CPU Scheduling and Process Synchronization

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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?

Process P3 will be allocated to the CPU. Signup and view all the answers

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

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

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

The average waiting time is 6.5 msec. Signup and view all the answers

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

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

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

Process P4 has the highest burst time of 654. Signup and view all the answers

What advantage does the Shortest Job First algorithm provide?

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

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

Process P4 starts executing after P2. Signup and view all the answers

What does TAT stand for in scheduling algorithms?

TAT stands for Turnaround Time. Signup and view all the answers

Why is preemption necessary in the SJF algorithm?

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

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

The completion time of Process P1 is 3. Signup and view all the answers

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

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

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

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

How does CPU utilization relate to scheduling performance?

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

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

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

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

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

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

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

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

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

List two criteria used to evaluate CPU scheduling algorithms.

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

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

14 units. 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)?

26 units. 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?

3 units. Signup and view all the answers

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

41 units. Signup and view all the answers

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

To allocate a small unit of CPU time to each process. 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?

73%. Signup and view all the answers

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

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

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

RT is calculated as 11 - 0 = 11. 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?

14 units. Signup and view all the answers

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

15 units. Signup and view all the answers

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

The formula is ( \tau_{n+1} = \alpha t_n + (1 - \alpha) \tau_n ). 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?

The predicted value is ( \tau_{n+1} = 0.5 \cdot 8 + 0.5 \cdot 10 = 9 ). 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?

The operating system removes the currently running process and allocates the CPU to the newly arrived process. 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?

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

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

A higher ( \alpha ) gives more weight to the most recent CPU burst, making predictions more responsive to recent activity. 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 )?

The final predicted burst time ( T4 ) is 7.5. 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?

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

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

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

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

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

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

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

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

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

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

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. Signup and view all the answers

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

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

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

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

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

The average waiting time was calculated as 7. Signup and view all the answers

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

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

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

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

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

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

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.

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Description

This quiz covers key concepts in CPU scheduling and process synchronization within operating systems. You'll explore various CPU scheduling algorithms, their evaluation criteria, and essential aspects of concurrent programming, including solutions to the critical-section problem. Test your knowledge on these foundational topics in computer science!