Chapter 2
Apply algorithm rules and compute waiting/turnaround using Gantt charts.
Each algorithm has a different rule for who runs next; this directly changes fairness and delay.
This topic is high-weight in papers and practical interviews because it combines concepts and calculation.
Clear explanation
For each algorithm: sort by rule, draw timeline, compute completion time, then derive turnaround and waiting.
What it really means
One consistent worksheet method prevents most arithmetic errors.
Example
Turnaround = completion - arrival; Waiting = turnaround - burst.
Key takeaway
Use a fixed solving pipeline for every question.
Clear explanation
No single algorithm wins all metrics; interactive systems often favor response while batch systems may favor throughput.
What it really means
Scheduling is policy design, not one-time formula memorization.
Key takeaway
Always justify algorithm choice using workload and metrics.
Clear explanation
Round Robin gives each ready process a fixed time quantum. If a process does not finish within quantum, it is preempted and pushed to queue tail.
What it really means
Like students answering one-by-one in class with a timer; unfinished student gets another turn later.
Example
For burst times [10,2,6,8,4] and quantum=3, the queue rotates repeatedly until each remaining burst reaches 0.
Key takeaway
Round Robin improves fairness and response, but quantum choice controls overhead.
Clear explanation
FCFS picks earliest arrival, SJF picks smallest next burst, SRTN keeps checking smallest remaining time, and Priority picks highest-priority process (preemptive or non-preemptive).
What it really means
These are different queue ordering rules applied at each scheduling decision point.
Example
If a short process arrives while a long one runs, SRTN may preempt immediately, but FCFS will not.
Key takeaway
Algorithm behavior is defined by selection rule + preemption policy.
Clear explanation
For P1(AT0,BT5), P2(AT1,BT3), P3(AT2,BT2), FCFS execution is P1(0-5), P2(5-8), P3(8-10).
What it really means
Arrival order is fixed, so later short jobs wait behind earlier long jobs.
Example
CT: [5,8,10], TAT: [5,7,8], WT: [0,4,6].
Key takeaway
Use CT -> TAT -> WT table format for scoring consistency.
Clear explanation
A typical RR sequence with quantum 2 can be shown as P1 -> P2 -> P3 -> P1 -> P2 -> P1, with each unfinished process returning to queue tail.
What it really means
RR shares CPU in fair time slices rather than full completion turns.
Example
After each slice, subtract executed quantum from remaining burst and redraw queue order.
Key takeaway
Marks are often awarded for correct rotation steps even before final averages.
Turnaround Time (TAT) = Completion Time (CT) - Arrival Time (AT)
Waiting Time (WT) = TAT - Burst Time (BT)
Response Time (RT) = First Start Time - Arrival Time
Use this as exam-ready explanation before solving numericals.
Rule: Run jobs in arrival order.
Good for: Simple batch systems and deterministic order.
Watch out: Convoy effect and poor average waiting time.
Rule: Each process gets a fixed time quantum in cyclic order.
Good for: Interactive systems needing fairness and response.
Watch out: Quantum too small => high context switching overhead.
Rule: Choose process with smallest burst among ready jobs.
Good for: Low average waiting time in predictable workloads.
Watch out: Needs burst estimate and may starve long jobs.
Rule: Always run job with shortest remaining time.
Good for: Fast completion of short jobs.
Watch out: Frequent preemption and starvation risk.
Rule: Run highest-priority process first.
Good for: Systems with critical and non-critical task classes.
Watch out: Low-priority starvation without aging.
What evaluators usually expect in structured exam answers.
Repeated pattern: Same 5-process RR/FCFS table is frequently repeated in papers.
Quick self-check from this topic before moving ahead.
Find average waiting time using FCFS and SJF for burst times 21,3,6,2.
Source: Summer 2023 Q2(C)
Answer focus: Correct Gantt sequence and metric calculation.
Apply Round Robin (q=3) for five processes and compute average TAT and WT.
Source: Summer 2024 Q2(A)
Answer focus: Correct rotation order with repeated arrivals at time 0.
These paper questions map directly to this topic. Solve now, then compare your structure with linked topics.
Calculate average waiting time using FCFS and SJF for burst times 21, 3, 6 and 2, all arrival time 0. Draw Gantt chart.
Calculate average turnaround and waiting time for Round Robin (q=3) and Priority Scheduling for five processes with given burst and priorities.
Explain SRTN in short and solve with processes P0..P3 with arrival 0,1,3,5 and burst 10,6,2,4. Draw Gantt chart and compute average TAT/WT.
Explain SJF in short and solve with processes P0..P3 with arrival 0,1,3,5 and burst 10,6,2,4. Draw Gantt chart and compute average TAT/WT.
Implement Round Robin (q=3) for five processes (same arrival) and find average turnaround and waiting time.
Using FCFS with given arrival and mixed I/O-CPU-I/O profile, find percentage of CPU idle time.
Implement Round Robin (q=3) for five processes and find average TAT and WT.
Explain FCFS and Round Robin algorithm with example.
Explain SJF and non-preemptive priority algorithm with example.
Implement FCFS for five processes (same arrival) and find average TAT and WT.
Explain FCFS algorithm with example.
Explain SJF algorithm with example.
Calculate average waiting time using FCFS and SJF for burst times 21, 3, 6 and 2, all arrival time 0. Draw Gantt chart.
Calculate average turnaround and waiting time for Round Robin (q=3) and Priority Scheduling for five processes with given burst and priorities.
Explain SRTN in short and solve with processes P0..P3 with arrival 0,1,3,5 and burst 10,6,2,4. Draw Gantt chart and compute average TAT/WT.
Explain SJF in short and solve with processes P0..P3 with arrival 0,1,3,5 and burst 10,6,2,4. Draw Gantt chart and compute average TAT/WT.
Implement Round Robin (q=3) for five processes (same arrival) and find average turnaround and waiting time.
Using FCFS with given arrival and mixed I/O-CPU-I/O profile, find percentage of CPU idle time.
Implement Round Robin (q=3) for five processes and find average TAT and WT.
Explain FCFS and Round Robin algorithm with example.
Explain SJF and non-preemptive priority algorithm with example.
Implement FCFS for five processes (same arrival) and find average TAT and WT.
Explain FCFS algorithm with example.
Explain SJF algorithm with example.
Summer 2023 Q2(C)
Calculate average waiting time using FCFS and SJF for burst times 21, 3, 6 and 2, all arrival time 0. Draw Gantt chart.
Answer pattern: define briefly, then solve stepwise and show final values/table.
Summer 2023 Q2(C) OR
Calculate average turnaround and waiting time for Round Robin (q=3) and Priority Scheduling for five processes with given burst and priorities.
Answer pattern: define briefly, then solve stepwise and show final values/table.
Winter 2023 Q2(C)
Explain SRTN in short and solve with processes P0..P3 with arrival 0,1,3,5 and burst 10,6,2,4. Draw Gantt chart and compute average TAT/WT.
Answer pattern: define briefly, then solve stepwise and show final values/table.
Winter 2023 Q2(C) OR
Explain SJF in short and solve with processes P0..P3 with arrival 0,1,3,5 and burst 10,6,2,4. Draw Gantt chart and compute average TAT/WT.
Answer pattern: define briefly, then solve stepwise and show final values/table.
Summer 2024 Q2(A)
Implement Round Robin (q=3) for five processes (same arrival) and find average turnaround and waiting time.
Answer pattern: concept -> intuition -> steps -> concluding point with one application.
Summer 2024 Q2(C) OR
Using FCFS with given arrival and mixed I/O-CPU-I/O profile, find percentage of CPU idle time.
Answer pattern: concept -> intuition -> steps -> concluding point with one application.
These are clearly marked additions, separate from source notes.