CPU Scheduling Gantt Chart: Visualizing Process Execution Order

What a CPU Scheduling Gantt Chart Shows

A CPU scheduling Gantt chart is a horizontal bar chart that visualizes which process runs on the processor at every point in time. Each bar represents a burst of execution, and the length of the bar corresponds to the burst duration. The chart makes it easy to compare how different scheduling algorithms order work, where idle gaps appear, and when context switches happen. For anyone studying operating systems or debugging a scheduler, the Gantt chart is the first place to look.

Reading a Gantt Chart

The horizontal axis is time, measured in milliseconds or arbitrary time units. The vertical axis lists process IDs or job names. A block that starts at time 2 and ends at time 5 means the process ran for three units beginning at time 2. If a gap appears between two blocks, the CPU was idle during that interval. Arrows or labels on the bars can indicate which scheduling decision caused a preemption or which process was swapped in next.

Key Metrics You Can Read Directly

• Completion time: where the last bar for a process ends.
• Turnaround time: completion time minus arrival time.
• Waiting time: time a process spends not running, visible as gaps before its bars.
• Response time: when a process first gets CPU time minus its arrival time.

How to Draw a Gantt Chart

Start by listing each process with its arrival time and burst time. Then apply the scheduling rule you are analyzing, picking the next process at every decision point. For non-preemptive algorithms, once a process starts it runs until its burst finishes. For preemptive ones, interrupt the running process when a higher-priority or time-slice event occurs. Draw a bar for each execution segment in chronological order, label the start and end times, and mark idle periods with a dashed or distinct line.

Example: FCFS with Three Processes

Suppose P1 arrives at time 0 with burst 4, P2 arrives at time 1 with burst 3, and P3 arrives at time 2 with burst 1. Under First-Come First-Served, P1 runs 0 to 4, P2 runs 4 to 7, and P3 runs 7 to 8. The Gantt chart shows three consecutive bars with no gaps. If P2 had arrived at time 0 instead, the order would change, and the chart would reflect the different waiting times each process experiences.

Common Scheduling Algorithms and Their Gantt Shapes

Different scheduling policies produce visually distinct Gantt charts. Shortest Job First tends to cluster short bursts together, reducing average waiting time but potentially starving long processes. Round Robin inserts many short bars of equal length, separated by context switches, which creates a characteristic striped pattern. Priority scheduling can produce irregular blocks wherever a high-priority process enters the queue and preempts the current one.

Context Switches and Idle Gaps

A Gantt chart makes context switches visible as vertical breaks or narrow markers between bars. Each switch consumes a small amount of CPU time that does not belong to any process. In heavily loaded Round Robin systems, these switches can dominate the chart. Idle gaps show when the ready queue is empty and the CPU has nothing to run, which is useful for capacity planning and for identifying underutilized systems.

Using the Chart to Compare Algorithms

Side-by-side Gantt charts for the same set of processes let you compare algorithms at a glance. You can see which schedule finishes all work sooner, which leaves the CPU idle, and which process waits longest. When calculating average turnaround or waiting time, the chart provides the raw completion times you need. For teaching and documentation, a well-labeled Gantt chart is often clearer than a table of numbers.

Limitations of Gantt Charts

A Gantt chart works best for a single CPU and a known set of processes with fixed arrival and burst times. It does not show memory usage, I/O waits within a burst, or dynamic changes that happen during real execution. For long-running or highly variable workloads, a statistical summary or a trace log may be more useful than a static chart.

Practical Tips for Accurate Charts

• Mark arrival times explicitly so you can see when processes become eligible.
• Use a consistent time scale so bar lengths are proportional.
• Label every switch point with the reason (preemption, I/O completion, quantum expiry) when comparing multiple algorithms.
• For preemptive algorithms, draw each new slice as a separate bar even if the same process resumes later.