Lecture 11

Question 1

Whats the objective of multi programming?

Answer 1

The objective of multiprogramming is to have some process running at all times, to maximize CPU utilization.

Question 2

What’s the idea of multi-programming?

Answer 2

1. Several processes are kept in memory at one time.
2. When one process has to wait, the operating system takes the CPU away from that process and gives the CPU to another process.

Question 3

What happens on a multi-core system?

Answer 3

On a multicore system, this concept of keeping the CPU busy is extended to all processing cores on the system.

Question 4

What are the states of process execution?

Answer 4

Process execution consists of a cycle of CPU execution(CPU burst) and I/O wait(I/O burst)

Question 5

How does a CPU schedular work?

Answer 5

1. Whenever the CPU becomes idle, the CPU scheduler selects one of the processes in the ready queue to be executed.
2. It then allocates the CPU to that process.

Question 6

The records in the queues are generally ___________ of the processes.

Answer 6

Process Control Blocks

Question 7

What are the four circumstances in which the CPU scheduling decisions take place?

Answer 7

1. When a process switches from the running state to the waiting state
2. When a process switches from the running state to the ready state
3. When a process switches from the waiting state to the ready state
4. When a process terminates

Question 8

What’s a non-preemptive scheme?

Answer 8

Under non-preemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases it either by terminating or by switching to the waiting state.

Question 9

Preemptive scheduling can result in ________ when data are shared among several processes.

Answer 9

race conditions

Question 10

What is a dispatcher?

Answer 10

The dispatcher is a module that gives control of the CPU’s core to the process selected by the CPU scheduler.

Question 11

What is dispatch latency?

Answer 11

The time it takes for the dispatcher to stop one process and start another running is known as the dispatch latency.

Question 12

What functions are performed by a dispatcher?

Answer 12

This function involves the following:
1. Switching context from one process to another
2. Switching to user mode
3. Jumping to the proper location in the user program to resume that program

Question 13

What are the criteria for comparing CPU- scheduling algorithms?

Answer 13

1. CPU utilization(maximizing)
2. Throughput(maximizing)
3. Turnaround time(minimizing)
4. Waiting time(minimizing)
5. Response time(minimizing)

Question 14

Discuss the FCFS algorithms of CPU scheduling.

Answer 14

Objective:
-The process that requests the CPU first is allocated the CPU first.
Implementation:
It is implemented with a FIFO queue.
1. When a process enters the ready queue, its PCB is linked to the tail of the queue.
2. When the CPU is free, it is allocated to the process at the head of the queue.
3. The running process is then removed from the queue.

Question 15

What is the disadvantage of the FCFS policy?

Answer 15

The average waiting time under the FCFS policy is often quite long.

Question 16

Discuss the shortest-job-first scheduling algorithm.

Answer 16

Objective:
It associates with each process the length of the process’s next CPU burst.
Implementation:
1. When the CPU is available, it is assigned to the process that has the smallest next CPU burst.
2. If the next CPU bursts of two processes are the same, FCFS scheduling is used to break the tie.
3. Note that a more appropriate term for this scheduling method would be the shortest-next-CPU-burst algorithm because scheduling depends on the length of the next CPU burst of a process, rather than its total length.

Question 17

What is the disadvantage of the SJF algorithm?

Answer 17

Although the SJF algorithm is optimal, it cannot be implemented at the level of CPU scheduling, as there is no way to know the length of the next CPU burst.

Question 18

How can we approximate SJF scheduling?

Answer 18

We may not know the exact length of the next CPU burst but we can predict its value. We expect the next CPU burst to be similar in length to the previous ones and then pick the process with the shortest predicted CPU burst.

Question 19

Preemptive scheduling is sometimes called ____________

Answer 19

shortest-remaining-time-first scheduling.