Scheduling Algorithms Overview
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Scheduling Algorithms Overview

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What characterizes nonpreemptive scheduling?

  • It only occurs when the ready queue is empty.
  • Processes can be interrupted to allow higher priority tasks to run.
  • Once allocated, a process retains the CPU until it terminates or waits. (correct)
  • The CPU can be taken away from a running process at any time.

    • Which of the following operating systems uses preemptive scheduling?

  • Classic Mac OS
  • Windows (correct)
  • DOS
  • Older UNIX systems

    • What is a potential consequence of preemptive scheduling?

  • Enhanced process priority management.
  • Race conditions when processes share data. (correct)
  • Deadlock resolution.
  • Increased CPU resource allocation.

    • When does scheduling become a preemptive process?

    <p>When choices exist in circumstances 2 or 3.</p> Signup and view all the answers

    What happens to a process in nonpreemptive scheduling once it is allocated the CPU?

    <p>It continues to execute until it releases the CPU voluntarily.</p> Signup and view all the answers

    In the context of the preemptive Shortest Remaining Time First (SRTF) scheduling algorithm, what does the average waiting time of 6.5 represent?

    <p>The average time processes waited in the queue before being executed</p> Signup and view all the answers

    What is the main characteristic of the Round Robin (RR) scheduling method?

    <p>Each process receives a fixed amount of CPU time before being preempted</p> Signup and view all the answers

    How does the time quantum 'q' in Round Robin scheduling influence process execution?

    <p>It limits the maximum time a process can use the CPU before being preempted</p> Signup and view all the answers

    What happens to a process in the Round Robin scheduling method after its time quantum has expired?

    <p>It is added to the end of the ready queue</p> Signup and view all the answers

    In the context of the preemptive Shortest Remaining Time First algorithm, which process is likely to wait the longest?

    <p>The process that keeps getting interrupted by others with shorter remaining times</p> Signup and view all the answers

    What is the CPU burst time for process P1?

    <p>25</p> Signup and view all the answers

    In Rate Monotonic Scheduling, what determines the priority of processes?

    <p>Period of the process</p> Signup and view all the answers

    Which scheduling class uses a FIFO strategy with no time-slicing for threads of equal priority?

    <p>SCHED_FIFO</p> Signup and view all the answers

    Under Earliest Deadline First Scheduling, what happens if two processes have the same deadline?

    <p>They are scheduled in a round-robin manner.</p> Signup and view all the answers

    Why did process P2 miss its deadline at time 80?

    <p>It has a longer CPU burst time than P1.</p> Signup and view all the answers

    What is one of the primary differences between SCHED_RR and SCHED_FIFO?

    <p>SCHED_RR allows time-slicing.</p> Signup and view all the answers

    For process scheduling, which scheduling policy gives priority based on the urgency of the deadlines?

    <p>Earliest Deadline First Scheduling</p> Signup and view all the answers

    If process P1 has a period of 50 and a CPU burst of 25, what can be said about its scheduling under Rate Monotonic Scheduling?

    <p>It has a higher priority than P2.</p> Signup and view all the answers

    What happens when the time quantum (q) is too small in a Round Robin scheduling algorithm?

    <p>Overhead due to context switching becomes too high.</p> Signup and view all the answers

    In priority scheduling, what defines the priority of a process?

    <p>The inverse of the predicted next CPU burst time.</p> Signup and view all the answers

    What is the typical average waiting time associated with Round Robin scheduling compared to Shortest Job First (SJF)?

    <p>Higher than SJF.</p> Signup and view all the answers

    Which statement regarding the context switch time is accurate for Round Robin scheduling?

    <p>It must be less than 10 microseconds to reduce overhead.</p> Signup and view all the answers

    What problem can arise from using priority scheduling?

    <p>Starvation of low priority processes may occur.</p> Signup and view all the answers

    What is the solution to prevent starvation in priority scheduling?

    <p>Implementing aging to increase process priority over time.</p> Signup and view all the answers

    How is the Gantt chart used to visualize process scheduling in Round Robin?

    <p>It plots time against the order of process execution.</p> Signup and view all the answers

    Which of the following statements best describes the average turnaround time in relation to time quantum?

    <p>It varies depending on the value of time quantum relative to CPU bursts.</p> Signup and view all the answers

    Given the burst times and priorities of several processes, how would you determine which process to execute next in priority scheduling?

    <p>Execute the process with the lowest numerical priority value.</p> Signup and view all the answers

    What is a common time quantum range for Round Robin scheduling?

    <p>10 to 100 milliseconds.</p> Signup and view all the answers

    What is the purpose of pthread_attr_setsched_policy in a POSIX environment?

    <p>To set the scheduling policy of a thread attribute.</p> Signup and view all the answers

    Which scheduling policy indicates that a thread will receive a higher priority when it is scheduled?

    <p>SCHED_FIFO</p> Signup and view all the answers

    In Linux scheduling, what is the range of the real-time priority?

    <p>0 to 99</p> Signup and view all the answers

    What characterizes the Completely Fair Scheduler (CFS) introduced in Linux 2.6.23?

    <p>Proportion of CPU time allocation.</p> Signup and view all the answers

    What is one known limitation of the scheduling system prior to Linux kernel version 2.5?

    <p>It had poor response times for interactive processes.</p> Signup and view all the answers

    What happens when a task's time slice expires in the Linux scheduling model?

    <p>It becomes un-runnable until all other tasks finish their slices.</p> Signup and view all the answers

    How does Linux track runnable tasks according to its scheduling algorithm?

    <p>Using a per-CPU runqueue data structure.</p> Signup and view all the answers

    What does the term 'nice value' indicate in the context of Linux scheduling?

    <p>A higher nice value corresponds to lower priority.</p> Signup and view all the answers

    What is the function of pthread_exit in the provided code?

    <p>It terminates the calling thread.</p> Signup and view all the answers

    What is the result when pthread_attr_getschedpolicy fails?

    <p>An error message is printed to stderr.</p> Signup and view all the answers

    Study Notes

    Scheduling Algorithms

    • Nonpreemptive Scheduling When the CPU is allocated, it remains with that process until it terminates or transitions to a waiting state.
    • Preemptive Scheduling The CPU is allocated to a process, but can be interrupted and taken away from it and given to another process. It is used by most operating systems, like Windows, MacOS, Linux, and Unix.
    • Race Conditions A problem that can arise with preemptive scheduling when multiple processes share data. One process may be preempted while updating data, leaving it in an inconsistent state when another process tries to read it.

    Scheduling Algorithms: Shortest-Remaining-Time-First (SJF)

    • A scheduling algorithm where the process with the shortest remaining processing time is chosen for execution.
    • In a preemptive SJF, a process can be interrupted if another process arrives with a shorter remaining processing time.

    Scheduling Algorithms: Round Robin (RR)

    • Each process is allocated a small time quantum, typically between 10-100 milliseconds.
    • After the quantum, the process is preempted and added to the end of the ready queue.
    • The scheduler then chooses the next process from the beginning of the queue.
    • If the time quantum is large, RR resembles First Come First Serve (FCFS).
    • If the time quantum is small, it resembles Round Robin with a shorter time slice and frequent context switching.

    Scheduling Algorithms: Priority Scheduling

    • Each process is assigned a priority number, with lower numbers representing higher priorities.
    • The CPU is then allocated to the process with the highest priority (smallest integer).
    • Priority scheduling can be preemptive or nonpreemptive.
    • A potential issue is "starvation" where low-priority processes may never get executed.
    • Aging can be used to address starvation by gradually increasing the priority of a process over time.

    Rate Monotonic Scheduling (RMS)

    • An algorithm for scheduling real-time tasks with fixed periods.
    • It assigns priorities based on the period of the task, giving the task with the shortest period the highest priority.
    • Can be applied to scheduling tasks with deadlines that are fixed and periodic.

    Earliest Deadline First Scheduling (EDF)

    • Also designed for real-time scheduling, EDF considers the deadlines of the tasks.
    • The task with the earliest deadline is given the highest priority.
    • It can handle tasks with varying deadlines.

    POSIX Real-Time Scheduling

    • A standard for real-time scheduling in Unix-like operating systems.
    • Defines two scheduling classes for real-time threads:
      • SCHED_FIFO: threads are scheduled using FCFS with a FIFO queue. No time-slicing for threads of equal priority.
      • SCHED_RR: similar to SCHED_FIFO, but time-slicing occurs for threads of equal priority.

    Linux Scheduling

    • Prior to kernel version 2.5, Linux used a variation of the standard UNIX scheduling algorithm.
    • Version 2.5 switched to a constant order O(1) scheduling time:
      • Preemptive, priority-based.
      • Two priority ranges: time-sharing and real-time.
      • Real-time range from 0 to 99, nice value from 100 to 140.
      • Mapping into global priority with lower values indicating higher priority.
      • Higher priority gets a larger time quantum.
      • Tasks are run-able as long as time left in the time slice (active).
      • If no time left (expired), they are not run-able until other tasks have used their slices.
      • All run-able tasks are tracked in a per-CPU runqueue data structure.

    Linux Scheduling in Version 2.6.23+

    • Completely Fair Scheduler (CFS) aims to provide fair use of the CPU to all processes.
    • It is based on the proportion of CPU time each process receives, rather than fixed time allotments.
    • Two scheduling classes are included:
      • Real-time.
      • Time-sharing.
    • Other scheduling classes can be added as needed.

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    ch5.pptx

    Description

    Explore the various scheduling algorithms used in operating systems, including nonpreemptive and preemptive scheduling, as well as the complexities introduced by race conditions. Dive into specific algorithms like Shortest-Remaining-Time-First and Round Robin to understand their mechanics and applications in CPU management.

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