Lecture 5

Question 1

What is context switching?

Answer 1

In a single‐processor, multi‐processing systems, processes time‐share the CPU resources, called context switching

Question 2

What is a CPU schedular?

Answer 2

A CPU Schedular (e.g. Short‐term Schedular) fetches processes from Ready‐Queue and allocates

Question 3

What is the aim of processor scheduling?

Answer 3

Aim is to assign processes to be executed by the processor in a way that meets system objectives, such as response time, throughput, and processor efficiency

Question 4

What are the 3 different types of processor scheduling?

Answer 4

Long term scheduling

Medium term scheduling

Short term scheduling

Question 5

What does the Long-term schedular do?

Answer 5

Determines which programs are admitted to the system for processing

Controls the degree of multi-programming

Question 6

What does the Medium-Term Scheduling do?

Answer 6

Part of the swapping function
• Swapping-in decisions are based on the need to
manage the degree of multi-programming
• Considers the memory requirements of the
swapped-out processes

Question 7

What does the short-Term Scheduling do?

Answer 7

• Known as the dispatcher
• Executes most frequently
• Makes the fine-grained decision of which process to execute next

Question 8

What is the short term scheduling criteria for User-oriented?

Answer 8

Response time in an interactive system

Relate to other behaviour of the system as perceived
by the individual user or process

Question 9

What is the short term scheduling criteria for System-oriented?

Answer 9

Performance: Focus is on effective and efficient utilization of the processor (rate at which processes
are completed)
 Minor importance on single-user systems

Question 10

What is nonpreemptive in Decision mode?

Answer 10

Nonpreemptive

• Once a process is in the running state, it will continue until it terminates or blocks itself for I/O

Question 11

Name 6 Scheduling algorithms

Answer 11

First-Come-First -Serve(FIFO)
Shortest job first
PRIOIRTY scheduling
Shortest remaining time
Round robin
Multi queue scheduling

Question 12

What is Preemptive in Decision mode?

Answer 12

Currently running process may be interrupted and moved to ready state by the OS

Question 13

What is the First-Come-First-Served (FCFS) algorithm?

Answer 13

FIFO(queue)

The process that has been in the ready queue the longest is selected for running

Question 14

Some principles of the FCFS Algorithm?

Answer 14

Performs much better for long processes than short ones

Tends to favour processor-bound processes over I/O bound processes

Question 15

What is the shortest job first Algorithm?

Answer 15

• A short process will jump to the head of the queue

* Possibility of starvation for longer processes

Question 16

2 Cons of Shortest job first Algorithm?

Answer 16

One difficulty is the need to know, or at least estimate, the required processing time of each process

If the programmer’s estimate is substantially under the actual running time, the system may abort the job

Question 17

What is the priority Scheduling Algorithm?

Answer 17

Always the process with the highest priority is being executed first

Uses one queue for each priority level.

Question 18

Cons of Priority Scheduling algorithm?

Answer 18

• Problem: Indefinite blocking or starvation
 A PS algorithm can leave some low-priority processes waiting indefinitely
 Low priority processes may wait too long to run on lightly loaded time,
 In a heavenly loaded system low-priority processes may never run due to system crash.

Question 19

What is the solution to the problems with Priority scheduling?

Answer 19

Solution: Aging

Question 20

What is the shortest remaining time first Algorithm?

Answer 20

Scheduler always chooses the process that has the shortest expected remaining processing time

Question 21

Con of Shortest remaining time first algorithm?

Answer 21

Risk of starvation of longer processes

Question 22

What is the round robin Algorithm?

Answer 22

Uses preemption based on a clock

• Also known as time slicing because each process is given a slice of time before being preempted

Question 23

1 advantage of round robin algorithm?

Answer 23

Particularly effective in a general-purpose time-sharing

system or transaction processing system

Question 24

1 con of Round Robin algorithm

Answer 24

Its relative treatment of processor-bound and I/O-bound processes

Question 25

What is Rate‐Monotonic Scheduling (RMS) in terms of Real Time Scheduling?

Answer 25

More frequent tasks are given higher priority

Question 26

What is Earliest Deadline First Scheduling (EDF) in terms of Real Time Scheduling?

Answer 26

Task with earliest deadline gets higher priority

Question 27

What is traditional UNIX Scheduling?

Answer 27

• Designed to provide good response time for interactive users while ensuring that low-priority background jobs do not starve
• Employs multilevel feedback using round robin within each of the priority queues