week 2 (2013-2014) operating system

8/13/2019 Week 2 (2013-2014) Operating System

http://slidepdf.com/reader/full/week-2-2013-2014-operating-system 1/30

ntroduction to Operating Systems

Lecture



Lecture 2 contentsProcess in the hardware.Process States and control blockProcess SchedulingScheduling: Behavior and Algorithms

2



Running process in the

hardware components.

3

Hard Disk :programs are

saved

Slowmechanical device

Main Memory :Processes areloaded in thememory and

ready/waitingfor execution

Running program

Fasterelectronicdevice

Processor :Run instruction by

instruction

Registry : Load a singleinstruction for a singlerunning process

Manages the allocation ofprocesses in the memory

Controls which process torun now in the processor

Cash Memory :Instructions aresaved to increasethe processing

Temporary memory , nearer to

the processor and faster than themain memory.

selected process

Operating System :

Fastest Memoryin the processor

Interrupt

Terminatingrunning process



Process states

4



5

PROCESS CONTROL BLOCK: is a block of data, loaded withthe process in the memory, that contains informationassociated with each process.

It's a data structure holding:

PC, CPU registers,

memory management information,

accounting ( time used, ID, ... )

I/O status ( such as file resources ),

scheduling data ( relative priority, etc. )Process State (so running, ready, etc. is simply a field inthe PCB ).

Process control block



3: Processes 6

The act of Scheduling a process means changing the active PCB pointed to by theCPU.

When CPU switches to another process, the system must save the state of the oldprocess into the memory and load the saved state for the new process from thememory via a context switch.

The CPU switchingfrom one process toanother.

Process Scheduling: Switching



7

Ready Q And IO Q’s

(Process is driven by events that are triggered by needs and availability )

Ready queue = contains those processes that are ready to run.

I/O queue (waiting state ) = holds those processes waiting for I/O service.

Process Scheduling: Queues



In many multitasking systems the processor scheduling subsystemoperates on three levels

Long term scheduling : which determines which programs areadmitted to the system for execution and when, and which onesshould be exited. selects which processes should be brought intothe ready queueShort term scheduling (or dispatching) : which determines whichof the ready processes can have CPU resources, and for how long.

selects which process should be executed next

3: Processes 8

Process Scheduling :

(short/long term scheduling)



Interrupt Handler

ShortTerm

Scheduler

ShortTermScheduler

Long TermScheduler

Long TermScheduler

Process Scheduling :(short/long term scheduling)



Scheduling : definitionInterleave the execution of processes to maximize CPUutilization while providing reasonable response time

The scheduler determines:o Who will runo When it will run

o For how long



Process Scheduling Interleave the execution of processes to maximize CPU utilizationwhile providing reasonable response time.

Fairness : Comparable processes should get comparable service

Efficiency: keep CPU and I/O devices busyResponse time : minimize, for interactive users

Turnaround time: average time for a job to complete

Waiting time: minimize the average over processes

Throughput: number of completed jobs per time unit



Types of Scheduling Non-preemptive scheduling, a process runs until it terminates .Preemptive scheduling, even if the current process is still running,a context switch will occur when its time slice is used up .

Bur st time is an assumption of how long a process requires the CPU between I/O waits. It can not be predicted exactly, before a processstarts.

It means the amount of time a process uses the CPU for a single time.(A process can use the CPU several times before complete the job inthe Preemptive schedul ing )

Process Schedulinga context switch is the process of storing andrestoring the state (context) of a process so thatexecution can be resumed from the same point ata later time.



14

Bursts of CPU usage alternate with periods of I/O waito CPU-bound processeso I/O bound processes

Scheduling : behavior types



Scheduling CriteriaCPU utilization – keep the CPU as busy as possible

Throughput – # of processes that complete theirexecution per time unit

Turnaround time – amount of time to execute a particularprocess

Waiting time – amount of time a process has beenwaiting in the ready queue

Response time – amount of time it takes from when arequest was submitted until the first response isproduced, not output (for time-sharing environment)

15



Scheduling: Algorithms1. FCFS : First-come-first-served (Non-Preempt)

2. SJF: Shortest job first (Non-Preempt)

3. SRTF: Shortest Remaining Time First (Preempt)4. RR : Round robin (Preempt)

5. Priority, Multilevel feedback queues (Preempt)

16



Process Burst time (milli)

P1

P2

P3

24

3

3

If they arrive in the order P1, P2, P3, we get:

0 24 27 30

P1 P2 P3

Average waiting time: (0+24+27) / 3 = 17

Scheduling : 1. FCFS Algorithm FCFS may cause long waiting times




P1

P2P3

24

33

If they arrive in the order P1, P2, P3, average waiting time 17

What if they arrive in the order P2, P3, P1?

0 3 30

P1P2 P3

Average waiting time:

6

(0+3+6) / 3 = 3

FCFS may cause long waiting times (cont’d) Scheduling : 1. FCFS Algorithm



19

The CPU is assigned to the process that has the smallest next CPUburstIn some cases, this quantity is known or can be approximated


ABC

524

D 1

CB

0 5

A Average waiting time: 5.75

7

D

11 12

D AB

0 1

Average waiting time: 2.75 7

C

12

SJF schedule

3

FCFS schedule

Schedu ling : 2. SJF Algorithm



21

Process Arrival timeP1P2P3

024

P4 5

P10

Preemptive SJF schedule

Burst time7415

2

P2

4

P3

7

P4

11 16

P1

Average waiting time: (9+1+0+2) / 4 = 3

P1(5) P2(2)

5

P2

P4(4)

Schedu ling : 3. SRTF Algorithm



Each process gets a small unit of CPU time ( time-quantum ),usually 10-100 milliseconds

Context switch overhead Time-quantum ~ switching time (or smaller)

Fast response : relatively large waste of CPU time

Time-quantum >> switching timelong response (waiting) times, FCFS

Scheduling : 4. Round Robin



23

Context switch overheadContext Switching is time consuming

Choosing a long time quantum to avoid switching:o Deteriorates response time.

o Increases CPU utilization

Choosing a short time quantum:o Faster response

o CPU wasted on switches




24




Select runnable process with highest priority

When there are classes of processes with the samepriority:

o group prioritieso each priority group may uses round robin

schedulingo A process from a lower priority group runs only

if there is no higher priority process waiting

Scheduling : 5. Priority Alg.



26

Multi-Level Queue SchedulingHighest priority

system processes

Interactive editing processes

batch processes

Low-priority batch processes

interactive processes

Lowest priority

Runnableprocesse

Disadvantage?

Schedulin g : 5. Priority Alg.

5 M



27

Dynamic multi level scheduling

The main drawback of priority scheduling: starvation !

To avoid it:

Prevent high priority processes from running indefinitely bychanging priorities dynamically :

o Each process has a base priorityo increase priorities of waiting processes at each clock tick

Approximation SJF schedulingo increase priorities of I/O bound processes

(decrease those of CPU bound processes) priority = 1/f ( f is the fraction of time-quantum used)

5. M

Schedulin g : 5. Priority Alg.



Sample QuestionIn SJF (Shortest Job First) Scheduling method.How to calculate Average Waiting Time and averageTurn-around time?

Is the following Gannt Chart correct ?

28



Operating Systems

Lecture

week 2 (2013-2014) operating system

Documents