Operating System CPU Scheduling

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Questions and Answers

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

Soft real-time systems cannot have periodic tasks.

Hard real-time systems must meet deadlines strictly. (correct)

Soft real-time systems guarantee task service at fixed intervals.

Hard real-time systems prioritize low overhead.

Which type of latency is described as the time from interrupt arrival to the start of the interrupt service routine?

Dispatch latency

Interrupt latency (correct)

Service latency

Context latency

In Rate Monotonic Scheduling, how is priority assigned to tasks?

Based on the amount of resources they require.

Based on their task completion history.

Based on their execution time.

Based on the inverse of their required period. (correct)

Which of the following scenarios can lead to missed deadlines in a task scheduling context?

Tasks that have longer processing times than their deadlines. Signup and view all the answers

What defines the rate of a periodic task in a real-time scheduling system?

1/p Signup and view all the answers

What is the main disadvantage of having a time quantum too small in a scheduling algorithm?

It results in increased context switch overhead. Signup and view all the answers

In a round-robin scheduling algorithm, if there are 4 processes in the ready queue and the time quantum is set to 5 units, how long will a process wait at maximum for CPU time?

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

Which of the following statements about multilevel queue scheduling is true?

Each queue can use a different scheduling algorithm. Signup and view all the answers

What is the recommended percentage of CPU bursts that should be shorter than the time quantum q?

80% Signup and view all the answers

What role does a timer interrupt play in preemptive scheduling?

It invokes the scheduler to change the current running process. Signup and view all the answers

In proportional share scheduling, how are shares allocated to applications?

Applications receive N shares where N is dependent on their priorities. Signup and view all the answers

What occurs during a CPU–I/O burst cycle?

CPU execution is followed by I/O wait. Signup and view all the answers

Which algorithm is typically used for the foreground queue in a multilevel queue scheduling system?

Round Robin (RR) Signup and view all the answers

In non-preemptive scheduling, when can CPU scheduling decisions take place?

When a process switches from running to waiting state. Signup and view all the answers

Which statement correctly describes thread scheduling?

Threads are generally scheduled instead of processes. Signup and view all the answers

Which of the following is true of preemptive scheduling?

Processes can be switched based on timer interrupts. Signup and view all the answers

What role does the dispatcher play in CPU scheduling?

It gives control of the CPU to the selected process. Signup and view all the answers

What does dispatch latency refer to in CPU scheduling?

The time it takes for a dispatcher to switch contexts between processes. Signup and view all the answers

Which of these metrics is targeted for optimization in CPU scheduling?

CPU utilization Signup and view all the answers

Which event can trigger a process to switch from waiting to ready state in preemptive scheduling?

Completion of an I/O operation. Signup and view all the answers

Which of the following scheduling types does NOT involve preemption?

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

What is the main characteristic of process-contention scope (PCS) in thread scheduling?

Competition is limited to threads within a process. Signup and view all the answers

Which PTHREAD scope is specifically used for kernel-managed threads?

PTHREAD_SCOPE_SYSTEM Signup and view all the answers

What limitation is imposed by Linux and Mac OS X regarding thread scheduling scope?

Both systems restrict to PTHREAD_SCOPE_SYSTEM only. Signup and view all the answers

What type of function is 'runner' in the provided code snippet?

A worker function that each thread executes. Signup and view all the answers

What is the purpose of the 'pthread_attr_getscope' function in the code?

To retrieve the scheduling scope of a thread. Signup and view all the answers

In the context of real-time CPU scheduling, what does 'soft real-time' mean?

Real-time processes may be delayed but still receive preference. Signup and view all the answers

What is the purpose of using 'pthread_create' in the provided code?

To create a new thread and specify its execution parameters. Signup and view all the answers

What error is indicated by the message 'Illegal scope value' in the code?

An unsupported value for thread scope was retrieved. Signup and view all the answers

What defines the throughput of a system?

The number of processes that complete execution per time unit Signup and view all the answers

In First-Come, First-Served (FCFS) scheduling, what is the waiting time for P3 when processes arrive in the order P1, P2, P3?

27 Signup and view all the answers

What is a significant disadvantage of FCFS scheduling?

It can lead to the convoy effect. Signup and view all the answers

What does the Shortest-Job-First (SJF) scheduling algorithm rely on to minimize average waiting time?

The burst time of the next CPU request of each process Signup and view all the answers

Under which condition does the SJF scheduling NOT function optimally?

When burst lengths can only be estimated Signup and view all the answers

Which statement about predicting the next CPU burst is TRUE?

Exponential averaging can provide better predictions. Signup and view all the answers

What is the maximum waiting time for process P1 in the priority scheduling example provided?

8.2 Signup and view all the answers

In Round Robin scheduling, what determines how long a process runs before being preempted?

The assigned time quantum Signup and view all the answers

What is the main solution to the problem of starvation in priority scheduling?

Aging of priority levels over time Signup and view all the answers

What is the concept of 'convoy effect' in scheduling?

Long processes delaying the execution of shorter ones. Signup and view all the answers

If the weight $eta$ is set to 1 in exponential averaging, which outcome occurs?

Only the actual length of the last CPU burst counts. Signup and view all the answers

What is indicative of a short burst time in scheduling?

Processes that are I/O-bound Signup and view all the answers

What defines the average waiting time in a scheduling system?

The sum of waiting times of all processes divided by the total number of processes Signup and view all the answers

Study Notes

CPU Utilization

Maximum CPU utilization is achieved through multiprogramming.

CPU-I/O Burst Cycle

Process execution involves a cycle of CPU execution and I/O wait.
CPU burst is followed by an I/O burst.
The distribution of CPU bursts is a key concern in scheduling.

CPU Burst Times Histogram

A histogram illustrates the frequency of CPU burst durations.
The distribution typically shows a higher frequency of shorter bursts.

CPU Scheduler

The short-term scheduler selects a process from the ready queue and allocates the CPU to it.
Scheduling methods include non-preemptive techniques (e.g., FCFS, SJF, priority) and preemptive scheduling (e.g., round robin)
CPU scheduling decisions occur when a process:
- Switches from running to waiting (blocked) state, often due to I/O or resource requests.
- Switches from running to ready state (e.g., timer interrupt).
- Switches from waiting to ready state (e.g., completion of I/O).
- Terminates

Dispatcher

The dispatcher gives control of the CPU to a selected process.
This involves context switching, switching to user mode, and jumping to the correct location in the user program.
Dispatch latency is the time it takes the dispatcher to pause one process and start another.

Scheduling Criteria

CPU utilization: Keeping the CPU busy executing applications.
Throughput: Number of processes completed per unit time.
Turnaround time: Total time taken from process arrival to completion.
Waiting time: Time spent in the ready queue.
Response time: Time from submission of a request to the first response.

First-Come, First-Served (FCFS) Scheduling

Processes are executed in the order of arrival.
A Gantt chart illustrates this schedule.
Average waiting time is calculated for a set of processes.
Convoy effect can occur when short processes are delayed behind long processes.

Shortest-Job-First (SJF) Scheduling

Processes are scheduled based on the length of their next CPU burst.
It's an optimal algorithm, minimizing average waiting time for a given set of processes.
The challenge is accurately predicting the next CPU burst length.
Shortest-remaining-time-first (preemptive version) is also considered.

Determining Length of Next CPU Burst

Burst length estimation is based on prior burst lengths for the same process.
Methods like exponential averaging use previous CPU bursts to predict future ones.
The formula for exponential averaging involves weight (α), varying from 0 to 1 and the current and prior burst length values.

Examples of SJF Scheduling

SJF scheduling chart and associated waiting times calculations are provided.

Example of Shortest-Remaining-Time-First

Example Gantt chart and calculation of average waiting time are included.

Priority Scheduling

Processes are assigned priority numbers, and the CPU is given to the process with the highest priority.
Preemptive and non-preemptive versions are possible.
SJF is a specific priority scheduling type where priority is inversely related to the predicted next CPU burst.
Starvation (low-priority processes might never execute) is a potential issue.
Aging (increasing priority over time) is a solution.

Example of Priority Scheduling

Gantt chart and average waiting time calculations are given.

Round Robin (RR) Scheduling

Each process receives a small time quantum (fixed duration).
After the quantum, the process is preempted, and the scheduler moves to the next process in the ready queue.
The time quantum should be larger than the context switch time to balance performance and overhead.
Performance is affected by the time quantum.

Example of Round Robin

Example with a Gantt chart illustrating the round-robin scheduling for different processes.

Time Quantum and Context Switch Time

Comparison between time quantum and context switch times for better understanding of the relationship.

Turnaround Time Varies with the Time Quantum

Graph showing how the average turnaround time varies based on the time quantum, providing insight into the tradeoffs.

Each process is allocated a share of the processing time. The scheduler ensures each application receives a proportional share of the processor time.

Multilevel Queue

Ready queue is subdivided into separate queues (e.g., foreground and background) organized into different categories of processes.
Each queue can use varying scheduling algorithms to optimize performance for the specific type of process.

Thread Scheduling

Distinction between user-level and kernel-level threads exists.
Kernel-managed threads are included in the system-contention scope.
User-level threads operate within the current process scope for scheduling.

Pthread Scheduling

API for specifying the scheduling scope (either process-contention scope or system-contention scope).
Methods like PTHREAD_SCOPE_PROCESS and PTHREAD_SCOPE_SYSTEM are available for thread creation.

Real-Time CPU Scheduling

Soft and hard real-time systems are differentiated.
Soft real-time systems give preference to real-time processes, but not guaranteed execution times.
Hard real-time systems require tasks completed by their deadlines.
Interrupt latency and dispatch latency affect real-time performance.
Potential conflicts may occur when scheduling real-time processes.

Priority-based Scheduling

Real-time scheduling needs preemptive priority scheduling to manage deadlines effectively.
Hard real-time systems involve periodic processes.
Rate Monotonic Scheduling is a priority-based algorithm that assigns priorities to processes based on their period. Shorter periods mean higher priority.

Rate Monotonic Scheduling

Priority assignment method for real-time tasks.
Process with shorter periods are assigned higher scheduling precedence.
Example scenarios for different tasks are discussed, demonstrating the concept using deadlines for better comprehension.

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Description

This quiz covers fundamental concepts of CPU utilization, the CPU-I/O burst cycle, and CPU scheduling methods. Key topics include the distribution of CPU burst times and the role of the CPU scheduler in process allocation. Test your understanding of non-preemptive and preemptive scheduling techniques.