Lecture 7: CPU Scheduling P1

Question 1

Define: CPU-I/O Burst-Cycle

Answer 1

Process execution consists of a cycle of CPU execution and I/O wait time.

Question 2

When does CPU Scheduling decisions occur?

Answer 2

1) When a process switches from running to waiting state
2) When a process switches from running to ready state
3) When a process terminates

Question 3

Define Non-preemptive vs preemptive.

Answer 3

Preemptive: process relinquishes CPU voluntarily after time interval before it finishes or until it finishes if the time it takes to finish is shorter than the time interval
Non-preemptive: process relinquishes CPU only after it finishes

Question 4

Are OS-specific processes (tied to OS services for example) allowed to be preempted?

Answer 4

No

Question 5

List the Scheduling Evaluation Metrics used to evaluate a scheduling algorithms.

Answer 5

1) CPU Utilization
2) Throughput
3) Turnaround Time
4) Waiting Time
5) Response Time
6) Predictability
7) Faireness

Question 6

Define: CPU Utilization Metric

Answer 6

Percentage of time the CPU is not idle

Question 7

Define: Throughput (as a metric for evaluating scheduling algorithms

Answer 7

Completed processes per time unit

Question 8

Define: Turnaround Time

Answer 8

Submission to completion

Question 9

Define: Waiting Time

Answer 9

Time spent on the ready queue

Question 10

Define: Response Time

Answer 10

Response Latency

Question 11

Define: Predictability

Answer 11

Variance in any of 6 other metrics used to evaluate scheduling algorithms

Question 12

Define: Fairness

Answer 12

Minimize process starvation

Question 13

List the Scheduling Algorithms we’ve learned:

Answer 13

1) First Come, First Served (FCFS)
2) Last in, First Out (LIFO)
3) Shortest Job First
4) Round Robin (RR)
5) May use priorities to determine who runs next, (dynamic vs. static algorithms)
6) Multilevel Queueing
7) Multilevel Queueing with Feedback
8) Lottery

Question 14

Describe First Come, First Served (FCFS)

Answer 14

• The process that requests the CPU first is allocated to the CPU first.
• Can calculate average wait time if you know the time duration of the CPU burst for each process
• shorter processes executing first is better because they can then enter the I/O burst while the next process enters a CPU burst ie. the CPU is never idle. If a long slow process enters CPU burst first, then it will leave for I/O bust, along with the next shorter processes leaving for I/O burst resulting in the CPU sitting idle.

Question 15

Describe Last In Last Out (LIFO)

Answer 15

• New process are placed at head of ready queue; improves response time for nerdy created processes.
• This may lead to CPU starvation for the early processes