Scheduling Criteria

Summary

This lecture delves into the critical aspects of scheduling criteria for evaluating the efficiency of different scheduling algorithms in operating systems. It introduces five key metrics: CPU utilization, throughput, turnaround time, waiting time, and response time. The presentation explains each criterion's significance, emphasizing the importance of maximizing CPU usage, minimizing process waiting times, and considering the time to first response in interactive systems. The goal is to provide a clear understanding of how these criteria can guide the choice and assessment of scheduling algorithms.

Takeaways

• 📊 CPU Utilization: The goal is to keep the CPU as busy as possible, with utilization ideally ranging from 40% in lightly loaded systems to 90% in heavily used systems.
• 🔄 Throughput: Measures the amount of work done by the CPU, defined as the number of processes completed per unit time.
• ⏱️ Turnaround Time: Considered from the perspective of a single process, it's the total time from submission to completion, including waiting and execution times.
• 🕒 Waiting Time: Refers to the total time a process spends waiting in the ready queue, influenced by the efficiency of the CPU scheduling algorithm.
• 📈 Response Time: Important in interactive systems, it's the time from request submission to the first response, not the time to complete the task or output.
• 🔍 Efficiency Evaluation: Scheduling algorithms are compared based on criteria such as CPU utilization, throughput, turnaround time, waiting time, and response time.
• 🔄 Work Measurement: Work done by the CPU is measured not only by how busy it is but also by the number of processes it completes within a given time frame.
• 🔄 Process States: Turnaround time includes all states a process goes through, such as ready, waiting, and execution.
• 👨‍💻 Scheduler's Role: The scheduler's role is to assign the CPU to processes efficiently, aiming to minimize the waiting time in the ready queue.
• 🖥️ System Performance: In interactive systems, response time can be a better measure of efficiency than turnaround time, especially when processes can output results quickly while continuing to compute.
• 🔄 Output Speed: The turnaround time can be affected by the speed of the output device, which may not always reflect the true efficiency of the scheduling algorithm.

Q & A

What are the five scheduling criteria discussed in the lecture?

The five scheduling criteria discussed are CPU utilization, throughput, turnaround time, waiting time, and response time.

What does CPU utilization represent in terms of scheduling?

CPU utilization represents how much the CPU is being used or how busy it is with executing processes, ideally aiming to keep it as busy as possible to maximize efficiency.

What is the range of CPU utilization in a real system?

In a real system, CPU utilization should range from 40% for a lightly loaded system to 90% for a heavily used system.

How is throughput defined in the context of CPU scheduling?

Throughput is defined as the number of processes completed per unit time, serving as a measure of the work done by the CPU.

What does turnaround time measure in terms of a process?

Turnaround time measures the interval from the time a process is submitted until it is completed, including all states the process goes through, such as waiting, ready, and execution.

How does waiting time differ from other scheduling criteria?

Waiting time specifically refers to the sum of periods a process spends waiting in the ready queue, and it is the aspect that CPU scheduling algorithms can most directly influence.

Why is response time a relevant criterion in interactive systems?

Response time is relevant in interactive systems because it measures the time from the submission of a request until the first response is produced, which can be a better indicator of efficiency than turnaround time in fast systems where output speed may not be the limiting factor.

What is the difference between turnaround time and response time?

Turnaround time is the total time from process submission to completion, including all waiting and execution times. Response time, on the other hand, is the time from request submission until the first output is produced, focusing on the initial response rather than the completion of the process.

How can a scheduling algorithm affect the waiting time of a process?

A scheduling algorithm can affect the waiting time of a process by determining how quickly and frequently the CPU is assigned to the process. An inefficient algorithm may cause a process to wait longer in the ready queue, increasing its waiting time.

What is the significance of keeping the CPU as busy as possible?

Keeping the CPU as busy as possible is significant because it maximizes CPU utilization, ensuring that the CPU is working on executing processes and not remaining idle, which in turn increases the overall efficiency of the system.

Why might a process be in a waiting state according to the lecture?

A process might be in a waiting state because it is waiting for the CPU to become available or waiting for an input/output operation to complete, which are scenarios independent of the CPU execution time.