Disadvantage
Disadvantage of FCFS is convoy effect.
Convoy effect is that where larger processes come first and penalize short processes then the total average waiting time is high.
Example: 1 (Convoy effect)
TAT = CT - AT and WT = TAT - BT
Average waiting time is high because of the convoy effect.
Note: If the arrival time of the processes are same (here P2 and P3) then schedule the process which one has the lowest process id.
Example 2: (No convoy effect)
TAT = CT - AT and WT = TAT - BT
Average waiting time is less because larger processes coming late in the ready queue.
Example (FCFS)
Carefully checks Arrival Time of every process before drawing at GANTT Chart.