Operating System Concepts: Multilevel Queue Scheduling

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Questions and Answers

What is the purpose of the target latency in the CFS scheduler?

To determine the priority of a task

To calculate the virtual run time of a task

To determine the number of active tasks

To determine the time interval during which a task should run at least once (correct)

What is the effect of increasing the number of active tasks on the target latency in the CFS scheduler?

Target latency decreases

Target latency becomes zero

Target latency remains the same

Target latency increases (correct)

What is the role of the decay factor in the CFS scheduler?

To prioritize tasks based on their priority

To determine the target latency of a task

To associate a lower priority with a higher decay rate (correct)

To calculate the virtual run time of a task

What is the scheduling algorithm used in Windows?

Priority-based preemptive scheduling Signup and view all the answers

In a symmetric multiprocessing system, how do processors access the system data structures?

Each processor accesses the system data structures Signup and view all the answers

What is the purpose of the priority classes in Windows?

To combine with relative priority to give a numeric priority Signup and view all the answers

What is the effect of waiting on a resource in Windows?

Priority is boosted Signup and view all the answers

In an asymmetric multiprocessing system, which processor accesses the system data structures?

Only one processor accesses the system data structures Signup and view all the answers

What is processor affinity in multiprocessing?

The process has a soft affinity for the processor on which it is currently running Signup and view all the answers

What is the purpose of the dispatch table in Solaris?

To convert class-specific priorities into a global priority Signup and view all the answers

What is the function of the dispatcher module in an operating system?

To give control of the CPU to the process selected by the short-term scheduler Signup and view all the answers

What is the effect of having multiple threads at the same priority in Solaris?

The threads are selected via Round Robin scheduling Signup and view all the answers

What is the purpose of load balancing in multiple-processor scheduling?

To distribute workload evenly among all CPUs Signup and view all the answers

What is the purpose of the global priority in Solaris?

To select the next task to run Signup and view all the answers

What is dispatch latency?

The time it takes for the dispatcher to stop one process and start another Signup and view all the answers

What is push migration in load balancing?

Periodic task checks load on each processor, and if found pushes task from overloaded CPU to other CPUs Signup and view all the answers

What is the advantage of multicore processors?

They are faster and consume less power Signup and view all the answers

What is the criteria for selecting a CPU-scheduling algorithm for an OS?

The algorithm's suitability for the OS's requirements Signup and view all the answers

What is the goal of maximizing CPU utilization?

To keep the CPU as busy as possible Signup and view all the answers

What is the convoy effect in FCFS scheduling?

A short process is delayed by a long process Signup and view all the answers

What is the purpose of thread scheduling in multicore processors?

To utilize multiple threads per core Signup and view all the answers

What is the advantage of using Shortest-Job-First (SJF) scheduling?

It is optimal and gives minimum average waiting time Signup and view all the answers

What is the difference between hard and soft real-time systems?

Hard real-time systems have a guarantee as to when critical real-time process will be scheduled, while soft real-time systems have no guarantee Signup and view all the answers

What is load balancing in multiple-processor scheduling?

To keep all CPUs loaded for efficiency Signup and view all the answers

How does the SJF scheduling algorithm determine the length of the next CPU burst?

By estimating the length based on previous CPU bursts Signup and view all the answers

What is the purpose of pull migration in load balancing?

Idle processors pull waiting tasks from busy processors Signup and view all the answers

What is the main difference between FCFS and SJF scheduling?

FCFS schedules based on arrival time, while SJF schedules based on burst time Signup and view all the answers

In a multiprocessor system, what is the advantage of symmetric multiprocessing?

Each processor accesses the system data structures, alleviating the need for data sharing Signup and view all the answers

What is the purpose of the scheduling criteria?

To optimize system performance Signup and view all the answers

What is the goal of minimizing waiting time in scheduling?

To reduce the time a process has been waiting in the ready queue Signup and view all the answers

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

To select the next process to execute Signup and view all the answers

Study Notes

CFS Scheduler

Target Latency: The CFS scheduler aims to maintain a desired average response time for interactive tasks.
Increasing Active Tasks: More active tasks lead to a higher target latency. This is because each task gets less CPU time, resulting in slower responses.
Decay Factor: This adjusts the target latency dynamically. A higher decay factor means quicker adjustments to changes in the number of active tasks.

Windows Scheduling

Scheduling Algorithm: Windows uses a priority-based preemptive scheduling algorithm.
Priority Classes: They group processes into categories, allowing system-critical tasks to run first, followed by user-critical tasks, and then background processes.
Waiting on Resources: Waiting on a resource (like a file lock) suspends the process. When the resource becomes available, the process is moved back to the ready queue.

Multiprocessing Systems

Symmetric Multiprocessing (SMP): All processors can access system data structures directly. This allows for efficient resource sharing and parallel execution.
Asymmetric Multiprocessing (AMP): One processor (typically the master) manages all system data structures. The other processors (slaves) depend on the master for data access.
Processor Affinity: Allows a process to be bound to a particular CPU core for optimal performance, especially for processes that are known to have high affinity to a specific core.

Solaris Scheduling

Dispatch Table: Stores information about all runnable processes, facilitating efficient process switching.
Dispatcher Module: Responsible for choosing the next process to run and switching control to it.
Multiple Threads at Same Priority: Solaris uses a fair share policy to ensure that threads of equal priority get a fair share of CPU time.

Multiple-Processor Scheduling

Load Balancing: Distributes processes evenly across available processors, maximizing processor utilization and minimizing response times.
Push Migration: Transferring a process from a heavily loaded processor to a less loaded one, proactively balancing the load.
Global Priority: A single priority value for each process, used to compare its priority with other processes across all processors.

Dispatch Latency

Definition: The time taken to select and switch to the next process for execution.

Multicore Processor Advantages

Increased Performance: Multiple cores offer parallel processing capabilities, leading to faster execution of tasks.
Improved Responsiveness: By distributing tasks among multiple cores, responsiveness can be improved, as other tasks can run while one task is waiting for I/O.

CPU Scheduling Criteria

Maximizing CPU Utilization: This aims to keep the CPU busy as much as possible, reducing idle time.
Minimizing Waiting Time: This focuses on keeping processes waiting for the CPU for as short a time as possible.
Minimizing Response Time: This aims to provide quick responses to interactive tasks, ensuring a pleasant user experience.

Scheduling Algorithms

FCFS (First-Come, First-Served): Processes are served in the order they arrive in the ready queue. It's simple but can lead to long wait times for shorter tasks.
SJF (Shortest-Job-First): Selects the process with the shortest estimated burst time next. It's optimal for minimizing average waiting time but requires accurate burst time estimates.
Convoy Effect (FCFS): A long process can block shorter processes behind it in the FCFS queue, leading to inefficient resource usage.
Thread Scheduling: In multicore systems, separate threads can be assigned to different cores, allowing for concurrent execution and improved performance.

Real-Time Systems

Hard Real-Time: Requires tasks to be completed within strict deadlines, with no tolerance for delays.
Soft Real-Time: Deadlines are important but occasional missed deadlines are tolerated.

Load Balancing (Multiple-Processor Scheduling)

Pull Migration: A process is moved to a less loaded processor when it requests additional resources.

Key Differences

FCFS vs. SJF: FCFS prioritizes arrival order, while SJF prioritizes estimated burst time.

Multiprocessor Systems

Symmetric Multiprocessing: All processors are equal and share the same resources, offering better scalability than AMP systems.

Scheduling Criteria

Minimizing Turnaround Time: This aims to reduce the total time spent by a process in the system, from arrival to completion.
Maximizing Throughput: This aims to run as many processes as possible in a given time period, increasing the overall efficiency of the system.

Short-Term Scheduler

Role: Responsible for selecting the next process to be loaded into memory and executed, making decisions based on scheduling criteria.

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Description

Test your knowledge on multilevel queue scheduling, a CPU scheduling algorithm that allows processes to move between queues. Learn about time slicing, foreground and background processes, and how aging is implemented. Based on Operating System Concepts, 9th Edition by Silberschatz, Galvin, and Gagne.