Operating Systems: Multiple-Processor Scheduling

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Questions and Answers

What is the primary characteristic that differentiates Multilevel Queue (MLQ) from Multilevel Feedback Queue (MLFQ)?

MLFQ requires all jobs to complete in the first queue.

MLQ executes all jobs only in one pass.

MLQ uses only one scheduling algorithm.

MLFQ allows processes to move between queues. (correct)

Which of the following statements regarding process scheduling in MLFQ is accurate?

A process must complete its execution in the first queue.

A process can be preempted and moved to a lower-priority queue. (correct)

Processes cannot upgrade to a higher priority queue.

All queues in MLFQ use the same time quantum.

In the MLFQ example provided, what time quantum is allocated to Queue 0?

8 milliseconds (correct)

Not specified

16 milliseconds

4 milliseconds

Which scheduling algorithm is used in Queue 2 of the MLFQ example?

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

What parameter is NOT typically defined in a multilevel-feedback-queue scheduler?

Method used for executing system calls Signup and view all the answers

What happens to a job in Queue 0 if it does not complete within its time slice?

It moves to Queue 1. Signup and view all the answers

In the given example of MLFQ, what is the time quantum for Queue 1?

16 milliseconds Signup and view all the answers

Which of the following correctly describes the behavior of MLFQ in terms of process management?

Processes may be aged to prevent starvation. Signup and view all the answers

What happens to a process that completes 4 units of execution time but has remaining burst time?

Its priority is reduced and it is shifted to Queue 2. Signup and view all the answers

In Queue 2, what is the time quantum used for process scheduling?

8 units Signup and view all the answers

What is a characteristic of symmetric multiprocessing (SMP)?

All processes are in a common ready queue. Signup and view all the answers

What method does pull migration employ in load balancing?

An idle processor requests a waiting task from a busy processor. Signup and view all the answers

What scheduling algorithm is used in the last queue of a multi-level feedback queue (MLFQ)?

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

In a multilevel queue scheduling system, which of the following statements is true regarding scheduling between queues?

Fixed priority scheduling can lead to starvation in lower queues. Signup and view all the answers

In what situation can a lower priority process be executed in a multi-level feedback queue?

When higher priority queues are empty. Signup and view all the answers

What is one of the changes introduced in Linux scheduling from version 2.5?

Constant order O(1) scheduling time. Signup and view all the answers

Which queue scheduling algorithm is typically used for foreground processes?

Round Robin (RR) Signup and view all the answers

What happens to a thread in Windows when it uses its time slice without being blocked or preempted?

It goes back to the end of its queue. Signup and view all the answers

What is the purpose of load balancing in symmetric multiprocessing?

To keep workload evenly distributed across all CPUs. Signup and view all the answers

Which type of thread can preempt a non-real-time thread in Windows scheduling?

Real-time threads Signup and view all the answers

What enhancement was introduced in Windows 7 regarding thread management?

User-mode scheduling (UMS) Signup and view all the answers

Which scenario exemplifies the concept of processor sets?

A group of processors dedicated to a specific set of tasks. Signup and view all the answers

In multilevel queue scheduling, which algorithm is typically assigned to background processes?

First-Come, First-Served (FCFS) Signup and view all the answers

What does processor affinity imply in a multiprocessor system?

A process prefers to run on the processor it is currently executing on. Signup and view all the answers

Study Notes

Multiple-Processor Scheduling

CPU scheduling is more complex when multiple CPUs are available.
Symmetric multiprocessing (SMP) is the most common, where each processor has its own private queue of ready processes.
Processor affinity refers to a process's preference for the processor it's currently running on.
- Soft affinity allows the process to migrate to other processors.
- Hard affinity prevents migration.

Multiple-Processor Scheduling - Load Balancing

Load balancing aims to distribute workload evenly across CPUs for efficiency.
Push migration involves a periodic task checking each processor's load and moving tasks from overloaded CPUs to others.
Pull migration involves idle processors pulling tasks from busy processors.

Multilevel Queue Scheduling

The ready queue is divided into separate queues, each with its own scheduling algorithm.
Common examples include foreground (interactive) and background (batch) queues.
Scheduling methods between queues:
- Fixed-priority scheduling: Processes are served from higher priority queues to lower priority queues.
- Time slice: Each queue receives a specific amount of CPU time to schedule its processes.

Multilevel Queue (MLQ)

Processes are placed in different queues based on their priority.
Higher priority processes are usually in the top-level queue.
Processes in lower-level queues are scheduled after the higher priority queues are completed.

Multilevel Feedback Queue (MLFQ)

Processes can move between queues.
MLFQ aims to separate processes by their CPU burst characteristics.
A process can move to a lower priority queue if it uses too much CPU time.
MLFQ dynamically analyzes process behavior and adjusts their priorities.

Example of Multilevel Feedback Queue

Three queues:
- Q0: Round Robin (RR) with 8-millisecond time quantum.
- Q1: RR with 16-millisecond time quantum.
- Q2: First Come, First Served (FCFS).
Scheduling:
- A newly arriving process enters Q0 and is served in FCFS.
- If a process doesn't complete within its time slice, it's moved to the next lower priority queue.

Linux Scheduling Through Version 2.5

Earlier versions used variations of the standard UNIX scheduling algorithm.
Version 2.5 introduced constant-order O(1) scheduling time for better performance:
- Preemptive, priority-based.
- Two priority ranges: time-sharing and real-time.
- Higher priority tasks have larger time slices.
- Run-able tasks are tracked in per-CPU run queue data structures.

Windows Scheduling

Employs priority-based preemptive scheduling.
The dispatcher acts as the scheduler.
Threads run until they:
- Block.
- Use their entire time slice.
- Are preempted by a higher-priority thread.
Real-time threads can preempt non-real-time threads.

Windows Priority Classes

Windows has a 32-level priority scheme:
- Variable class: 1-15.
- Real-time class: 16-31.
Priority 0 belongs to the memory-management thread.
A queue exists for each priority level.
When no runnable thread is available, the idle thread runs.
Priority boosting is applied when a wait occurs, based on the reason for the wait.
Foreground windows receive a 3x priority boost.
Windows 7 introduced User-Mode Scheduling (UMS) for increased efficiency.

Windows Priority Classes (Continued)

UMS allows applications to create and manage threads independently of the kernel.
UMS schedulers are often provided by programming language libraries, such as the C++ Concurrent Runtime (ConcRT) framework.

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Description

This quiz covers key concepts in multiple-processor scheduling within operating systems. You'll explore CPU scheduling complexities, load balancing strategies, and the nuances of multilevel queue scheduling techniques. Prepare to test your understanding of symmetric multiprocessing and processor affinity.