Operating System - Process Scheduling

Question 1

Maximise CPU use

Answer 1

Quickly switch processes onto CPU for time sharing

Question 2

Under what condition does the process “gives” up the CPU?

Answer 2

• I/O requests

-After N units of time have elapsed

Question 3

Ready Queue

Answer 3

Once a process gives up the CPU it is added to a ready queue

Question 4

Progress scheduler

Answer 4

Selects among available processes in the ready queue for next execution on CPU

Question 5

Queing diagram

Answer 5

Represents queue, resources, flows

Question 6

Job queue

Answer 6

Set all processes in the system

Question 7

Ready Queue

Answer 7

Set of all processes residing in main memory, ready and waiting to execute

Question 8

Device queues

Answer 8

Set of processes waiting for an I/O device

Question 9

How are the scheduling queues maintained

Answer 9

The operating system maintains scheduling queues of processes.

Processes migrate among the various queues.

Question 10

Short-term scheduler (cpu scheduler)

Answer 10

Selects which process should be executed next and allocates a CPU

Sometimes the only scheduler in the program

Is invoked frequently (milliseconds) must be fast

Question 11

Long-term scheduler (Job scheduler) selects which processes should be brought into the ready queue

Answer 11

Long-term scheduler is invoked infrequently(seconds, minutes) => may be slow

The long-term scheduler controls the degree of multiprogramming

Question 12

Types of processes

Answer 12

-I/O bound process - spends more time doing I/O than computations, many short CPU bursts

-CPU-bound process - spends more time doing computations, few very long CPU bursts

Question 13

Context Switch

Answer 13

When the CPU switches to another process, the system must save the state of the old process and load the saved state for the new process via a context switch

Question 14

Dispatch Events

Answer 14

The events which will cause the OS to switch from one process to another

Question 15

Clock Interrupt

Answer 15

A type of Dispatch Event

Occurs after a specified time interval usually 3-10ms

Execution of processes interrupted, control goes back to OS

The current process is added to the ready queue

Frequency of such an interrupt important system parameter

Balance overhead of context switch vs responsiveness

Question 16

Memory Fault/ Page fault

Answer 16

Another type of dispatch event

Executing process refers to a virtual memory address that is not allocated a physical memory location (data still on disk)

Currently executing process is switched to blocked state

Switch to another process from the ready queue

When I/O completed blocked process moves back to ready queue

Question 17

Dispatcher

Answer 17

An OS function that allocates CPU to processes, switches CPU from one process to the next