Operating Systems Chapter 6: CPU Scheduling

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Questions and Answers

What is the primary purpose of CPU scheduling in multiprogrammed operating systems?

To maximize CPU utilization (correct)

To reduce memory usage

To increase power consumption

To manage I/O devices

Which of the following best describes a CPU–I/O burst cycle?

An error state of the CPU when waiting for I/O

A continuous process execution without interruptions

A repetitive pattern of computing followed by input/output operations (correct)

A method of enhancing CPU speed beyond limits

What triggers CPU scheduling decisions within an operating system?

Switching between CPU and memory states

Switching from running to waiting state (correct)

Switching between user and kernel modes

Switching from idle to active state

What is primarily evaluated when selecting a CPU scheduling algorithm?

The average response time for processes Signup and view all the answers

Which of the following is NOT a state change that leads to CPU scheduling decisions?

Switch from ready to blocked state Signup and view all the answers

Which of the following scheduling types is nonpreemptive?

First-Come, First-Served Signup and view all the answers

What does the dispatcher module NOT do?

Allocate system resources to a process Signup and view all the answers

Which scheduling criterion focuses on keeping the CPU as busy as possible?

CPU utilization Signup and view all the answers

What is one of the drawbacks of First-Come, First-Served (FCFS) scheduling?

It can lead to increased average waiting time due to long processes. Signup and view all the answers

In Shortest-Job-First (SJF) scheduling, what is a major challenge?

Determining the length of the next CPU burst accurately. Signup and view all the answers

What is the average waiting time when processes P1, P2, and P3 arrive in that order with burst times 24, 3, and 3, respectively?

17 Signup and view all the answers

Which of the following is NOT a goal of scheduling algorithm optimization criteria?

Maximizing turnaround time Signup and view all the answers

What average waiting time does the SJF scheduling produce with the processes P1, P2, P3, and P4 having arrival times 0.0, 2.0, 4.0, and 5.0 with burst times 6, 8, 7, and 3 respectively?

7 Signup and view all the answers

What is the effect called when short processes get delayed by long processes in FCFS scheduling?

Convoy effect Signup and view all the answers

Which of the following best describes the role of the dispatcher in process scheduling?

It transfers control to the selected process and manages context switching. Signup and view all the answers

What effect does setting alpha ($\alpha$) to 0 have on the prediction of the next CPU burst?

The predicted value remains constant. Signup and view all the answers

In priority scheduling, what happens to a low priority process over time?

Its priority may increase due to aging. Signup and view all the answers

What does SJF stand for in the context of CPU scheduling?

Shortest Job First Signup and view all the answers

What is the main characteristic of preemptive scheduling?

Higher priority processes can interrupt currently running processes. Signup and view all the answers

Given the following processes: P1 with priority 3, P2 with priority 1, P3 with priority 4, P4 with priority 5, P5 with priority 2, which process gets executed first?

P2 Signup and view all the answers

How is the average waiting time calculated in the example of preemptive SJF?

By averaging the time that each process waits in the queue. Signup and view all the answers

What is the typical time quantum range for Round Robin scheduling?

10-100 milliseconds Signup and view all the answers

What is the potential problem associated with priority scheduling?

Starvation of low priority processes. Signup and view all the answers

What happens when $\alpha$ is set to 1 for predicting the next CPU burst?

Only the last CPU burst is emphasized. Signup and view all the answers

What happens to a process after its time quantum elapses in a Round Robin scheduling?

It is added to the end of the ready queue. Signup and view all the answers

In a Round Robin schedule, if there are n processes and the time quantum is q, what is the maximum time a process can wait before it executes again?

(n-1)q time units Signup and view all the answers

What is a potential downside of having a small time quantum in Round Robin scheduling?

Increased average turnaround time Signup and view all the answers

What is the typical range for time quantum in a Round Robin scheduling algorithm?

10ms to 100ms Signup and view all the answers

In a Multilevel Queue scheduling system, how is the ready queue organized?

Partitioned into separate queues based on process priority Signup and view all the answers

Which scheduling algorithms are typically associated with the foreground and background queues in a Multilevel Queue system?

Foreground - Round Robin, Background - FCFS Signup and view all the answers

What is meant by context switch time in the context of Round Robin scheduling?

The time taken to switch the CPU from one process to another Signup and view all the answers

For effective operation of Round Robin scheduling, what should the burst times of CPU bursts be in relation to the time quantum?

80% of CPU bursts should be shorter than q Signup and view all the answers

What is the time quantum for queue Q0 in a multilevel feedback queue?

8 milliseconds Signup and view all the answers

In a multilevel feedback queue, what happens to a job that does not complete in Q0 after 8 milliseconds?

It moves to queue Q1 Signup and view all the answers

Which scheduling algorithm is typically used in the background queue of a multilevel queue scheduling system?

First-Come, First-Served Signup and view all the answers

What types of threads can be scheduled in a many-to-many threading model?

Both user-level and kernel-level threads Signup and view all the answers

What does the PTHREAD_SCOPE_PROCESS setting specify during thread creation?

Scheduling competition within the process Signup and view all the answers

In symmetric multiprocessing (SMP), which statement is true regarding how processes are scheduled?

Each processor has its own scheduling queue Signup and view all the answers

What is a characteristic of processor affinity?

It designates specific CPUs for process execution Signup and view all the answers

Which method in load balancing pushes tasks from an overloaded CPU?

Push migration Signup and view all the answers

Which statement is true regarding multithreaded multicore systems?

They often use multiple threads per core Signup and view all the answers

When upgrading a process in a multilevel feedback queue system, what factor can be implemented?

Time spent in a low-priority queue Signup and view all the answers

What does aging in a multilevel feedback queue help manage?

It ensures long-waiting processes receive attention Signup and view all the answers

What library function is used to set the scheduling scope in Pthread?

pthread_attr_setscope Signup and view all the answers

In which scenario would many-to-one thread models be most applicable?

In cases of single-threaded processes Signup and view all the answers

Study Notes

Basic Concepts

CPU scheduling is essential for multiprogrammed operating systems to manage process execution efficiently.
Processes alternate between CPU execution and I/O wait, forming a CPU-I/O burst cycle.
It’s crucial to analyze the distribution of CPU burst times for effective scheduling.

CPU Scheduler

Short-term scheduler allocates CPU to processes in the ready queue based on different criteria.
Scheduling decisions occur during state transitions: running to waiting, running to ready, waiting to ready, or when a process terminates.
Nonpreemptive scheduling occurs when processes terminate or switch from running to waiting, while preemptive scheduling occurs otherwise.

Dispatcher

The dispatcher transfers CPU control to the selected process, involving context switching and user mode transitions.
Dispatch latency refers to the time taken for the dispatcher to stop one process and start another.

Scheduling Criteria

CPU Utilization: Aims to keep the CPU as occupied as possible.
Throughput: Measures the number of processes completed in a time unit.
Turnaround Time: Total time taken to execute a process.
Waiting Time: Duration a process remains in the ready queue.
Response Time: Time from request submission to the first response production.

Scheduling Algorithm Optimization Criteria

Maximize CPU utilization and throughput, while minimizing turnaround, waiting, and response times.

FCFS Scheduling

Processes are served in the order of their arrival, leading to potentially high average waiting times due to the convoy effect with CPU-bound processes.

Shortest-Job-First (SJF) Scheduling

SJF schedules processes based on the length of their next CPU burst, minimizing average waiting time when the next burst is known or estimated.
Preemptive SJF is termed shortest-remaining-time-first, allowing ongoing processes to be preempted by shorter ones.

Priority Scheduling

Processes are assigned priority levels, allowing higher priority (smaller integer) processes to preempt lower priority processes.
A potential problem of starvation for low-priority processes can be mitigated through aging, which increases the priority of long-waiting processes.

Round Robin (RR)

Each process receives a fixed time quantum (q), cycling through the ready queue for better responsiveness but potentially higher turnaround times.
Ideal quantum size should be larger than the context switch time to avoid excessive overhead.

Multilevel Queue Scheduling

The ready queue is divided into multiple queues, each with its own scheduling algorithm, such as RR for foreground and FCFS for background processes.
Scheduling types can include fixed priority allocation or time-slice distribution between queues.

Multilevel Feedback Queue

Processes can transition between queues based on their execution behavior, incorporating aging to adjust process priorities dynamically.
Defined through parameters such as number of queues, scheduling algorithms, and promotion/demotion criteria.

Thread Scheduling

Distinction between user-level and kernel-level threads, utilizing many-to-one or many-to-many models.
Thread scheduling can be based on process-contention scope (PCS) or system-contention scope (SCS).

Multiple-Processor Scheduling

Requires more complex scheduling as processes may run on multiple homogeneous processors.
In asymmetric multiprocessing, one processor manages system data, while symmetric multiprocessing (SMP) allows all processors to schedule independently.
Processor affinity, either soft or hard, refers to a process's tendency to run on the same processor.

Load Balancing in Multiprocessors

Load balancing is crucial in SMP environments for efficiency, utilizing push migration to redistribute tasks from overloaded CPUs, or pull migration where idle CPUs fetch tasks from busy ones.

Multicore Processors

Trend towards integrating multiple processor cores in a single chip for improved performance and reduced power consumption.
Multithreading capability per core is on the rise to enhance throughput during memory operations.

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Description

This quiz covers Chapter 6 on CPU Scheduling from Operating Systems. It includes topics such as scheduling criteria, algorithms, thread scheduling, and real-time CPU scheduling. Test your understanding of various CPU-scheduling concepts and their evaluation criteria.