📝 First Come First Served Scheduling Algorithm in Operating System (41 MCQs)

📖 From Operating System • 6. CPU Scheduling • 41 questions available

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What is First Come First Served Scheduling Algorithm in Operating System?

Definition:
FCFS is a non-preemptive algorithm where the CPU is allocated to the process that requests it first, following FIFO order with average waiting time $W = \frac{1}{n}\sum_{i=1}^{n} W_i$ .

Example:
If Process P1 (burst=24ms), P2 (burst=3ms), and P3 (burst=3ms) arrive in that order, P2 must wait 24ms despite needing only 3ms of CPU time.

Reason:
While simple to implement, FCFS suffers from the convoy effect where short processes get stuck behind long ones, resulting in poor average waiting times and low suitability for time-sharing systems.

Easy

Medium

Hard

📝 All First Come First Served Scheduling Algorithm in Operating System MCQs

Q1. What is the fundamental principle of the First-Come, First-Served (FCFS) scheduling algorithm?

A.The shortest process gets CPU first

B.The process that requests CPU first is allocated CPU first ✅

C.The process with highest priority gets CPU first

D.The CPU is allocated in round-robin fashion

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: FCFS operates on a simple principle: the process that requests the CPU first receives it first. This is similar to a queue where early arrivals get service before later arrivals, regardless of their size or priority.

Q2. Which data structure is typically used to implement the FCFS scheduling algorithm?

A.Stack

B.Priority queue

C.FIFO queue ✅

D.Circular buffer

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: FCFS is implemented using a FIFO (First-In, First-Out) queue where processes are added to the tail and removed from the head. This straightforward implementation reflects the algorithm's simplicity and the order of service.

Q3. In FCFS scheduling, when a process enters the ready queue, its PCB is:

A.Removed from the queue

B.Linked onto the tail of the queue ✅

C.Placed at the head of the queue

D.Discarded

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The PCB (Process Control Block) of a new process is linked to the tail of the ready queue, maintaining the FIFO order. This ensures that processes are served in the exact order they arrived, with no reordering.

Q4. When the CPU becomes free in an FCFS system, how is the next process selected?

A.It is selected randomly

B.It is allocated to the process at the head of the ready queue ✅

C.It selects the process with the shortest burst

D.It selects based on priority

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: In FCFS, the CPU is allocated to the process at the head of the ready queue. This process has been waiting the longest, following the FIFO principle. After allocation, the process is removed from the queue.

Q5. Which of the following best describes the code complexity of FCFS scheduling?

A.Extremely complex

B.Moderately complex

C.Simple to write and understand ✅

D.Depends on the operating system

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: FCFS is known for being simple to write and understand. Its straightforward FIFO queue implementation requires minimal code, making it an ideal introductory scheduling algorithm despite its performance limitations.

Q6. What is the average waiting time for processes P1(24ms), P2(3ms), P3(3ms) if they arrive in P1, P2, P3 order?

A.17ms ✅

B.10ms

C.20ms

D.15ms

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Waiting times: P1=0ms, P2=24ms, P3=27ms. Average = (0+24+27)/3 = 17ms. P1 waits because it runs first, P2 waits for P1 to finish, and P3 waits for both P1 and P2, resulting in the 17ms average.

Q7. For the same processes P1(24ms), P2(3ms), P3(3ms), what is the average waiting time if they arrive in P2, P3, P1 order?

A.17ms

B.3ms ✅

C.10ms

D.5ms

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Arrival order P2, P3, P1: P2=0ms, P3=3ms, P1=6ms. Average = (0+3+6)/3 = 3ms. This substantial reduction demonstrates how arrival order dramatically affects FCFS performance.

Q8. What is the convoy effect in FCFS scheduling?

A.All processes move together through the system

B.One large CPU-bound process causes others to wait ✅

C.Processes form a queue for I/O devices

D.Processes communicate with each other

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The convoy effect occurs when one CPU-bound process holds the CPU while many I/O-bound processes wait in the ready queue. This leads to low resource utilization as I/O devices remain idle while processes are waiting for CPU.

Q9. Is the FCFS scheduling algorithm preemptive or nonpreemptive?

A.Fully preemptive

B.Nonpreemptive ✅

C.Partially preemptive

D.Both depending on configuration

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: FCFS is strictly nonpreemptive. Once a process gets the CPU, it continues running until it either terminates or requests I/O. This nonpreemptive nature is a key characteristic that leads to many of FCFS's performance issues.

Q10. What happens to the CPU in FCFS when a process requests I/O?

A.The CPU continues executing the same process

B.The CPU is allocated to the next waiting process ✅

C.The CPU becomes idle until I/O completes

D.The process is moved to the head of the queue

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: When a process requests I/O, it releases the CPU voluntarily. The CPU is then allocated to the next waiting process in the ready queue. The I/O-bound process moves to the I/O queue and returns to the ready queue when I/O completes.

Q11. Why is FCFS particularly troublesome for time-sharing systems?

A.It uses too much memory

B.It doesn't allow regular CPU sharing among users ✅

C.It requires complex hardware

D.It consumes too much power

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: FCFS is nonpreemptive and can allow one process to hold the CPU for extended periods, preventing regular CPU sharing. Time-sharing systems require each user to get CPU time at regular intervals, which FCFS cannot guarantee.

Q12. In the scenario with one CPU-bound and many I/O-bound processes in FCFS, what causes lower device utilization?

A.CPU processes too quickly

B.I/O-bound processes wait in ready queue while I/O devices idle ✅

C.I/O devices are too slow

D.CPU-bound processes use too much memory

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The convoy effect causes I/O-bound processes to wait in the ready queue while the CPU-bound process runs. During this time, I/O devices are idle because processes that need I/O are waiting for CPU, resulting in low I/O device utilization.

Q13. What is the waiting time for process P2 in the sequence P1(24), P2(3), P3(3) arriving in P1, P2, P3 order?

A.0ms

B.24ms ✅

C.27ms

D.3ms

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: P2 arrives second and waits for P1 to complete its 24ms CPU burst. Since P1 runs first, P2's waiting time equals P1's burst time of 24ms. P2 then runs for 3ms, making its completion time 27ms.

Q14. What condition causes the average waiting time to vary substantially in FCFS?

A.When all processes have equal burst times

B.When CPU burst times vary greatly ✅

C.When all processes arrive simultaneously

D.When the queue is empty

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: FCFS performance varies substantially when CPU burst times differ greatly. Processes with short bursts can wait behind long processes, causing high variance in waiting times. Equal burst times or simultaneous arrival still produce variation, but it's the burst time differences that create the most significant variance.

Q15. In FCFS, how does the CPU-bound process affect I/O-bound processes in terms of execution order?

A.I/O-bound processes execute before CPU-bound

B.I/O-bound processes get CPU immediately

C.I/O-bound processes wait behind CPU-bound processes ✅

D.CPU-bound processes are executed last

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: The CPU-bound process holds the CPU, causing I/O-bound processes to wait in the ready queue. This convoy effect means I/O-bound processes get delayed behind the CPU-bound process, leading to inefficient resource utilization.

Q16. What happens to I/O devices during the convoy effect in FCFS?

A.They remain busy

B.They become idle ✅

C.They process data faster

D.They switch to CPU operations

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: During the convoy effect, I/O devices become idle because I/O-bound processes are waiting in the ready queue for the CPU. Since they can't proceed to I/O until they get CPU time, the I/O devices have no work, causing low utilization.

Q17. Which of the following is a characteristic of FCFS that makes it unsuitable for real-time systems?

A.It uses too much CPU

B.It is nonpreemptive ✅

C.It requires too much memory

D.It causes excessive context switching

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: FCFS is unsuitable for real-time systems because it's nonpreemptive and cannot guarantee deadlines. A long process can delay critical real-time processes, causing missed deadlines. Preemptive algorithms are necessary to ensure timely execution of time-critical tasks.

Q18. What determines when a process releases the CPU in FCFS?

A.Time quantum expires

B.Higher priority process arrives

C.Process terminates or requests I/O ✅

D.Another process interrupts

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: In FCFS, a process keeps the CPU until it either terminates or requests I/O. No preemption occurs based on time or priority. This voluntary release mechanism is why FCFS is nonpreemptive and can lead to long delays for other processes.

Q19. What is the effect of the convoy effect on I/O device utilization in FCFS?

A.I/O device utilization increases

B.I/O device utilization decreases significantly ✅

C.I/O device utilization remains constant

D.I/O device utilization becomes maximum

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The convoy effect causes I/O devices to become idle while I/O-bound processes wait in the ready queue for the CPU. Since they cannot proceed to I/O operations until they get CPU time, I/O device utilization drops significantly, creating inefficiency.

Q20. Which of the following correctly describes the process of adding a new process to the FCFS ready queue?

A.The process is inserted at the head of the queue

B.The process is inserted at a position based on priority

C.The process is linked to the tail of the queue ✅

D.The process replaces the current process

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: In FCFS, a new process is added to the tail of the FIFO queue. This maintains the arrival order, ensuring that the process that has been waiting longest gets CPU first. Inserting at the head would reverse the FCFS principle.

Q21. What happens to the ready queue when the CPU is allocated to a process in FCFS?

A.The process is removed from the head of the queue ✅

B.The process is moved to the tail of the queue

C.The process remains in the queue

D.The process is discarded

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: When a process is allocated the CPU, it is removed from the head of the ready queue. This reflects the FIFO nature of FCFS: once a process starts, it's no longer waiting in the queue.

Q22. Which scenario best illustrates the convoy effect in FCFS?

A.Multiple CPU-bound processes running simultaneously

B.One CPU-bound process holding CPU while I/O-bound processes wait ✅

C.All processes completing in short bursts

D.Processes with equal burst times

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The convoy effect specifically refers to one CPU-bound process holding the CPU while many I/O-bound processes wait in the ready queue. This creates a convoy where all processes follow the slow CPU-bound process, leading to poor resource utilization.

Q23. What is the fundamental problem with FCFS scheduling in a dynamic system with mixed process types?

A.It causes high memory usage

B.It leads to low CPU and device utilization due to convoy effect ✅

C.It increases context switch overhead

D.It requires complex hardware

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The fundamental problem is the convoy effect in mixed environments. CPU-bound processes cause I/O-bound processes to wait, leading to CPU idle time when the CPU-bound process does I/O and I/O devices idle when processes wait for CPU. This dual-idle scenario significantly reduces overall system utilization.

Q24. In FCFS, what would happen if a CPU-bound process has a 100ms burst and 10 I/O-bound processes have 1ms bursts each, all arriving together?

A.All processes complete quickly

B.I/O-bound processes wait 100ms before running ✅

C.CPU processes first

D.Processes execute in parallel

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The CPU-bound process runs first for 100ms while all I/O-bound processes wait in the ready queue. Each I/O-bound process then runs for 1ms, so they are delayed by approximately 100ms each, demonstrating the convoy effect where many short processes wait behind one long process.

Q25. What would be the average waiting time if processes P1(8), P2(6), P3(4), P4(2) arrive in FCFS order P1, P2, P3, P4?

A.10ms ✅

B.15ms

C.12ms

D.8ms

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Waiting times: P1=0, P2=8, P3=14, P4=18. Sum = 0+8+14+18=40. Average = 40/4 = 10ms. This demonstrates that in FCFS, waiting time increases as the process position in the queue increases, with earlier processes accumulating longer waiting times for later arrivals.

Q26. Which of the following is a valid criticism of FCFS scheduling?

A.It is too complex to implement

B.It favors shorter processes over longer processes

C.The average waiting time can vary dramatically based on arrival order ✅

D.It causes too many context switches

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: FCFS's average waiting time varies dramatically based on arrival order, as shown by the P1(24), P2(3), P3(3) example where average waiting time changed from 17ms to 3ms based solely on arrival order. This unpredictability is a major criticism.

Q27. In FCFS scheduling, the ready queue is managed as a FIFO queue. What would be the consequence of using a LIFO (stack) implementation instead?

A.It would improve performance

B.Process arrival order would be reversed ✅

C.It would have no effect

D.It would eliminate waiting times

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A LIFO implementation would reverse the order, meaning the last process to arrive would get CPU first. This would drastically change the behavior from FCFS to Last-In-First-Out, which could cause starvation for earlier-arriving processes and completely change the scheduling characteristics.

Q28. What is the waiting time for the last process in a sequence of n processes in FCFS?

A.Always 0

B.Sum of all previous CPU bursts ✅

C.Equal to its own burst time

D.Always n times the average

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: For the last process in FCFS, its waiting time equals the sum of all previous processes' CPU burst times. In the example P1(24), P2(3), P3(3), P3's waiting time is 24+3=27ms. This accumulation effect is why FCFS can cause long waiting times.

Q29. A system with FCFS has processes: P1(20ms), P2(5ms), P3(10ms), P4(2ms) arriving in that order at time 0. What is the waiting time for P3?

A.15ms

B.20ms

C.25ms ✅

D.5ms

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: P3 waits for P1(20) + P2(5) = 25ms. P3 doesn't start until both P1 and P2 complete. P3's waiting time is 25ms, and then it executes for 10ms, completing at 35ms. This shows how waiting time accumulates from all previous processes regardless of their lengths.

Q30. Which of the following best describes the FCFS scheduling algorithm's behavior with multiple processes of varying burst lengths?

A.It always minimizes average waiting time

B.It tends to favor longer processes over shorter ones ✅

C.It ensures fairness in CPU allocation

D.It guarantees minimal response time

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: FCFS tends to favor longer processes because they arrive earlier and hold the CPU, causing shorter processes to wait behind them. This is the convoy effect in action, where long processes delay many shorter processes, making FCFS unfavorable for mixed workloads.

Q31. What is the relationship between process arrival order and waiting time variance in FCFS?

A.Arrival order has no effect on waiting time variance

B.Waiting time variance increases with later arrivals ✅

C.Waiting time variance is highest for early arrivals

D.Waiting time variance is determined by burst length distribution

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: In FCFS, later arrivals experience greater variance in waiting time because they are affected by the cumulative burst times of all previous processes. The first process always waits 0, while the last process waits the sum of all previous bursts, creating increasing variance with position in the queue.

Q32. What would be the effect on FCFS performance if the time quantum were introduced?

A.It would no longer be FCFS ✅

B.It would improve significantly

C.It would work exactly the same

D.It would cause more context switches

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Introducing a time quantum would change the algorithm from FCFS (nonpreemptive) to Round Robin (preemptive). FCFS is defined by its nonpreemptive nature and FIFO queue; any preemption would fundamentally change the algorithm's behavior and characteristics.

Q33. In the scenario with one CPU-bound and many I/O-bound processes in FCFS, what sequence of events occurs after the CPU-bound process finishes its burst?

A.I/O-bound processes execute quickly and go to I/O, leaving CPU idle ✅

B.I/O-bound processes wait for CPU indefinitely

C.CPU-bound process immediately restarts

D.All processes terminate

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: After the CPU-bound process completes its burst and goes to I/O, the I/O-bound processes (with short CPU bursts) execute quickly and also go to I/O. This leaves the CPU idle while the CPU-bound process is doing I/O and the I/O-bound processes wait for their I/O to complete, perpetuating the convoy effect.

Q34. Which of the following is an advantage of FCFS scheduling?

A.It ensures minimal average waiting time

B.It provides excellent interactive performance

C.It is simple to implement and understand ✅

D.It gives priority to I/O-bound processes

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: FCFS's main advantage is its simplicity. The code is easy to write, the FIFO queue is straightforward to manage, and the behavior is predictable in its ordering. However, this simplicity comes at the cost of poor performance in mixed workloads and interactive systems.

Q35. What would be the effect of the convoy effect on response time in an interactive system using FCFS?

A.Response time would be minimal

B.Response time would be excellent

C.Response time would be poor for interactive processes ✅

D.Response time would be unaffected

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The convoy effect causes interactive (I/O-bound) processes to wait behind CPU-bound processes, leading to poor response times. Users of interactive systems would experience delays as their processes wait for the CPU-bound process to finish, making FCFS unsuitable for time-sharing environments.

Q36. In FCFS, what happens to a process that is currently running and a new process arrives with a shorter CPU burst?

A.The running process is preempted

B.The running process continues to run ✅

C.The new process starts immediately

D.Both processes share the CPU

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Since FCFS is nonpreemptive, the running process continues even if a new process with a shorter burst arrives. The new process joins the tail of the ready queue and waits its turn. This nonpreemptive behavior is why short processes can experience long waiting times behind long processes.

Q37. What is the completion time of the last process in a sequence of 4 processes with bursts 5, 10, 15, 20 arriving in that order?

A.50ms ✅

B.45ms

C.40ms

D.55ms

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Completion time of last process = sum of all bursts = 5+10+15+20 = 50ms. The last process completes at 50ms, having waited 30ms (5+10+15) and executed for 20ms. This total completion time represents the makespan of the schedule.

Q38. What is the effect of the convoy effect on the CPU scheduling process in FCFS?

A.It improves overall system throughput

B.It reduces CPU and device utilization ✅

C.It eliminates context switching

D.It increases process priority

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The convoy effect reduces both CPU and device utilization. CPU utilization drops when the CPU-bound process does I/O and I/O-bound processes execute quickly, leaving the CPU idle. Device utilization drops when processes wait for CPU, leaving I/O devices idle. This dual reduction makes the system inefficient.

Q39. In FCFS scheduling, what is the waiting time for process i in terms of previous burst times?

A.b_i

B.b_1 + b_2 + ... + b_{i-1} ✅

C.b_1 + b_2 + ... + b_i

D.0

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: For process i in FCFS, waiting time equals the sum of all previous burst times: b_1 + b_2 + ... + b_{i-1}. This cumulative formula shows why later processes experience longer waiting times and why FCFS can cause substantial delays for processes that arrive late.

Q40. Which of the following would help mitigate the convoy effect in FCFS?

A.Increasing the time quantum

B.Using a shorter CPU burst

C.Allowing preemption ✅

D.Increasing memory size

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Allowing preemption would fundamentally change FCFS to a different scheduling algorithm (like Round Robin). Preemption would prevent a single CPU-bound process from holding the CPU indefinitely, allowing I/O-bound processes to get CPU time and breaking the convoy effect. However, this would no longer be FCFS.

Q41. What is the relationship between FCFS and the Shortest Process Next (SPN) algorithm?

A.They are identical

B.FCFS is a special case of SPN

C.They are completely different scheduling approaches ✅

D.SPN is more complex than FCFS

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: FCFS and SPN are different scheduling approaches. FCFS serves processes in arrival order regardless of burst length, while SPN selects the process with the shortest burst. They have different goals: FCFS focuses on fairness (arrival order), while SPN optimizes average waiting time by favoring short processes.

🔗 Related Topics (MCQs)

📝 CPU Scheduling Basic Concepts MCQs 📝 CPU I/O Burst Cycle MCQs 📝 CPU Scheduler in Operating System MCQs 📝 Preemptive Scheduling in Operating System MCQs 📝 Dispatcher in CPU Scheduling MCQs

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