The convoy effect is a phenomenon that can occur in scheduling algorithms that use the shortest-job-first (SJF) approach. It refers to a situation in which a number of short jobs are followed by a long job, and the long job causes all of the short jobs to be delayed. This can happen because the long job occupies the CPU for a long time, preventing the short jobs from being executed including the shortest job first (SJF) algorithm. The convoy effect occurs when a long running process blocks a number of shorter processes from being executed, causing them to wait in a "convoy" behind the longer process. This can result in poorer performance for the shorter processes, as they have to wait for the longer process to complete before they can be executed.
It is correct that the shortest job first (SJF) scheduling algorithm does not typically suffer from the convoy effect. The convoy effect, also known as the convoy phenomenon, is a situation in which a group of processes or jobs with similar execution times are all scheduled to run consecutively, resulting in poor overall performance. This effect is most commonly associated with first-come, first-served (FCFS) scheduling, where the first job to arrive at the ready queue is the first one to be executed.
SJF, on the other hand, is a scheduling algorithm that selects the job with the shortest execution time for execution, regardless of when it arrived in the ready queue. This means that, in SJF, the order in which jobs arrive does not affect their execution order, and thus the convoy effect is not an issue.
It is worth noting, however, that the SJF algorithm can only work effectively if the execution times of the jobs are known in advance. In cases where this information is not available, the algorithm may not perform as well as other scheduling algorithms. Additionally, the SJF algorithm can be subject to the so-called "starvation" effect, where long-running jobs are continually preempted by shorter jobs and are never able to complete. This can be mitigated by using a variant of the SJF algorithm called "shortest remaining time first" (SRTF), which considers the remaining execution time of each job rather than its total execution time.