Convoy Effect in Scheduling Operating System

Steps are as following below which leads to convoy effect:

• The I/O bound processes are first allocated CPU time ,quickly get executed and goto I/O queues as they are less CPU intensive.
• 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 other CPU intensive process still hasn’t finished. This leads to I/O devices being idle and  hence performing inefficiently.
• When the CPU intensive process gets executed, 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, 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.

Hence in Convoy Effect, one slow process affects and slows down the performance of the other processes, and leads to wastage of CPU time and other resources.

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.