Scheduling Algorithms Overview
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Scheduling Algorithms Overview

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Questions and Answers

What is the average waiting time for the processes P1, P2, and P3 when they arrive in the order P1, P2, P3?

  • 3
  • 10
  • 17 (correct)
  • 24

    • What effect can occur in a First-Come, First-Served (FCFS) scheduling scenario with a CPU-bound process and multiple I/O-bound processes?

  • Convoy effect (correct)
  • Thrashing
  • Latency
  • Starvation

    • Which of the following criteria is NOT used to optimize scheduling algorithms?

  • Max throughput
  • Min waiting time
  • Min response time
  • Min energy consumption (correct)

    • In the Gantt chart for P2, P3, P1 where P2 arrives first, what is the waiting time for process P3?

    <p>3</p> Signup and view all the answers

    What is the characteristic of the Shortest-Job-First (SJF) scheduling algorithm?

    <p>Minimizes average waiting time for a set of processes</p> Signup and view all the answers

    In FCFS scheduling, which process has the longest waiting time when they arrive in the order P1, P2, P3?

    <p>P3</p> Signup and view all the answers

    What is the main disadvantage of First-Come, First-Served (FCFS) scheduling?

    <p>Long average waiting time for short processes</p> Signup and view all the answers

    When determining the next CPU burst length for the Shortest-Job-First (SJF) scheduling, which approach is NOT correct?

    <p>Measure the current burst length only</p> Signup and view all the answers

    What is the main characteristic of deterministic modeling in algorithm evaluation?

    <p>It defines performance based on a predetermined workload.</p> Signup and view all the answers

    What is the average waiting time for the Non-preemptive Shortest Job First (SFJ) algorithm for the given processes?

    <p>13ms</p> Signup and view all the answers

    What type of input does deterministic evaluation require?

    <p>Exact numerical values.</p> Signup and view all the answers

    In queueing models, what common probability distribution is often used to describe process arrivals?

    <p>Exponential distribution</p> Signup and view all the answers

    Which of the following metrics can be computed from queueing models?

    <p>Average queue length</p> Signup and view all the answers

    What is the impact of a large time quantum on process scheduling?

    <p>It approaches First-Come, First-Served behavior.</p> Signup and view all the answers

    What happens if the time quantum is too small compared to context switch time?

    <p>Overhead increases excessively.</p> Signup and view all the answers

    What is the primary issue that can arise with priority scheduling?

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

    What is the typical effect of using shorter time quantums in scheduling?

    <p>Increased overhead from context switching.</p> Signup and view all the answers

    In priority scheduling, how is Shortest Job First (SJF) classified?

    <p>As a non-preemptive scheduling algorithm.</p> Signup and view all the answers

    What is a recommended time quantum range to enhance performance?

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

    How can starvation of low priority processes be mitigated?

    <p>By using aging to increase their priority over time.</p> Signup and view all the answers

    What does a time quantum usually need to be in relation to CPU bursts for optimal performance?

    <p>Large compared to context switch time.</p> Signup and view all the answers

    If 80% of CPU bursts should be shorter than the time quantum, what implication does this have on scheduling?

    <p>Longer time quantums will lead to increased waiting time.</p> Signup and view all the answers

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

    <p>To maintain an even distribution of workload</p> Signup and view all the answers

    Which type of migration involves idle processors pulling tasks from busy processors?

    <p>Pull migration</p> Signup and view all the answers

    What does processor affinity refer to in the context of CPU scheduling?

    <p>A thread's tendency to remain on the same processor it started on</p> Signup and view all the answers

    What is soft affinity in CPU scheduling?

    <p>The operating system makes efforts to keep a thread on the same processor, but without guarantees</p> Signup and view all the answers

    In a hard real-time system, what must happen to tasks?

    <p>Tasks must be serviced by their specified deadlines</p> Signup and view all the answers

    What does NUMA stand for and what does it imply about CPU scheduling?

    <p>Non-Uniform Memory Access, meaning memory is assigned based on CPU proximity</p> Signup and view all the answers

    What is event latency in the context of real-time CPU scheduling?

    <p>The time from when an event occurs until it is serviced</p> Signup and view all the answers

    What distinguishes hard affinity from soft affinity?

    <p>Hard affinity guarantees processor usage; soft affinity does not</p> Signup and view all the answers

    What is a key feature of a multilevel-feedback-queue scheduler?

    <p>It can specify methods for upgrading and demoting processes.</p> Signup and view all the answers

    In the example of the multilevel-feedback queue, what happens if a process does not finish in Q0 within 8 milliseconds?

    <p>It is moved to Q1 and receives 16 additional milliseconds.</p> Signup and view all the answers

    What distinguishes many-to-one and many-to-many threading models?

    <p>The number of user threads mapped to kernel threads.</p> Signup and view all the answers

    What API call can be used to set the scheduling scope for Pthreads?

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

    What does PTHREAD_SCOPE_SYSTEM indicate in regard to thread scheduling?

    <p>Threads compete for CPU resources globally across the system.</p> Signup and view all the answers

    What is a characteristic of symmetric multiprocessing (SMP)?

    <p>Each processor has the ability to self-schedule.</p> Signup and view all the answers

    How is aging implemented in multilevel feedback queues?

    <p>By increasing the priority of processes over time.</p> Signup and view all the answers

    Which of the following is not a component defining a multilevel-feedback-queue scheduler?

    <p>The total number of processes</p> Signup and view all the answers

    What happens to a process if it does not complete in Q1 after receiving additional 16 milliseconds?

    <p>It is moved to queue Q2 and preempted.</p> Signup and view all the answers

    When creating threads, how does 'pthread_attr_getscope' help the programmer?

    <p>It allows the identification of the thread’s scheduling scope.</p> Signup and view all the answers

    What is the purpose of the 'runner' function in the pthread scheduling API example?

    <p>It serves as a placeholder to allow thread completion.</p> Signup and view all the answers

    What is a characteristic of a many-to-one threading model?

    <p>All user threads share a single system thread.</p> Signup and view all the answers

    Which of the following scheduling characteristics applies to multithreaded cores?

    <p>They can handle multiple threads simultaneously.</p> Signup and view all the answers

    What disadvantage can arise from using user-level threads in a many-to-many model?

    <p>Inefficient management of blocking calls.</p> Signup and view all the answers

    Study Notes

    Scheduling Algorithm Optimization Criteria

    • Maximize CPU utilization
    • Maximize throughput
    • Minimize turnaround time
    • Minimize waiting time
    • Minimize response time

    First-Come, First-Served (FCFS) Scheduling

    • The first process that enters the ready queue is the first process that gets scheduled
    • Scheduling processes based on the order they arrive
    • FCFS can lead to the convoy effect, where a short process gets stuck behind a long process

    Shortest-Job-First (SJF) Scheduling

    • Schedules the job with the shortest burst time first
    • Can be preemptive, known as shortest-remaining-time-first
    • The length of the next CPU burst is used to determine the priority

    Round-Robin Scheduling

    • Each process is allocated a time quantum, and the CPU cycles through the processes
    • When a time quantum is exhausted, the process is put back in the queue and the next job is scheduled
    • Time quantum should be significantly larger than the context switch time
    • The time quantum can affect the average turnaround time and response time
    • If time quantum is too small, the system will be bogged down by context switching

    Priority Scheduling

    • Each process is assigned a priority number
    • The process with the highest priority gets scheduled (smaller number is higher priority)
    • Can be preemptive or non-preemptive
    • Problem: Starvation, a low-priority process may never get scheduled
    • Solution: Aging - incrementally increase a process's priority over time

    Multilevel Feedback Queue

    • Multiple queues with different scheduling algorithms
    • A process can move between queues based on its behavior
    • Aging can be implemented using multilevel feedback queues

    Thread Scheduling

    • Distinction between user-level and kernel-level threads
    • Threads are scheduled instead of processes
    • There are numerous models for scheduling, such as many-to-one and many-to-many
    • Process-contention scope (PCS) is when threads within one process compete for the CPU
    • System-contention scope (SCS) is when threads across all processes compete for the CPU

    Pthread Scheduling

    • Pthreads allows specifying either PCS or SCS during thread creation
    • PTHREAD_SCOPE_PROCESS schedules threads using PCS
    • PTHREAD_SCOPE_SYSTEM schedules threads using SCS
    • Some OSes may limit the scheduling scope

    Multiple-Processor Scheduling

    • More complex when multiple CPUs are available
    • Must consider:
      • Multicore CPUs
      • Multithreaded cores
      • NUMA systems
      • Heterogeneous multiprocessing

    Load Balancing

    • The goal is to keep all CPUs equally loaded for efficiency
    • Push migration - moves a task from an overloaded CPU to a less loaded CPU
    • Pull migration - an idle CPU pulls a waiting task from a busy CPU

    Processor Affinity

    • When a thread runs on a specific CPU, it caches memory accesses
    • Load balancing may disrupt this affinity
    • Soft affinity - the OS tries to keep a thread on the same CPU, but no guarantees
    • Hard affinity - the process defines a set of CPUs it can run on

    NUMA and CPU Scheduling

    • If the OS is NUMA-aware, it assigns memory to the CPU nearest to the running thread

    Real-Time CPU Scheduling

    • Scheduling can be very challenging
    • Soft real-time systems - critical tasks get the highest priority, but execution is not guaranteed
    • Hard real-time systems - tasks must meet their deadlines
    • Event Latency - the time between an event occurring and when it is serviced

    Deterministic Modeling

    • Analytic evaluation
    • Based on a predetermined workload, calculates the performance of each scheduling algorithm
    • Simple and fast, but specific to the input parameters

    Queuing Models

    • Describe the arrival of processes and their bursts probabilistically
    • Used to calculate average throughput, utilization, waiting time, etc.

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    Related Documents

    ch5.pptx

    Description

    Explore various scheduling algorithms used in operating systems, such as First-Come, First-Served, Shortest-Job-First, and Round-Robin. Understand the optimization criteria that enhance CPU utilization and minimize wait times. This quiz will test your knowledge of process scheduling methods and their implications.

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