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Disadvantage: Convoy effect 

 

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.