CPU Scheduling Basics

Summary

This video explores how operating systems schedule tasks using different time scales and algorithms. It discusses the long-term, mid-term, and short-term schedulers, focusing on the short-term scheduler's role in efficiently executing tasks. The video introduces scheduling concepts like CPU-bound vs. I/O-bound processes, cooperative vs. pre-emptive schedulers, and quantum. It also covers scheduling algorithms like First-Come, First-Serve (FCFS), Round-Robin, and Shortest Job First (SJF), highlighting their advantages and disadvantages.

Takeaways

• 😀 Operating systems manage task scheduling on different time scales, including long-term, mid-term, and short-term schedulers.
• 🔄 The long-term scheduler decides which processes are allowed to run on the system, potentially with admission control.
• 🔄 The mid-term scheduler determines how many processes are actively using the CPU and memory, deciding which to keep in memory and which to page out to disk.
• 🏃 The short-term scheduler focuses on executing tasks efficiently to minimize CPU idleness.
• 🤔 The script introduces the concept of CPU-bound and I/O-bound processes, which are differentiated by their usage patterns of CPU and I/O operations.
• 👥 Schedulers can be either cooperative, where tasks run to completion, or pre-emptive, which can interrupt tasks after a set time quantum.
• 🕰 The choice of time quantum in pre-emptive scheduling affects system responsiveness and efficiency.
• 🤖 Modern CPUs and operating systems often use pre-emptive scheduling due to the unpredictability of downloaded software.
• 📝 Good schedulers should be fair, efficient, responsive, and avoid starvation for low-priority tasks.
• 🛠️ The script discusses three basic scheduling algorithms: First-Come, First-Serve (FCFS), Round-Robin, and Shortest Job First (SJF).
• 📊 SJF, when pre-emptive, can significantly reduce average response and wait times, especially beneficial for interactive programs.
• 🔄 SJF can lead to starvation of longer tasks if not managed properly, and its effectiveness depends on the accuracy of predicting task lengths.

Q & A

What are the three time scales on which modern operating systems schedule tasks?

Modern operating systems schedule tasks on three different time scales: the long-term scheduler, the mid-term scheduler, and the short-term scheduler.

What is the primary function of the long-term scheduler in an operating system?

The long-term scheduler decides which processes are going to run on the system. It may allow any processes to run or have admission control systems that make decisions about when to accept a task.

What does the mid-term scheduler in an operating system control?

The mid-term scheduler decides how many processes are going to be running on the CPU actively at any given time, aiming to use the CPU and memory as efficiently as possible.

What is the role of the short-term scheduler in operating system task scheduling?

The short-term scheduler's job is to execute tasks as efficiently as possible, ideally ensuring that the CPU is not idle if there is work to be done.

What are the two main types of schedulers mentioned in the script, and how do they differ?

The two main types of schedulers mentioned are cooperative schedulers and pre-emptive schedulers. Cooperative schedulers run tasks to completion, while pre-emptive schedulers set a timer and interrupt a process after a certain amount of time, giving them the choice to run the same or a different job.

What is the significance of the 'quantum' in the context of pre-emptive scheduling?

The quantum is the amount of time a process is allowed to run before the scheduler interrupts it. It affects the responsiveness and efficiency of the system, as a very long quantum can make the system less responsive, while a very short quantum can lead to excessive switching overhead.

What are the three scheduling algorithms discussed in the script, and how do they differ?

The three scheduling algorithms discussed are first-come, first-serve (FIFO), round-robin, and shortest job first (SJF). FIFO executes tasks in the order they arrive, round-robin runs each job for one quantum before switching to the next, and SJF selects the job with the least amount of remaining time to execute first.

Why might a first-come, first-serve (FIFO) scheduling algorithm result in high wait times for some tasks?

In a FIFO scheduling system, tasks are executed in the order they arrive. If a long task arrives first, subsequent tasks must wait for it to complete, leading to potentially high wait times for those tasks.

What is the potential problem with the shortest job first (SJF) scheduling algorithm?

The potential problem with SJF is that it can lead to starvation for longer jobs. If short jobs keep arriving, they will keep interrupting longer jobs, potentially delaying them indefinitely.

How can operating systems prevent starvation in scheduling algorithms like SJF?

Operating systems can prevent starvation by aging priorities, where the longer a task waits, the higher its priority becomes, or by using randomization, where tasks are selected based on assigned probabilities that ensure lower priority tasks eventually run.

What is the concept of 'nice values' in Linux, and how do they affect process scheduling?

In Linux, 'nice values' determine the priority of a process, with a higher nice value indicating lower priority. Processes with a lower nice value (e.g., -20) are given more CPU time, while those with a higher nice value (e.g., 19) are given less CPU time.