Convoy Effect in Operating Systems

The Convoy Effect is a phenomenon associated with the First Come First Serve (FCFS) algorithm, in which the whole Operating System slows down due to a few slow processes.

What is the Convoy Effect?

FCFS algorithm is non-preemptive in nature, that is, once CPU time has been allocated to a process, other processes can get CPU time only after the current process has finished. This property of FCFS scheduling leads to the situation called Convoy Effect. Suppose there is one CPU-intensive (large burst time) process in the ready queue, and several other processes with relatively fewer burst times but are Input/Output (I/O) bound (Need I/O operations frequently).

👁 Convoy Effect

Steps Involved in Convoy Effect in Operating Systems

Steps are as following below:

• The I/O bound processes are first allocated CPU time. As they are less CPU intensive, they quickly get executed and goto I/O queues.
• Now, the CPU intensive process is allocated CPU time. As its burst time is high, it takes time to complete.
• While the CPU intensive process is being executed, the I/O bound processes complete their I/O operations and are moved back to ready queue.
• However, the I/O bound processes are made to wait as the CPU intensive process still hasn't finished. This leads to I/O devices being idle.
• When the CPU intensive process gets over, it is sent to the I/O queue so that it can access an I/O device.
• Meanwhile, the I/O bound processes get their required CPU time and move back to I/O queue.
• However in Convoy Effect, they are made to wait because the CPU intensive process is still accessing an I/O device. As a result, the CPU is sitting idle now.

Measures to Avoid the Convoy Effect in Operating System

Hence in Convoy Effect, one slow process slows down the performance of the entire set of processes, and leads to wastage of CPU time and other devices. To avoid Convoy Effect, preemptive scheduling algorithms like Round Robin Scheduling can be used - as the smaller processes don't have to wait much for CPU time - making their execution faster and leading to less resources sitting idle.

Examples of Convoy Effect in Operating Systems

Consider there are three processes, P1, P2, and P3. The burst time of process P3 is the highest.

The following formulas in the table are used to compute the waiting time and turnaround time:

Waiting Time = Turn Around Time - Burst Time

Turn Around Time = Completion Time - Arrival Time

Process P3 is the first in the queue in the first scenario, yet it has the longest burst time of all the processes. Because we are using the FCFS scheduling technique, Process P3 will be carried out by the CPU first.

The CPU's average waiting time will be rather long according to this schedule. The convoy effect is the cause of this. P1 and P2, the other processes, will have to wait 46 units of time for their turn. Whatever the case their burst time is very low. Now this schedule suffers from starvation.

Process ID	Arrival Time	Brust Time	Completion Time	Turn Around Time	Waiting Time
P1	1	1	42	41	40
P2	1	3	45	44	41
P3	0	42	46	46	4

Avg waiting Time of processes = 85/3.

In the second example, the starving issue would not have arisen if Process P1 had arrived at the end of the queue and the other processes P2 and P3 had finished before then.

Let's look at one more example to see how the waiting periods in the two circumstances differ from one another. Even though the schedule has the same length (45 units), there will be less waiting time with this one.

Process ID	Arrival Time	Brust Time	Completion Time	Turn Around Time	Waiting Time
P1	0	2	4	2	0
P2	0	1	3	3	2
P3	1	42	45	44	2

Avg Waiting Time = 4/3