FCFS Algorithm

The Problem

In a traditional single-processor system, the CPU is strictly limited to executing only one single process at any given moment in time.
While that specific task is running, all other loaded processes are forced into an idle waiting state, completely unable to make progress.

⏳

Whenever a running process has to wait for an I/O operation, the CPU goes idle. It’s a massive waste of processing power with zero efficiency.

The Solution: CPU Scheduling

To solve the issue of wasted time, the operating system relies on a simple rule: never let the CPU sit idle!
To maximize CPU utilization, several processes are kept in memory at the same time. When one process has to wait (typically for an I/O request), the OS immediately takes the CPU away and gives it to another ready process.

To manage these rapid switches, the OS relies on two main components: the CPU Scheduler, which makes the decision, and the Dispatcher, which executes the switch in a tiny timeframe known as Dispatch Latency.

Ready Queue (in Main Memory)

CPU

I/O Waiting Queue

I/O Device

EXIT

Click Play to see how the Scheduler completely eliminates CPU idle time!

As you can see, the CPU Scheduler successfully keeps the processor busy. But this leaves one crucial question: When there are multiple processes ready in the queue, how does the scheduler decide who goes first?

The answer to this question lies in:
Scheduling Algorithms ⬇

CPU Scheduling Algorithms

CPU Scheduling
- Preemptive
  - Priority Scheduling
  - Shortest Remaining Job First
  - Longest Remaining Job First
  - Round Robin
- Non-Preemptive
  - First Come First Serve
  - Shortest Job First
  - Longest Job First
  - Highest Response Ratio Next

Preemptive Scheduling

In preemptive scheduling, the operating system can interrupt a running process to allocate the CPU to another process usually due to priority rules or time-sharing policies. A process may be moved from Running → Ready state before it finishes.

READY QUEUE

CPU

I/O WAITING QUEUE

I/O Device

EXIT

Non-Preemptive Scheduling

In non-preemptive scheduling, once a process starts using the CPU, it runs until it finishes or moves to a waiting state. The OS cannot forcibly take away the CPU.

READY QUEUE

CPU

I/O WAITING QUEUE

I/O Device

EXIT

Now that we know the types, let's explore the first scheduling algorithm: FCFS.

Scenario 1

Arrival Time (ms)

CPU Burst Time (ms)

I/O Burst Time (ms)

Note: CPU Burst Time values must be between 1 and 20 ms. * This restriction is purely to keep the simulation visually clear and manageable. Real-world operating systems have no such arbitrary limits.

READY QUEUE

CPU

EXIT

Gantt Chart

Turnaround Time (TAT)

is the total time from arrival to completion

TAT = Completion Time - Arrival Time

Waiting Time (WT)

is the sum of the periods spent waiting in the ready queue.

WT = Turnaround Time - CPU Burst Time

Process

Completion Time

Turnaround Time

Waiting Time

Average Turnaround Time:

Measures how long processes take to finish overall
➔ Used to evaluate system responsiveness

= ∑ TAT / Number of processes
= ?? / 4 = ?? ms

Average Waiting Time:

Shows how long processes stay in the ready queue
➔ Used to evaluate scheduling efficiency

= ∑ WT / Number of processes
= ?? / 4 = ?? ms

Makespan:

Total time required to complete all processes
➔ Used to measure total execution duration

= Last Completion Time
= ?? ms

Throughput:

Number of processes completed per unit time
➔ Used to measure system productivity

= Number of processes / Makespan
= 4 / ?? = ?? processes / ms

These metrics are used to evaluate the performance and efficiency of scheduling algorithms.

🎯 How to evaluate scheduling efficiency?:

↓ Minimize (Lower is better): Average Waiting Time, Average Turnaround Time, and Makespan.
↑ Maximize (Higher is better): Throughput.

Scenario 2

Arrival Time (ms)

CPU Burst Time (ms)

I/O Burst Time (ms)

Notes:

•Arrival Time values must be between 0 and 20 ms.
•CPU Burst Time values must be between 1 and 20 ms.

READY QUEUE

CPU

EXIT

Gantt Chart

If 2 processes arrive at the same time, the process with the smaller ID is scheduled first.

Process

Completion Time

Turnaround Time

Waiting Time

Average Turnaround Time

= ∑ TAT / Number of processes
= ?? / 4 = ?? ms

Average Waiting Time

= ∑ WT / Number of processes
= ?? / 4 = ?? ms

Makespan

= Last Completion Time
= ?? ms

Throughput

= Number of processes / Makespan
= 4 / ?? = ?? processes / ms

Scenario 3

Arrival Time (ms)

CPU Burst Time (ms)

I/O Burst Time (ms)

Notes:

•Arrival Time values must be between 0 and 20 ms.
•CPU Burst Time values must be between 1 and 20 ms.
•I/O Burst Time values must be between 0 and 20 ms, where 0 indicates no I/O.
•When I/O is involved, the CPU Burst is divided into two phases: one executes before the I/O, and the other finishes after it.

READY QUEUE

CPU

I/O WAITING QUEUE

I/O Device

EXIT

Gantt Chart

Process

Completion Time

Turnaround Time

Waiting Time

Average Turnaround Time

= ∑ TAT / Number of processes
= ?? / 4 = ?? ms

Average Waiting Time

= ∑ WT / Number of processes
= ?? / 4 = ?? ms

Makespan

= Last Completion Time
= ?? ms

Throughput

= Number of processes / Makespan
= 4 / ?? = ?? processes / ms

Advantages & Disadvantages

Classify each statement as an Advantage or a Disadvantage of FCFS.

1 / 8

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Conclusion

You've Mastered FCFS! 🏆

You have successfully explored the foundational building block of CPU scheduling.
First-Come, First-Serve is beautifully simple, it guarantees strict fairness and ensures that no process is ever left behind.

However, as you witnessed with the Convoy Effect, simplicity comes at a cost. In the real world of operating systems, there is no single "perfect" algorithm. Every scheduling decision is a delicate trade-off between fairness, waiting time, and overall efficiency.

Understanding FCFS gives you the essential baseline. It shows you exactly why modern operating systems have to be so incredibly smart to balance these trade-offs every single millisecond.

FCFS Algorithm

First-Come, First-Serve

The Problem

The Solution: CPU Scheduling

CPU Scheduling Algorithms

Preemptive Scheduling

Non-Preemptive Scheduling

First Come, First Serve (FCFS)

Scenario 1

Gantt Chart

Turnaround Time (TAT)

Waiting Time (WT)

Average Turnaround Time:

Average Waiting Time:

Makespan:

Throughput:

Scenario 2

Gantt Chart

Average Turnaround Time

Average Waiting Time

Makespan

Throughput

Scenario 3

Gantt Chart

Average Turnaround Time

Average Waiting Time

Makespan

Throughput

Advantages & Disadvantages

Your Results:

Conclusion

You've Mastered FCFS! 🏆