CPU Scheduling and Process Synchronization

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to Lesson

Podcast

Play an AI-generated podcast conversation about this lesson
Download our mobile app to listen on the go
Get App

Questions and Answers

What is the average waiting time (WT) for processes P1, P2, and P3 under Shortest Job First (SJF) scheduling?

The average waiting time (WT) is 11 units.

How long does process P3 take to return to the CPU after its initial I/O operation?

Process P3 will return after 5 units of time.

If process P2 requires 30 units of time, how many units of time does P3 require in total?

Process P3 requires 10 units of time in total.

Which process will be allocated to CPU between P2 and P3 if they are ready at the same time?

<p>Process P3 will be allocated to the CPU.</p> Signup and view all the answers

What is the average turnaround time (TAT) for the processes given their scheduling?

<p>The average turnaround time (TAT) is 19 units.</p> Signup and view all the answers

What is the average waiting time for the processes in the Gantt chart?

<p>The average waiting time is 6.5 msec.</p> Signup and view all the answers

How does the Shortest Job First (SJF) algorithm determine process execution order?

<p>The SJF algorithm prioritizes processes with the shortest burst time first.</p> Signup and view all the answers

What process has the highest burst time in the given example?

<p>Process P4 has the highest burst time of 654.</p> Signup and view all the answers

What advantage does the Shortest Job First algorithm provide?

<p>SJF is provably optimal as it minimizes the average waiting time for a given set of processes.</p> Signup and view all the answers

In the preemptive SJF Gantt chart, which process starts executing after P2?

<p>Process P4 starts executing after P2.</p> Signup and view all the answers

What does TAT stand for in scheduling algorithms?

<p>TAT stands for Turnaround Time.</p> Signup and view all the answers

Why is preemption necessary in the SJF algorithm?

<p>Preemption allows a process to be interrupted to allow a shorter job to execute.</p> Signup and view all the answers

What is the completion time (CT) of Process P1?

<p>The completion time of Process P1 is 3.</p> Signup and view all the answers

What defines the difference between non-preemptive and preemptive scheduling?

<p>Non-preemptive scheduling does not allow a running process to be interrupted, while preemptive scheduling enables interrupts to switch tasks.</p> Signup and view all the answers

What are the key roles of the dispatcher in process management?

<p>The dispatcher switches context, changes to user mode, and jumps to the correct location in the user program to restart execution.</p> Signup and view all the answers

How does CPU utilization relate to scheduling performance?

<p>CPU utilization aims to keep the CPU as busy as possible to maximize efficiency and performance of the scheduling algorithm.</p> Signup and view all the answers

What is meant by turnaround time in the context of process scheduling?

<p>Turnaround time refers to the total time taken to execute a process from submission to completion.</p> Signup and view all the answers

What is the significance of response time in time-sharing environments?

<p>Response time measures the time from request submission until the first response is produced, critical for user satisfaction.</p> Signup and view all the answers

Describe the First-Come, First-Served (FCFS) scheduling algorithm.

<p>FCFS scheduling processes jobs in the order they arrive, resembling a queue structure.</p> Signup and view all the answers

What factors should be considered when preempting a process in kernel mode?

<p>Access to shared data and the potential for interrupts occurring during crucial OS activities are key factors.</p> Signup and view all the answers

List two criteria used to evaluate CPU scheduling algorithms.

<p>Throughput and waiting time are two important criteria for evaluating scheduling algorithms.</p> Signup and view all the answers

What is the total waiting time (WT) for process P1?

<p>14 units.</p> Signup and view all the answers

If process P2 has a total turnaround time (TAT) of 41 units and its burst time (BT) is 30 units, what is its waiting time (WT)?

<p>26 units.</p> Signup and view all the answers

How many units of time does process P3 spend utilizing the CPU for the first 30% of its processing time?

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

What is the completion time (CT) of procedure P1 based on the Gantt chart provided?

<p>41 units.</p> Signup and view all the answers

In the Round Robin (RR) scheduling approach, what is the purpose of the time quantum?

<p>To allocate a small unit of CPU time to each process.</p> Signup and view all the answers

What is the average CPU utilization given a total expected CPU time of 30 units and actual CPU time of 41 units?

<p>73%.</p> Signup and view all the answers

What does the term 'turnaround time' (TAT) refer to in process scheduling?

<p>The total time taken from arrival to completion of a process.</p> Signup and view all the answers

How is the response time (RT) for process P2 determined?

<p>RT is calculated as 11 - 0 = 11.</p> Signup and view all the answers

For process P1, if it waits for 3 units and then for 11 units before resuming, what is the total time waited?

<p>14 units.</p> Signup and view all the answers

What proportion of its processing time does P2 spend performing I/O operations?

<p>15 units.</p> Signup and view all the answers

What is the formula used to calculate the predicted value of the next CPU burst?

<p>The formula is ( \tau_{n+1} = \alpha t_n + (1 - \alpha) \tau_n ).</p> Signup and view all the answers

If the actual length of the nth CPU burst, ( t_n ), is 8 and ( \alpha ) is set to 0.5, what is the predicted value for the next CPU burst given ( \tau_n ) of 10?

<p>The predicted value is ( \tau_{n+1} = 0.5 \cdot 8 + 0.5 \cdot 10 = 9 ).</p> Signup and view all the answers

What outcome occurs if a newly arrived process has a shorter burst time than the remaining burst time of the currently running process in SJF with preemption?

<p>The operating system removes the currently running process and allocates the CPU to the newly arrived process.</p> Signup and view all the answers

In the context of shortest job first (SJF), which of the following would serve first: processes with burst times of 4, 7, 8, and 16?

<p>The process with a burst time of 4 would serve first.</p> Signup and view all the answers

How does the value of ( \alpha ) affect the predicted CPU burst time in the exponential averaging formula?

<p>A higher ( \alpha ) gives more weight to the most recent CPU burst, making predictions more responsive to recent activity.</p> Signup and view all the answers

What is the final predicted burst time ( T4 ) after processing burst times of 4, 7, and 8 using ( \alpha = 0.5 )?

<p>The final predicted burst time ( T4 ) is 7.5.</p> Signup and view all the answers

According to the prediction method discussed, what will ( T3 ) equal if ( T2 ) and the burst time are both 7?

<p>The value of ( T3 ) will also be 7.</p> Signup and view all the answers

Why is SJF considered a non-preemptive technique, and what does it imply?

<p>SJF is considered non-preemptive because once a process starts execution, it runs to completion unless it is finished or its time slice expires.</p> Signup and view all the answers

What is the primary principle behind Shortest Job First (SJF) scheduling?

<p>SJF scheduling selects the process with the smallest burst time for execution next.</p> Signup and view all the answers

In SJF scheduling, what happens when multiple processes have the same burst time?

<p>When multiple processes have the same burst time, the First-Come, First-Served (FCFS) technique is applied.</p> Signup and view all the answers

How is the Turnaround Time (TAT) calculated in process scheduling?

<p>Turnaround Time (TAT) is calculated as TAT = WT + BT, where WT is the waiting time and BT is the burst time.</p> Signup and view all the answers

Define the difference between non-preemptive and preemptive scheduling in the context of SJF.

<p>Non-preemptive scheduling does not allow a running process to be interrupted, while preemptive scheduling allows the system to forcibly remove a process to run another.</p> Signup and view all the answers

How would you calculate the average waiting time for a set of processes in SJF?

<p>The average waiting time is calculated by summing the waiting times of all processes and dividing by the number of processes.</p> Signup and view all the answers

What is the significance of the burst time in the SJF scheduling algorithm?

<p>Burst time determines how long a process will take to execute, which is crucial for selecting the next process in SJF scheduling.</p> Signup and view all the answers

In the example provided, what was the average waiting time for the processes?

<p>The average waiting time was calculated as 7.</p> Signup and view all the answers

What role does arrival time play in the SJF scheduling algorithm?

<p>Arrival time influences the order in which processes are considered for execution, as processes must arrive before they can be scheduled.</p> Signup and view all the answers

Explain how SJF scheduling might decide between two processes with equal burst times.

<p>In case of equal burst times, SJF scheduling will use First-Come, First-Served (FCFS) to determine which process to execute first.</p> Signup and view all the answers

What is the expected outcome of using the SJF scheduling technique compared to other scheduling methods?

<p>The expected outcome of SJF scheduling is reduced average waiting time and turnaround time for processes.</p> Signup and view all the answers

Flashcards

Waiting Time (WT)

The time a process spends waiting in the ready queue before being allocated to the CPU.

Turnaround Time (TAT)

The total time a process takes from its arrival in the system until its completion.

Response Time (RT)

The time required for a process to complete its first CPU burst.

Shortest Job First (SJF)

A scheduling algorithm where the process with the shortest remaining burst time is executed first.

Signup and view all the flashcards

CPU Burst Time

The time a process spends actively using the CPU during its execution.

Signup and view all the flashcards

Process transition: Running to Waiting

When a process transitions from the running state to a waiting state, it is usually due to an event, such as I/O request or waiting for a resource.

Signup and view all the flashcards

Process transition: Running to Ready

A process switches from the running state to the ready state when it is interrupted or voluntarily yields the CPU.

Signup and view all the flashcards

Process transition: Waiting to Ready

A process transitions from the waiting state to the ready state when the event it was waiting for has occurred, making it eligible to run again.

Signup and view all the flashcards

Process Termination

When a process terminates, it is no longer eligible to run and its resources are released back to the system.

Signup and view all the flashcards

What does the Dispatcher do?

The dispatcher is responsible for switching the CPU to the process selected by the scheduler. This involves context switching, changing to user mode, and restarting the user program.

Signup and view all the flashcards

What is Dispatch Latency?

Dispatch latency refers to the time taken by the dispatcher to switch from one process to another, making the CPU available for the next process to run.

Signup and view all the flashcards

CPU Utilization

CPU utilization measures the percentage of time the CPU is actively used for processing.

Signup and view all the flashcards

Process Throughput

Throughput is a measure of how many processes complete their execution per unit of time.

Signup and view all the flashcards

Shortest Remaining Time First (SRTF)

This variant of SJF allows preemption, meaning the CPU can be taken away from a process if a new process with a shorter remaining time arrives.

Signup and view all the flashcards

Non-preemptive SJF

In SJF, once the CPU is assigned to a process, it cannot be interrupted until the process finishes.

Signup and view all the flashcards

Burst Time (BT)

The estimated time a process needs to complete execution.

Signup and view all the flashcards

Exponential Averaging

An algorithm used to determine the next CPU burst time for a process, based on previous bursts.

Signup and view all the flashcards

Ready Queue

A pool of processes that are ready to be scheduled when the current process completes.

Signup and view all the flashcards

Round Robin (RR)

A scheduling algorithm that assigns a fixed time slice (quantum) to each process before switching to the next.

Signup and view all the flashcards

Gantt Chart

A visual representation of the execution flow of processes, showing their start and end times.

Signup and view all the flashcards

Average Waiting Time

Algorithm to calculate the average waiting time for multiple processes.

Signup and view all the flashcards

Alpha (α) in Exponential Averaging

A parameter in the exponential averaging formula that determines the weight given to the current CPU burst in predicting the next one. A value closer to 1 emphasizes the current burst, while a value closer to 0 emphasizes past predictions.

Signup and view all the flashcards

Predicted Burst Time (Ï„n+1)

The predicted length of the next CPU burst, calculated using the exponential averaging formula.

Signup and view all the flashcards

Actual Burst Time (tn)

The actual length of the nth CPU burst.

Signup and view all the flashcards

Execution Order in SJF

The specific order in which processes are executed in a non-preemptive scheduling algorithm like SJF, where the shortest job is processed first.

Signup and view all the flashcards

Process Arrival Time

The point in time when a new process arrives in the system, potentially causing a preemption in SRTF scheduling.

Signup and view all the flashcards

Preemptive Shortest Job First

Preemptive SJF is an extension of SJF where if a new process arrives with a shorter burst time than the currently running process, the running process is preempted and the new process is executed.

Signup and view all the flashcards

Arrival Time (AT)

The time at which a process enters the system.

Signup and view all the flashcards

Study Notes

CPU Scheduling

  • CPU scheduling is the basis for multiprogrammed operating systems
  • Various CPU scheduling algorithms exist
  • Evaluation criteria for selecting a CPU scheduling algorithm include maximizing CPU utilization, throughput, minimizing turnaround time, waiting time, and response time.
  • Short-term scheduler selects a process from the ready queue and allocates the CPU.
  • CPU scheduling decisions take place when a process:
    • Switches from running to waiting state
    • Switches from running to ready state
    • Switches from waiting to ready
    • Terminates

Process Synchronization

  • Process synchronization is a crucial aspect of concurrent programming.
  • The critical-section problem deals with issues that occur when processes share resources, such as data or access to devices.
  • Peterson's solution is an example of a hardware-based solution to the critical-section problem.
  • Mutex locks and semaphores are software solutions to coordinate access to shared resources.
  • Classical problems of synchronization, including the producer-consumer problem and the readers-writers problem, illustrate scenarios where synchronization is vital.
  • Monitors provide a higher-level abstraction for process synchronization.

Process State Transition Diagram

  • A diagram illustrating the various states a process can be in (new, ready, run, blocked, wait, suspend/ready, suspend/wait, suspend).
  • Processes transition between these states as they are created, scheduled, execute, wait for I/O, and terminate.
  • The diagram shows how a process moves from one state to another, including transitions due to events like scheduling, I/O completion, and priority changes.

Scheduling Criteria

  • CPU utilization: keep the CPU as busy as possible.
  • Throughput: number of processes that complete their execution per time unit.
  • Turnaround time: amount of time to execute a process (arrival time to completion time).
  • Waiting time: the amount of time a process has been waiting in the ready queue.
  • Response time: the time from when a request was submitted until the first response is produced (for time-sharing environments).

Scheduling Algorithm Optimization Criteria

  • Maximize CPU Utilization, Throughput
  • Minimize Turnaround time, Waiting time, and Response time

First-Come, First-Served (FCFS) Scheduling

  • Schedules jobs according to their arrival time.
  • The job that arrives first gets the CPU first.
  • FCFS scheduling can lead to starvation if the first process has a very long burst time, potentially holding up subsequent shorter jobs.

Shortest-Job-First (SJF) Scheduling

  • Selects the waiting process with the smallest execution time to execute next.
  • SJF can be non-preemptive or preemptive (SRTN).
  • Sorting processes by arrival time and burst time is crucial for proper scheduling.
  • Preemptive SJF (SRTN) allows for interrupting a running process if a shorter process arrives.

Priority Scheduling

  • Associates a priority number with each process, with lower numbers representing higher priority.
  • The CPU is allocated to the process with the highest priority.
  • Priority scheduling can lead to starvation of low-priority processes if higher-priority processes consistently arrive. Aging is a technique to increase the priority of a process over time to prevent starvation.

Round Robin (RR) Scheduling

  • Each process gets a small unit of CPU time (time quantum), often 10-100 milliseconds.
  • After the time quantum, the process is preempted and moved to the end of the ready queue.
  • Performance is good in time-sharing systems.
  • Larger time quantum are similar to FCFS, and smaller ones lead to higher context switching.

Dispatching

  • Dispatcher module gives control of the CPU to the selected process (switching contexts, user mode, restarting user program).
  • Dispatch latency refers to the time it takes the dispatcher to stop one process and start another.

Convoy Effect

  • Occurs in FCFS scheduling when a long process holds up shorter processes in the queue, reducing system throughput, especially when there are many I/O-bound processes.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Quiz Team

Related Documents

Lecture 5: CPU Scheduling PDF

More Like This

Use Quizgecko on...
Browser
Open
Browser
Browser