What Are the Main Process Scheduling Algorithms?
Compare the main process scheduling algorithms — FCFS, Shortest Job First, Priority Scheduling, Round Robin — and their tradeoffs between fairness and speed.
Expected Interview Answer
Process scheduling algorithms are the rules an operating system uses to decide which ready process gets the CPU next, with common ones being First-Come-First-Served, Shortest Job First, Priority Scheduling, and Round Robin.
First-Come-First-Served (FCFS) runs processes strictly in arrival order, which is simple but can cause long waits behind a slow job, known as the convoy effect. Shortest Job First (SJF) picks the process with the smallest estimated burst time next, minimizing average waiting time but risking starvation of longer jobs. Priority Scheduling runs the highest-priority ready process first, which can also starve low-priority tasks unless aging is applied. Round Robin gives every process a fixed time slice in rotation, which keeps the system responsive and fair for interactive workloads, though a poorly chosen time quantum hurts efficiency. Real operating systems often blend these ideas into multilevel feedback queues.
- Different algorithms trade off fairness, throughput, and responsiveness
- FCFS is simple and predictable to implement
- SJF minimizes average waiting time in theory
- Round Robin gives fair, responsive time-sharing
- Priority scheduling lets critical tasks run first
AI Mentor Explanation
Scheduling algorithms are like ways a groundsman picks which nets session gets the pitch next. First-come-first-served lets whoever arrived first bat on, shortest-job-first lets a quick drill go first to clear the queue, priority scheduling lets the national team go before club players, and round robin gives every batter fixed overs before rotating.
Step-by-Step Explanation
Step 1
FCFS
Processes run strictly in arrival order using a simple queue; easy to implement but suffers the convoy effect.
Step 2
Shortest Job First
The scheduler picks the ready process with the smallest estimated burst time, minimizing average wait but risking starvation.
Step 3
Priority Scheduling
Each process gets a priority number; the highest-priority ready process runs first, with aging used to prevent starvation.
Step 4
Round Robin
Each process gets a fixed time quantum in a circular queue, giving fair, responsive time-sharing for interactive systems.
Step 5
Multilevel Feedback Queue
Real OSes combine these ideas, moving processes between queues based on behavior to balance fairness and throughput.
What Interviewer Expects
- Names at least three concrete algorithms (FCFS, SJF, Priority, Round Robin)
- Explains the tradeoff each algorithm makes
- Mentions starvation and how aging addresses it
- Understands the convoy effect in FCFS
- Knows real OSes use multilevel feedback queues combining ideas
Common Mistakes
- Naming only one algorithm without explaining tradeoffs
- Confusing SJF with priority scheduling
- Forgetting that starvation can occur without aging
- Assuming Round Robin performance doesn't depend on the time quantum
Best Answer (HR Friendly)
“Process scheduling algorithms are the rules a computer's operating system uses to decide which waiting task gets the processor next. Some rules serve tasks in the order they arrived, some prioritize the quickest task, some let important tasks jump the queue, and some give every task a fair, rotating turn — each approach balances fairness against speed differently.”
Code Example
from collections import deque
def round_robin(processes, quantum):
queue = deque(processes) # each: {'pid': int, 'remaining': int}
timeline = []
while queue:
p = queue.popleft()
run = min(quantum, p['remaining'])
timeline.append((p['pid'], run))
p['remaining'] -= run
if p['remaining'] > 0:
queue.append(p)
return timeline
procs = [{'pid': 1, 'remaining': 5}, {'pid': 2, 'remaining': 3}]
print(round_robin(procs, quantum=2))
Follow-up Questions
- What is the convoy effect and which algorithm causes it?
- How does aging prevent starvation in priority scheduling?
- How do you choose a good time quantum for Round Robin?
- What is a multilevel feedback queue scheduler?
- How does preemptive scheduling differ from non-preemptive?
MCQ Practice
1. Which scheduling algorithm can cause the convoy effect?
FCFS runs strictly in arrival order, so a long process at the front makes every process behind it wait, causing the convoy effect.
2. What is a downside of pure Shortest Job First scheduling?
SJF favors short processes, so a steady stream of short jobs can leave a long job waiting indefinitely — starvation.
3. What technique prevents starvation in priority scheduling?
Aging gradually increases the priority of waiting processes over time, ensuring they eventually get scheduled.
Flash Cards
What is FCFS scheduling? — Processes run strictly in the order they arrived in the ready queue.
What is Shortest Job First? — The scheduler runs the ready process with the smallest estimated burst time next.
What does Round Robin guarantee? — Every process gets a fixed time quantum in rotation, giving fair, responsive sharing.
What fixes starvation in priority scheduling? — Aging — gradually raising the priority of processes that have waited a long time.