The Convoy Effect in FCFS Scheduling: Understanding and Mitigating Performance Bottlenecks

Learn about the convoy effect, a significant performance issue in First-Come, First-Served (FCFS) scheduling. This tutorial explains how long-running processes can disproportionately delay shorter processes, leading to increased average waiting times and potential system inefficiencies.

Convoy Effect in FCFS (First-Come, First-Served) Scheduling

Understanding the Convoy Effect

The convoy effect is a phenomenon that can occur in First-Come, First-Served (FCFS) scheduling, where a long-running process at the front of the ready queue significantly delays shorter processes behind it. It's similar to a real-world convoy—a long line of vehicles can delay faster vehicles behind it. In this case, the long process is analogous to a long vehicle in a real-world convoy. The other processes are delayed even though they require much less processing time. This can lead to significant increases in average waiting time, a situation often referred to as starvation.

Example Illustrating the Convoy Effect

Let's consider three processes (P1, P2, P3) with the following arrival and burst times:

In this scenario, the long-running process (P1) causes significant delays for P2 and P3, even though they have much shorter burst times. The average waiting time is high (39 + 42) / 3 = 40.33.

If the order of processes were changed (P2, P3 then P1), the average waiting time would be significantly less.

Average waiting time: (0 + 3 + 3) / 3 = 2

This illustrates how the order of arrival in FCFS can drastically impact waiting times, highlighting the convoy effect.