemption, so SRTF is a preemptive algorithm; it is the preemptive version of Shortest Job First.
Step 3: Check FCFS.
First Come First Serve simply runs processes strictly in the order they arrive in the ready queue. Once a process starts executing, FCFS has no rule that ever interrupts it for a later-arriving process, no matter how short or urgent that new process is; the running process keeps the CPU until it finishes or blocks by itself. Because there is no mechanism in FCFS to take the CPU away early, FCFS is defined to be non-preemptive, and it cannot be operated in a preemptive form; a version that reordered or interrupted arrivals would no longer be "first come first serve".
Step 4: Check Round Robin.
Round Robin gives every process a fixed time quantum and forcibly switches to the next process in the queue when that quantum expires, even if the running process has not finished. This forced switch at the end of the quantum is preemption by definition, so Round Robin is preemptive.
Step 5: Check Priority Scheduling.
Priority scheduling can be implemented in either a preemptive or a non-preemptive way. In its preemptive form, if a new process arrives with higher priority than the one currently running, the CPU is immediately taken from the running process and given to the higher priority arrival. So priority scheduling CAN be preemptive, though it does not have to be.
Step 6: Conclusion.
SRTF, Round Robin, and Priority scheduling can all run in a preemptive form. FCFS is the only one of the four that is inherently non-preemptive by its very definition and can never preempt a running process.
Final Answer:
First Come First Serve (FCFS) Scheduling cannot be preemptive. \[ \boxed{\text{Option (B)}} \]
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