Scheduling Algorithms - Shortest Job First (SJF)

Summary

This lecture delves into the Shortest Job First (SJF) scheduling algorithm, contrasting it with the First Come First Serve (FCFS) method. It explains SJF's advantage in reducing average waiting time by prioritizing processes with shorter CPU bursts, highlighting both non-preemptive and preemptive versions. The non-preemptive version is straightforward, while the preemptive version, also known as Shortest Remaining Time First, can be complex due to preemption. The script also addresses the challenge of predicting CPU burst lengths and suggests approximation as a practical solution, concluding that despite its drawbacks, SJF is generally more efficient than FCFS.

Takeaways

• 😀 The Shortest Job First (SJF) scheduling algorithm prioritizes processes based on the length of their next CPU burst, allocating the CPU to the process with the shortest burst time first.
• 📚 SJF is an improvement over the First Come First Serve (FCFS) algorithm, as it considers the process length, aiming to reduce the average waiting time for processes.
• 🔄 SJF can be implemented in both preemptive and non-preemptive ways. In the preemptive version, a process can be interrupted and the CPU can be given to another process with a shorter remaining burst time.
• 📉 In non-preemptive SJF, once a process starts executing, it is not interrupted, leading to a Gantt chart where processes execute in the order of their shortest burst times without interruption.
• 📈 The average waiting time for processes under SJF is calculated by considering the time a process spends waiting for the CPU, taking into account the execution time already completed and the arrival time.
• 🔄 For preemptive SJF, the waiting time is calculated by subtracting the time a process has already executed and its arrival time from the total waiting time observed on the Gantt chart.
• 📝 The main challenge with SJF is predicting the exact length of the next CPU burst for a process, which is essential for the algorithm to work effectively.
• 🔮 To overcome the unpredictability of CPU burst times, an approximation can be used, assuming that the next CPU burst will be similar in length to the previous one.
• 🚀 Despite its challenges, SJF can still be applied by using the approximation method to predict CPU burst times, allowing for a more efficient process scheduling compared to FCFS.
• 📚 The script provides a detailed example of how to calculate waiting times and average waiting time for both preemptive and non-preemptive SJF, illustrating the algorithm's application and comparison with FCFS.
• 👍 The lecture concludes that while SJF has its disadvantages, particularly in knowing the length of the next CPU request, it offers a more efficient scheduling approach than FCFS when implemented correctly.

Q & A

What is the Shortest Job First (SJF) scheduling algorithm?

The Shortest Job First (SJF) scheduling algorithm is a CPU scheduling method where the process that requires the least amount of CPU time for its next burst is given priority to execute. It is designed to minimize the average waiting time for processes.

How does SJF differ from First Come First Serve (FCFS) in terms of process selection?

In FCFS, the process that arrives first is the first to get the CPU, regardless of the length of its execution time. In contrast, SJF selects the process with the shortest next CPU burst time, which means the length of the process matters in SJF but not in FCFS.

What is the concept of CPU burst time in the context of SJF scheduling?

CPU burst time in SJF refers to the amount of time a process is expected to use the CPU for its next execution. SJF scheduling focuses on the next CPU burst time rather than the entire length of the process.

Can SJF scheduling be both preemptive and non-preemptive?

Yes, SJF scheduling can be implemented as either preemptive or non-preemptive. In a non-preemptive SJF, once a process starts executing, it cannot be interrupted until it completes or goes to a waiting state. In a preemptive SJF, a process can be interrupted and the CPU can be given to another process with a shorter remaining burst time.

How is the tie broken in SJF scheduling when two processes have the same CPU burst time?

If two processes have the same CPU burst time, the First Come First Serve (FCFS) principle is used to break the tie, meaning the process that arrived first will be scheduled to execute first.

What is the main advantage of using SJF over FCFS in terms of average waiting time?

SJF generally results in a lower average waiting time compared to FCFS because it prioritizes shorter processes, reducing their waiting times and consequently the overall average waiting time for all processes.

What is the formula used to calculate the waiting time for processes in a preemptive SJF scheduling?

The waiting time for a process in a preemptive SJF scheduling is calculated as the total waiting time minus the number of milliseconds the process executed, minus the arrival time of the process.

Why is it difficult to implement SJF scheduling at the level of short-term CPU scheduling?

It is difficult to implement SJF at the short-term CPU scheduling level because it requires knowledge of the length of the next CPU burst for each process, which is often hard to predict accurately.

How can SJF scheduling be approximated in practice when the exact next CPU burst time is unknown?

SJF scheduling can be approximated by predicting the next CPU burst time based on the assumption that it will be similar in length to previous bursts. This allows for an estimation to be made and the process with the shortest predicted CPU burst to be selected.

What is another name for the preemptive version of the SJF scheduling algorithm?

The preemptive version of the SJF scheduling algorithm is also known as the Shortest Remaining Time First (SRTF) scheduling.