Lecture 8: CPU Scheduling 2

Question 1

Shortest-Job-First (SJF) - Non-Preemptive Policy

Answer 1

Rank processes by CPU time requests.

Optimizes (minimizes) average waiting time

Question 2

Shortest-Job-First (SJF) - Preemptive Policy

Answer 2

Currently running process can be preempted if new process arrives with shorter remaining CPU burst

Question 3

Exponential Averaging

Answer 3

Uses “history” of previous CPU bursts to predict length of next CPU burst
tn:Actual length of nth CPU burst
tn+1: Predicted length of next CPU burst
ɑ:“Weight”:
→ How much emphasis to put on recent past
→ 0 ≤ a ≤1

Question 4

States in the Ready Queue

Answer 4

New: Process is being created
Running: Instructions are being executed
Waiting: Process is waiting for some event to occur
Ready: Process is waiting to be assigned the CPU
Terminated: Process has finished execution

Question 5

Priority Policy

Answer 5

•Priority Number (Integer) Associated With Each Process
•CPU Allocated to Process With Highest Priority (Lowest Priority Number)…
3a. Non-preemptive:
Whenever running process blocks or terminates
3b. Preemptive:
Whenever new process arrives or running process blocks or terminates

Question 6

Round-Robin

Answer 6

•Each process gets small unit of CPU time (time quantum).
•After time quantum has elapsed, process is
preempted and added to end of ready queue
•Preemptive only: Process switch based on timer interrupts

Question 7

Multilevel Queues

Answer 7

Ready queue is partitioned into separate queues, each with own scheduling algorithm

Question 8

Inter-queue Scheduling (Scheduling of Queues)

Answer 8

1. Fixed Priority:
   Serve all from high-priority queue before servicing lower-priority
   queue (–: starvation)
2. Time-slice:
   Each queue gets reserved slice of CPU time to schedule amongst its
   processes (e.g.: 80% for hi-priority, 20% low)

Question 9

Scheduling Metrics

Answer 9

• CPU utilization, throughput, turnaround time, wait time, response time
• Also care about: fairness, dispatch time