FCFS (First Come First Served) in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does FCFS stand for in operating systems?

FCFS stands for First Come First Served. It is a simple scheduling algorithm where the process that arrives first gets executed first.

beginner

How does the FCFS scheduling algorithm decide which process to run next?

FCFS runs processes in the order they arrive. The process that comes first is served first, without preemption.

intermediate

What is a major drawback of the FCFS scheduling algorithm?

A major drawback is the "convoy effect," where short processes wait a long time if a long process is running first, causing poor average waiting time.

beginner

In FCFS, if Process A arrives at time 0 and needs 5 units of CPU, and Process B arrives at time 1 needing 2 units, which process finishes first?

Process B finishes first because Process A starts at time 0 and runs for 5 units, finishing at time 5. Process B starts at time 5 and runs for 2 units, finishing at time 7. So Process A finishes first.

beginner

Is FCFS a preemptive or non-preemptive scheduling algorithm?

FCFS is a non-preemptive scheduling algorithm. Once a process starts running, it runs until it finishes without interruption.

What is the main principle of FCFS scheduling?

A Serve processes randomly

B Serve processes in the order they arrive

C Serve the process with highest priority first

D Serve the shortest process first

FCFS schedules processes based on their arrival time, serving the earliest arriving process first.

Which problem is commonly associated with FCFS scheduling?

A Convoy effect

B Starvation

C Deadlock

D Priority inversion

The convoy effect happens in FCFS when short processes wait behind long ones, causing delays.

Is FCFS scheduling preemptive or non-preemptive?

A Preemptive

B Both

C Non-preemptive

D Depends on the process

FCFS is non-preemptive; once a process starts, it runs until completion.

If Process X arrives before Process Y, which process will FCFS run first?

A Randomly chosen

B Process Y

C The one with shorter burst time

D Process X

FCFS runs the process that arrives first, so Process X runs before Process Y.

Which of these is NOT a characteristic of FCFS?

A Processes can be interrupted

B Can cause long waiting times

C Simple to implement

D Non-preemptive scheduling

FCFS does not interrupt processes once they start; it is non-preemptive.

Explain how the FCFS scheduling algorithm works and mention one advantage and one disadvantage.

Think about the order processes arrive and how that affects waiting times.

Describe a real-life situation that is similar to FCFS scheduling.

Imagine a queue where everyone waits their turn.

Practice

1. What does FCFS (First Come First Served) scheduling mean in operating systems?

easy

A. Processes with the shortest time are handled first.

B. Processes are handled in the order they arrive.

C. Processes are handled randomly.

D. Processes with the highest priority are handled first.

Solution

1. Step 1: Understand FCFS concept

   FCFS means tasks are processed in the order they come, like a queue.

2. Step 2: Compare options

   Only Processes are handled in the order they arrive. describes this order-based processing correctly.

3. Final Answer:

   Processes are handled in the order they arrive. -> Option B

4. Quick Check:

   FCFS = Order of arrival [OK]

Hint: Remember: FCFS is like waiting in line [OK]

Common Mistakes:

• Confusing FCFS with priority scheduling
• Thinking shortest tasks go first
• Assuming random order

2. Which of the following is the correct way to describe FCFS scheduling?

easy

A. Processes are scheduled in the order they arrive without preemption.

B. Processes are scheduled based on their priority levels.

C. Processes are scheduled by shortest remaining time first.

D. Processes are scheduled randomly to balance load.

Solution

1. Step 1: Identify FCFS scheduling traits

   FCFS schedules tasks in arrival order and does not interrupt running tasks.

2. Step 2: Match options to traits

   Processes are scheduled in the order they arrive without preemption. correctly states no preemption and order of arrival.

3. Final Answer:

   Processes are scheduled in the order they arrive without preemption. -> Option A

4. Quick Check:

   FCFS = Arrival order + no preemption [OK]

Hint: FCFS runs tasks fully in arrival order [OK]

Common Mistakes:

• Mixing FCFS with priority or shortest job scheduling
• Assuming tasks can be interrupted
• Thinking scheduling is random

3. Given three processes arriving at times 0, 2, and 4 with burst times 5, 3, and 1 respectively, what is the completion time of the second process using FCFS?

medium

A. 10

B. 5

C. 8

D. 9

Solution

1. Step 1: Calculate completion of first process

   Process 1 arrives at 0 and runs for 5 units, finishing at time 5.

2. Step 2: Calculate completion of second process

   Process 2 arrives at 2 but waits until process 1 finishes at 5, then runs for 3 units, finishing at 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   Process 2 finishes at 5+3=8 [OK]

Hint: Add burst times in arrival order [OK]

Common Mistakes:

• Starting second process at its arrival time instead of after first finishes
• Adding arrival times incorrectly
• Ignoring waiting time

4. A student wrote this FCFS scheduling code but it gives wrong completion times. What is the likely error?

processes = [(0, 4), (1, 3), (2, 1)]  # (arrival, burst)
completion = []
current_time = 0
for arrival, burst in processes:
    if arrival > current_time:
        current_time = arrival
    completion.append(current_time)
    current_time += burst
print(completion)

medium

A. Not updating current_time after appending completion time.

B. Using a list instead of a queue for processes.

C. Not considering arrival time when updating current_time.

D. Appending completion time before updating current_time.

Solution

1. Step 1: Analyze code logic

   Completion time is appended before current_time is updated with burst time, causing wrong values.

2. Step 2: Identify correct order

   We must update current_time by adding burst before appending completion time to reflect actual finish time.

3. Final Answer:

   Appending completion time before updating current_time. -> Option D

4. Quick Check:

   Update time before recording completion [OK]

Hint: Update current time before saving completion [OK]

Common Mistakes:

• Appending completion time too early
• Ignoring arrival time adjustments
• Confusing process order

5. In an FCFS system, three processes arrive at times 0, 1, and 2 with burst times 4, 2, and 6. If the first process takes longer than expected and runs for 8 units instead of 4, how does this affect the waiting time of the third process?

hard

A. The third process waits longer because the first process delays the queue.

B. The third process waiting time remains the same.

C. The third process starts earlier due to the delay.

D. The third process is skipped and runs immediately.

Solution

1. Step 1: Understand FCFS impact of longer burst

   FCFS runs processes fully in arrival order, so a longer first process delays all others.

2. Step 2: Analyze waiting time effect on third process

   The third process must wait until the first and second finish, so longer first process increases its waiting time.

3. Final Answer:

   The third process waits longer because the first process delays the queue. -> Option A

4. Quick Check:

   Longer first task = longer wait for later tasks [OK]

Hint: Long first task delays all later tasks [OK]

Common Mistakes:

• Assuming later tasks start earlier
• Ignoring impact of burst time changes
• Thinking FCFS skips tasks