Convoy Effect in FCFS

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This class will try to understand the Convoy Effect in FCFS.

In our previous class, we already discussed the concept of the FCFS Scheduling algorithm.

Convoy Effect is one of the significant disadvantages of FCFS.

To understand the convoy effect, we will consider a real-world example.

For example, if the chief minister’s convoy passes by, the security will stop all the connecting road traffic.

The convoy will create a delay in our daily routine.

In the same way, if we choose the arrival time of the processes for scheduling, then the average waiting time of the processes will increase.

To understand this, we will consider the following example, as shown below.

In the above example, process P1’s burst time is 50 units in time and processes P2 and P3 have burst times of 1 and 2 units.

So, P2 and P3 processes have to wait for the process P1’s execution to complete.

Observe the waiting times of the processes P2 and P3. Both of them have long waiting times.

So, the average waiting time is (0 + 49 + 50)\2 = 33.

The above image shows us the exact arrival times for processes P2 and P3.

We must access the processes with lesser process numbers in such cases.

So, process P2 will be executed.

We will also consider the below scenario for a better understanding of this concept.

In the above scenario, processes P1 and P2 are accessed first then process P3 with the highest burst time will be executed.

In such cases, the average waiting time is less.

The average waiting time is (2 + 0 + 1)/2 = 1.