Scheduling Metrics & Criteria: Arrival, Burst, Turnaround & Waiting
“The golden formulas every systems engineer and GATE aspirant must master.”
1.The Scheduling Problem & Core Terminology
§ 3.1.Whenever the CPU becomes idle, the operating system's Short-Term Scheduler must select one process from the Ready Queue to execute. How does the OS evaluate whether a scheduling algorithm is 'good'?
To measure scheduler efficiency, systems engineers rely on mathematically precise time metrics for every process Pᵢ:
Arrival Time (AT): The precise time instant at which the process enters the Ready Queue.
Burst Time (BT): The total amount of CPU execution time required by the process to complete its current CPU burst.
Completion Time (CT): The time instant at which the process finishes executing its final instruction.
Turnaround Time (TAT): The entire lifespan of the process in the system, from arrival to completion (TAT = CT - AT).
Waiting Time (WT): The total time the process spent sitting idly in the Ready Queue waiting to be allocated the CPU (WT = TAT - BT).
Response Time (RT): The time elapsed from arrival until the process produces its first response / gets its very first CPU burst.
2. The Golden Mathematical Formulas & Derivations
| Arrival TimeAT | Burst TimeBT | Completion TimeCT |
Turnaround time represents total elapsed residence time in the system from arrival to completion.
Of the total time spent in the system (TAT), the process was actively executing for its Burst Time (BT). Everything else was spent waiting in the ready queue.
To evaluate overall scheduling fairness, compute the arithmetic mean over all N processes.
3. GATE Lens & Verified PYQs
Syllabus focus, recurring numerical patterns, common traps, and clearly labelled exam practice. Questions remain source-pending until an official paper and answer key are attached.
Scheduling metrics are a recurring GATE calculation area (2-4 Marks consistently) across virtually all exam papers.
- •Given a table of processes with Arrival and Burst times, construct the Gantt chart and compute Average Waiting Time or Average Turnaround Time.
- •Watch out for non-zero arrival times! If a process arrives at t = 2, its waiting time does NOT start counting from t = 0.
- •If the CPU sits idle between bursts, that idle slot MUST be drawn in the Gantt chart and accounted for in CT.
Exam Practice / Source Verification Required
1 problemConsider three processes P1, P2, and P3 with arrival times 0, 2, and 4 ms, and CPU burst times 4, 3, and 1 ms, respectively. If non-preemptive Shortest Job First (SJF) scheduling is used, what is the average turnaround time (in ms)?
4. 1-Minute Panic Revision
“Always build Gantt chart → Find Completion Time (CT) → TAT = CT - AT → WT = TAT - BT.”
- •TAT = CT - AT (Total time in system)
- •WT = TAT - BT (Time spent waiting in ready queue)
- •Response Time (RT) = First time getting CPU - AT
- •Never subtract AT from BT directly; WT is derived from TAT!
- •Account for CPU idle gaps in the Gantt chart if no process has arrived.
Primary Academic References & Exam Sources
Every concept in Notes From a B.Tech Brain is synthesized from authoritative academic texts, seminal peer-reviewed research, verified exam archives, and renowned university lectures.
Official Chapter 5 slides defining CPU utilization, throughput, turnaround time, waiting time, and response time.
Full chapter PDF introducing turnaround time vs response time metrics and basic scheduling policies.
If the textbook derivation did not click immediately, consult these targeted video lessons:
Clear breakdown of Arrival Time, Burst Time, Completion Time, TAT, WT, and CPU idle gap handling on Gantt charts.
Whiteboard explanations comparing throughput, CPU utilization, turnaround time, and waiting time definitions.
Curated lectures on Gantt Charts, Preemption, FCFS Convoy Effect, Round Robin Quantum & SJF/SRTF:
Step-by-step Gantt chart calculations with zero arithmetic ambiguity.
The most reliable demonstration of ready-queue tracking during simultaneous arrivals.
Structured textbook-level coverage with cleanly paced whiteboard walkthroughs.
Intense numerical speed practice and identifying examiner traps in Gantt charts.