L-2.3: First Come First Serve(FCFS) CPU Scheduling Algorithm with Example

Main ideas / lessons (FCFS CPU Scheduling)

• FCFS (First Come First Serve) schedules CPU tasks based on arrival time:
  • The process that arrives first gets the CPU first.
• FCFS here is non-preemptive:
  • Once the CPU starts a process, it runs to completion without interruption.
  • Only after it finishes does the scheduler choose the next process.
• A Gantt chart is used to visualize the timeline and compute:
  • Completion time
  • Turnaround time
  • Waiting time
  • Response time

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Numerical example walkthrough (from the subtitles)

Given processes

• Processes: P1, P2, P3, P4
• Arrival times (AT):
  • P1 arrives at 0
  • P2 arrives at 1
  • P3 arrives at 5
  • P4 arrives at 6
• Burst times (BT) / execution times:
  • P1 = 2
  • P2 = 2
  • P3 = 3
  • P4 = 4

Gantt chart execution logic

• Time 0 to 2
  • At time 0, only P1 has arrived → CPU runs P1 for 2 units
  • P1 completes at 2
• Time 2 to 4
  • At time 2, P2 has already arrived and is waiting → run P2 for 2 units
  • P2 completes at 4
• Time 4 to 5 (CPU idle)
  • Next arrivals are:
    • P3 at 5
    • P4 at 6
  • No process available between 4 and 5 → CPU is idle
• Time 5 to 8
  • At time 5, run P3 for 3 units
  • P3 completes at 8
• Time 8 to 12
  • At time 6, P4 arrived earlier, but it waits until the CPU is free
  • Run P4 for 4 units
  • P4 completes at 12

Completion time (CT)

• CT values:
  • P1: 2
  • P2: 4
  • P3: 8
  • P4: 12

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Time calculations (instructions / formulas)

1) Turnaround Time (TAT)

• Formula: TAT = CT − AT
• Calculations:
  • P1: 2 − 0 = 2
  • P2: 4 − 1 = 3
  • P3: 8 − 5 = 3
  • P4: 12 − 6 = 6

2) Waiting Time (WT)

• Formula relationship: Waiting time = Turnaround time − Burst time WT = TAT − BT

• Calculations:

  • P1: 2 − 2 = 0
  • P2: 3 − 2 = 1
  • P3: 3 − 3 = 0
  • P4: 6 − 4 = 2

3) Response Time (RT)

• Formula described: Response time = (time when CPU first starts the process) − arrival time

• Computed first-CPU start times (from the Gantt chart):

  • P1 first runs at 0 → 0 − 0 = 0
  • P2 first runs at 2 → 2 − 1 = 1
  • P3 first runs at 5 → 5 − 5 = 0
  • P4 first runs at 8 → 8 − 6 = 2
• Key note stated:
  • In non-preemptive FCFS, response time ends up equivalent to waiting time, so response time is not especially emphasized (more relevant in preemptive/round-robin cases).

4) Average Turnaround Time

• Formula: average TAT = (sum of TAT) / number of processes
• Subtitles indicate:
  • Total TAT = 14
  • Number of processes = 4
  • Average TAT = 14/4 = 3.5

5) Average Waiting Time

• Formula: average WT = (sum of WT) / number of processes
• Subtitles indicate:
  • Average waiting time result = 0.75
  • (Implied sum of WT = 0.75 × 4 = 3)

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Speakers / sources featured

• No individual speaker name is provided in the subtitles.
• Source is an educational video/channel: Gate smashers (referenced by the narrator).