SJF Algorithm Explained with Simple Animations

Welcome to an introduction to CPU scheduling with the Shortest Job First algorithm.CPU scheduling is a crucial part of operating system design, determining how processes share CPU time.Shortest Job First, or SJF, is a scheduling algorithm that prioritizes processes with the shortest execution time.Let's look at some example processes with different burst times. Each block's width represents its execution time.In SJF, these processes are arranged based on their burst times, from shortest to longest.Notice how the processes are reordered - Process 4 with 2 milliseconds goes first, followed by Process 2 with 3 milliseconds, and so on.SJF is a non-preemptive algorithm, meaning once a process starts executing, it runs to completion without interruption.It provides optimal average waiting time when all processes are available at the same time.However, it requires knowing the burst time of each process in advance, which is often impractical in real systems.To understand SJF's benefits, let's compare it with First Come First Serve scheduling.While FCFS executes processes in arrival order, SJF's approach of prioritizing shorter jobs often leads to better overall system performance.Let's compare SJF with First Come First Serve scheduling to understand its advantages.In FCFS, processes are executed in the order they arrive, regardless of their burst time.But in SJF, processes are ordered by their burst time, from shortest to longest.Let's analyze the waiting times for each process in both approaches.In FCFS, the average waiting time is higher because longer processes can delay shorter ones.SJF significantly reduces the average waiting time by prioritizing shorter processes.SJF provides several key benefits for process scheduling.One major challenge with Shortest Job First scheduling is the inability to accurately predict process burst times.In real systems, the CPU can only estimate how long a process will take, and these estimates are often inaccurate.Another significant issue with SJF is the starvation problem. Let's see what happens to a long process when shorter ones keep arriving.As we can see, the longer process keeps getting delayed as shorter processes take priority.These limitations make pure SJF scheduling impractical in real operating systems. Let's look at the specific challenges.These challenges led to the development of more practical variations of SJF scheduling.Let's explore SRTF, the preemptive version of Shortest Job First scheduling.In SRTF, if a new process arrives with a shorter remaining time than the current process, it preempts the CPU.When Process 2 arrives with a shorter burst time, it preempts Process 1, demonstrating the preemptive nature of SRTF.Modern systems use prediction mechanisms to estimate burst times, since exact times are usually unknown.The prediction formula uses weighted averaging of previous and actual burst times.Modern operating systems implement SJF-like features through multilevel feedback queues.Processes move between priority queues based on their behavior and execution time.SJF and its variations are best suited for specific scenarios.They work well in batch processing systems and environments with known burst times.And they're particularly effective in mixed workload environments where process times vary significantly.