Operating System Scheduling Quiz

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Questions and Answers

What is necessary for obtaining maximum CPU utilization?

  • Sequential processing
  • Single programming
  • Multiprogramming (correct)
  • Serial execution

During which of the following situations does the CPU scheduler have no choice in terms of scheduling?

  • When a process switches from running to ready
  • When a process terminates (correct)
  • When a process switches from waiting to ready
  • When a process switches from running to waiting (correct)

Which of the following is a key concern in the CPU burst distribution?

  • Frequency of inputs
  • Size of the process
  • Total execution time
  • Duration of CPU bursts (correct)

How does the CPU scheduler decide which process to allocate the CPU core to?

<p>Based on the ordering of the ready queue (B)</p> Signup and view all the answers

Which of the following accurately describes the CPU-I/O burst cycle?

<p>Process execution alternates between CPU execution and I/O wait. (C)</p> Signup and view all the answers

Which scheduling algorithm is considered optimal for minimizing average waiting time?

<p>Shortest-Job-First (SJF) (B)</p> Signup and view all the answers

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

<p>17 (C)</p> Signup and view all the answers

In the Gantt chart for the arrival order P2, P3, P1 using FCFS, what is the waiting time for process P1?

<p>6 (A)</p> Signup and view all the answers

What happens when a new process arrives in the scheduling of the Shortest Remaining Time First algorithm?

<p>The scheduling decision is redone according to SJN criteria. (D)</p> Signup and view all the answers

What is the average waiting time for the processes P1 (6), P2 (8), P3 (7), and P4 (3) using Shortest-Job-First scheduling?

<p>7 (C)</p> Signup and view all the answers

What is the purpose of the dispatcher in an operating system?

<p>To control the CPU and switch between processes (D)</p> Signup and view all the answers

Which of the following represents the time taken by the dispatcher to switch from one process to another?

<p>Dispatch latency (D)</p> Signup and view all the answers

Which scheduling criterion measures the number of processes that finish execution in a specific time frame?

<p>Throughput (C)</p> Signup and view all the answers

What does turnaround time indicate in the context of process scheduling?

<p>The total time taken to complete a process (D)</p> Signup and view all the answers

In an operating system, how is CPU utilization defined?

<p>The percentage of CPU time that is actively being used (B)</p> Signup and view all the answers

What characterizes nonpreemptive scheduling?

<p>CPU allocation remains with a process until it terminates or waits. (D)</p> Signup and view all the answers

Which type of scheduling is primarily used by modern operating systems?

<p>Preemptive scheduling (A)</p> Signup and view all the answers

What potential issue can arise from preemptive scheduling?

<p>Data inconsistency due to race conditions. (D)</p> Signup and view all the answers

Under what circumstances is scheduling considered nonpreemptive?

<p>When scheduling occurs only in specific conditions. (A)</p> Signup and view all the answers

In preemptive scheduling, what happens if a process is updating shared data and is then preempted?

<p>The second process may read data in an inconsistent state. (D)</p> Signup and view all the answers

Flashcards

CPU Utilization

Maximizing CPU utilization is a key goal in operating systems. It aims to keep the CPU busy as much as possible, preventing it from idling and wasting resources.

CPU-I/O Burst Cycle

The CPU-I/O Burst Cycle describes the typical behavior of a process alternating between CPU execution and I/O operations. This cycle reflects the common pattern of processes needing to interact with external devices like disks or networks.

CPU Scheduling

CPU Scheduling is the process of deciding which process from the ready queue should be allocated the CPU core at any given time. The scheduler aims to optimize various factors like performance and fairness.

Ready Queue

A ready queue holds processes that are ready to be executed. The order of processes in this queue can be determined by different scheduling algorithms.

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CPU Scheduling Decisions

When a process transitions from running to waiting, changes from running to ready, transitions from waiting to ready, or terminates, these are moments the CPU scheduler may need to re-evaluate and decide which process should run next.

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Dispatcher

The part of the operating system responsible for handing over control of the CPU to the next process chosen by the scheduler. It involves switching contexts, changing to user mode, and resuming the chosen program at the appropriate point.

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Dispatch Latency

The time taken for the dispatcher to stop one process and start another.

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Throughput

The number of processes that successfully complete execution within a specific time period.

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Turnaround Time

The total time it takes for a process to finish executing, from the moment it begins to when it ends.

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Nonpreemptive Scheduling

A scheduling scheme where a process keeps the CPU until it finishes or voluntarily releases it, without interruption from the OS.

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Preemptive Scheduling

A scheduling scheme where the OS can interrupt a running process and allocate the CPU to another process.

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Race Condition

A situation that occurs when multiple processes access and modify shared data simultaneously, resulting in inconsistent data states.

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Inconsistent Data State

A state where data shared by multiple processes is in an inconsistent state, typically due to preemption before a process has finished updating it.

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Timing Dependency

An unpredictable situation where the outcome of a program depends on the timing of competing processes, often caused by preemptive scheduling.

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First-Come, First-Served (FCFS)

A scheduling algorithm where the process that arrives first gets executed first, regardless of its CPU burst time.

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Shortest-Job-First (SJF)

This algorithm aims to minimize the average waiting time for all processes by selecting the process with the shortest CPU burst time to execute next.

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Shortest Remaining Time First (SRTF)

SJF scheduling, but preemptive. When a new process arrives, the scheduler re-evaluates which process to run based on the remaining burst time.

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Study Notes

CPU Scheduling

  • CPU scheduling algorithms manage the allocation of CPU time to processes.
  • Operating systems use scheduling to improve CPU utilization, throughput, turnaround time, waiting time, and response time.
  • Scheduling decisions can occur when a process switches states (running to waiting, running to ready, waiting to ready, terminating).
  • Nonpreemptive scheduling occurs when a process keeps the CPU until it releases it (terminates or switches to waiting).
  • Preemptive scheduling allows the OS to take the CPU away from a running process.
  • Preemptive scheduling can lead to race conditions when data is shared among multiple processes.
  • The dispatcher module controls the CPU transfer to the selected process.
  • Dispatch latency is the time it takes for the dispatcher to stop one process and start another.

Scheduling Criteria

  • CPU utilization: Keeping the CPU busy as much as possible.
  • Throughput: The number of processes completed per unit of time.
  • Turnaround time: The total time taken for a process to complete.
  • Waiting time: The total time a process spends in the ready queue.
  • Response time: The time from when a request is submitted until the first response is produced.

Scheduling Algorithm Optimization Criteria

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

First-Come, First-Served (FCFS) Scheduling

  • Processes are scheduled in the order they arrive.
  • Simple to implement.
  • Can lead to long average waiting times, especially when longer processes arrive early.
  • Example using P1, P2, P3 with respective burst times of 24, 3, and 3: Average waiting time = 17

Shortest-Job-First (SJF) Scheduling

  • Processes are scheduled based on their burst time.
  • Optimal approach for minimum average waiting time.
  • Difficult to predict process burst times.
  • Preemptive version: Shortest-Remaining-Time-First (SRTF).
  • Example of SJF with P1, P2, P3, P4: Average waiting time = 7

Shortest-Remaining-Time-First (SRTF) Scheduling

  • Preemptive version of SJF.
  • Schedules the process with the shortest remaining time.
  • Optimal, but involves greater complexity.

Round Robin (RR) Scheduling

  • Each process gets a time slice (quantum).
  • After the quantum expires, the process is put back in the ready queue.
  • Higher response times than SJF, but lower turnaround times for shorter jobs.
  • Time quantum should be large compared to context switch time.
  • Example with a quantum of 4: Gantt chart shows a sequence of processes, where P1 runs for 24, P2 for 3, and P3 for 3. Average wait time is a result of running P1, P2, P3, multiple times.

Priority Scheduling

  • Processes are assigned priorities.
  • Highest priority processes are run first.
  • Can lead to starvation for low-priority processes.
  • SJF can be implemented as a priority scheduling using the inverse of predicted next CPU burst time
  • Aging is used to prevent starvation: increasing the priority of processes over time.

Multilevel Queue

  • Ready queue is divided into separate queues.
  • Each queue has a different scheduling algorithm.
  • Processes are assigned to queues based on their characteristics.
  • Processes in different queues are prioritised by their type.

Multilevel Feedback Queue

  • Processes can move to different queues depending on their behavior.
  • Processes that use too much CPU time are demoted to lower priority queues.
  • Queues can have different scheduling algorithms or prioritize based on process type.
  • Example shows three queues (Q0, Q1, Q2), where Q0 has a lower time quantum than Q1.

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