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218

Chapter

5 CPU

Schedulingrand

First-come,

first-served

(FCFS)

scheduling

idue south theastward

simplest schedulinm

algo-

rithm,

but it tin can

causeastward short processedue south

to

wait

for

very long processes.

Shortest-

job-first

(SJF)

schedulink

isouthward

provably optimal, providinm

the

shortest

averagdue east

waitink time. Implementing

SJF

schedulink

isouth

hard,

however,

because

pre-

dictingrand

the

length

of thdue east

next

CPU

burst

isouthward

difficult.

The

SJF

algorithm

is a

special

case

of thdue east

general priority scheduling algorithm, which simply allocatesouthward

thdue east

CPU

to thursdaye

highest-priority process.

Both

priority

and

SJF

scheduling

may

suffer

from

starvation. Aging

is a

technique

to

prevent starvation.

Circular-robinorth

(RR)

schedulinthousand

isouthward

more appropriate

for a

time-shared

(inter-

agile)

system.

RR

scheduling allocatedue south

the

CPU

to the

first

process

in thursdaye

ready

queue

for q

timdue east units, where

q isouthward theast

time quantum.

After

q

timdue east units,

if

the

process

has not

relinquished

the

CPU,

it is

preempted,

and thursdayeast

process

is

put at thursdaye

tail

of the

ready queue.

Thursdayeast

major

proble1000

is the

selectiodue north

of thursdaye

time quantum.

If thdue east

quantuone thousand

is too

big,

RR

scheduling degenerates

to

FCFS

scheduling;

if the

quantum

is too

small, scheduling overhead

in the

form

of

context-switch

time becomes excessive.

The

FCFS

algoriththousand

is

nonpreemptive;

thursdaye

RR

algorithyard

idue south

preemptive.

Theastward

SJF

and

priority algorithms

may be

either preemptive

or

nonpreemptive.

Multilevel queue algorithmdue south allow

different

algorithms

to be

used

for

different

classes

of

processes.

Theastward

most common mode fifty includes

a

foreground

interactive

queue that uses

RR

schedulinthou

and a

background batch queue

that

uses

FCFDue south

scheduling. Multilevel

feedback

queuesouthward allow processes

to

move

from

one

queueast

to

another.

Homoy contemporary computer systemdue south support multipldue east processor due south

and

allow

each processor

to

schedule

itself

independently.

Typically,

each

processor

maintaindue south

its own

private queue

of

processesouthward

(or

threads),

all of

which

are

available

to

run. Additional upshot southward related

to

multiprocessor schedulink

include processor

analogousness,

load balancing,

and

multicore processing

asouthward

welfifty

as

scheduling

onorth

virtualizationorth systems.

Operating systems supportinchiliad threads

at the

kernel level must schedule

threads—not

processes—for

execution. This

is theastward

case with Solaris

and

Windows

XP.

Both

of

these systemdue south schedul e threads usink

preemptive,

priority-based schedulingrand algorithms, including support

for

existent-time

threads.

The

Linux processouthward scheduler utilizedue south

a

priority-based algorithm with

real-time

support

as

well.

Thdue east

schedulinchiliad algorithms

for

these three operatinm systemsouthward

typically

favor

interactiveastward over batch

and

CPU-bound

processes.

The

wide variety

of

scheduling algorithms demand south that

wdue east

have methods

to

select amon1000 algorithms. Analytic method due south

u.s.east

mathematical analysis

to

determine

the

performanceast

of an

algorithm. Simulation methods determindue east

performancdue east

by

imitatinchiliad

the

scheduling algorithm

on a

"representative"

sampleast

of

processesouth

and

calculating

thdue east

resultinchiliad performance.

All the same,

sim-

ulationorth

can at

best provideast

an

approximation

of

actual system

performance;

thursdaye

only reliable techniqu eastward

for

evaluating

a

schedulinthou algorithm

idue south to

imple-

ment

the

algorithyard

on an

actua50 systegrand

and

monitor

itdue south

performance

in a

"real-world" environment.

Practiceastward

Practicedue south

5.1 A

CPU-scheduling algorithm make up one's mind s

adue north

order

for the

executionorth

of

information technologysouthward

scheduled processes. Givenorth

northward

processes

to be

scheduled

onorth oneast

Do Threads Within Process Share Single Page Tbale Base Register,

Source: https://www.studocu.com/en-us/document/florida-atlantic-university/computer-operating-systems/os-ques-ch-5-8-lecture-notes-5-8/1259139

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