Chapter04 CPU Scheduling

Chng 4 Lp lch CPU

Th.s: H L Hoi Trung

2Khoa KTMT

n tp chng 3

Process

Trng thi process

To process

Hy Process

C ch giao tip process

3Khoa KTMT

Mc tiu

Ti sao phi nh thi.

Gii thiu cc b lp lch trong h thng a

chng

M t cc gii thut ca b lp lch

Cc tiu chun nh gi b lp lch

4Khoa KTMT

Ni Dung

Khai niem c ban

Cac bo nh thi

long-term, mid-term, short-term

Cac tieu chuan nh thi CPU

Cac giai thuat nh thi

First-Come, First-Served (FCFS) Shortest Job First (SJF) va Shortest Remaining

Time First (SRTF)

Priority Scheduling Round-Robin (RR) Highest Response Ratio Next (HRRN) Multilevel Queue Multilevel Feedback Queue

5Khoa KTMT

Khai niem c ban

Trong cac he thong multitasking

Thc thi nhieu chng trnh ong thi lam tang hieu suat he

thong.

Tai moi thi iem, ch co mot process c thc thi. Do o,

can phai giai quyet van e phan chia, la chon process thc

thi sao cho c hieu qua nhat chien lc nh thi CPU.

nh thi CPU

Chon mot process (t ready queue) thc thi.

Vi mot multithreaded kernel, viec nh thi CPU la do OS

chon kernel thread c chiem CPU.

6Khoa KTMT

Cac bo nh thi

ready

running

suspended

ready

suspended

blocked

new

terminatedblocked

Long-term

scheduling

Long-term

scheduling

Medium-term

scheduling

Medium-term

scheduling

Short-term

scheduling

7Khoa KTMT

Cac bo nh thi

Long-term scheduling

Xac nh chng trnh nao c chap nhan nap vao he thong e thc thi

ieu khien mc o multiprogramming cua he thong

Long term scheduler thng co gang duy tr xen lan CPU-bound va I/O-bound process

Medium-term scheduling

Process nao c a vao (swap in), a ra khoi (swap out) bo nh chnh

c thc hien bi phan quan ly bo nh va c thao luan phan quan ly bo nh.

8Khoa KTMT

Cac bo nh thi (tt)

Short term scheduling

Xac nh process nao trong ready queue se c chiem CPU

e thc thi ke tiep (con c goi la nh thi CPU, CPU

scheduling)

Short term scheduler con c goi vi ten khac la dispatcher

Bo nh thi short-term c goi moi khi co mot trong cac s

kien/interrupt sau xay ra:

Ngt thi gian (clock interrupt)

Ngat ngoai vi (I/O interrupt)

Li goi he thong (operating system call)

Signal

Chng nay se tap trung vao nh thi ngan han

9Khoa KTMT

Dispatcher

Dispatcher se chuyen quyen ieu khien CPU ve cho

process c chon bi bo nh thi ngan han

Bao gom:

Chuyen ng canh (s dung thong tin ng canh trong PCB)

Chuyen che o ngi dung (user mode)

Nhay en v tr thch hp trong chng trnh ng dung e khi ong lai chng trnh (chnh la program counter trong PCB)

Cong viec nay gay ra ph ton

Dispatch latency: thi gian ma dispatcher dng mot process va khi ong mot process khac

10Khoa KTMT

Cac tieu chuan nh thi CPU

User-oriented

Thi gian ap ng (Response time): khoang thi gian process

nhan yeu cau en khi yeu cau au tien c ap ng (time-

sharing, interactive system) cc tieu

Thi gian quay vong (hoan thanh) (Turnaround time): khoang thi

gian t luc mot process c nap vao he thong en khi process

o ket thuc cc tieu

Thi gian ch (Waiting time): tong thi gian mot process i trong

ready queue cc tieu

System-oriented

S dung CPU (processor utilization): nh thi sao cho CPU cang

ban cang tot cc ai

Cong bang (fairness): tat ca process phai c oi x nh nhau

Thong lng (throughput): so process hoan tat cong viec trong

mot n v thi gian cc ai.

11Khoa KTMT

Hai yeu to cua giai thuat nh thi

Ham chon la (selection function): dung e chon

process nao trong ready queue c thc thi (thng

da tren o u tien, yeu cau ve tai nguyen, ac iem

thc thi cua process,), v duw = tong thi gian i trong he thong

e = thi gian a c phuc vu

s = tong thi gian thc thi cua process (bao gom ca e)

12Khoa KTMT

Hai yeu to cua giai thuat nh thi (tt)

Che o quyet nh (decision mode): chon thi iem thc

hien ham chon la e nh thi. Co hai che o

Khong trng dung (Non-preemptive)

Khi trang thai running, process se thc thi cho en khi ket thuc hoac b blocked do yeu cau I/O

Trng dung (Preemptive)

Process ang thc thi (trang thai running) co the b ngat na chng va chuyen ve trang thai ready bi

he ieu hanh

Chi ph cao hn non-preemptive nhng anh oi lai bang thi gian ap ng tot hn v khong co trng

hp mot process oc chiem CPU qua lau.

13Khoa KTMT

Preemptive va Non-preemptive

Hm nh thi c thc hin khi

(1) process Chuyn t trng thi running sang waiting

(2) process Chuyn t trng thi running sang ready

(3) process Chuyn t trng thi waiting, new sang ready

(4) process Kt thuc thc thi

1 v 4 khng cn la chn loi nh thi biu, 2 v 3 cn

Trng hp 1, 4 c gi l nh thi nonpreemptive

Trng hp 2, 3 c gi l nh thi preemptive

Thc hin c ch nao kho hn? Tai sao?

14Khoa KTMT

Khao sat giai thuat nh thi

Service time = thi gian process can CPU trong mot chu ky

CPU-I/O

Process co service time ln la cac CPU-bound process

ProcessArrival

Time

Service

Time

1 0 3

2 2 6

3 4 4

4 6 5

5 8 2

load store

add store

read from file

wait for I/O

inc store

write to file

load store

add store

read from file

wait for I/O

wait for I/O

I/O burst

CPU burst

CPU burst

CPU burst

I/O burst

I/O burst

mot chu ky

CPU-I/O

15Khoa KTMT

1. First-Come First-Served (FCFS)

Ham la chon: Tien trnh nao yeu cau CPU trc se c cap phat

CPU trc; Process se thc thi en khi ket thuc hoac b blocked do I/O

Che o quyet nh: non-preemptive algorithm

Hien thc : s dung hang i FIFO (FIFO queues)

Tien trnh i vao c them vao cuoi hang i

Tien trnh c la chon e x ly c lay t au cua queues

0 5 10 15 20

P1

P2

P3P4P5

addrun

16Khoa KTMT

FCFS Scheduling

V du :

Process Burst Time

P1 24

P2 3

P3 3

Gia s th t vao cua

cac tien trnh la

P1, P2, P3

Thi gian ch

P1 = 0;

P2 = 24;

P3 = 27;

Thi gian ch trung

bnh

(0+24+27)/3 = 17

0 24 27 30

P1 P2 P3

Gantt Chart for Schedule

17Khoa KTMT

FCFS Scheduling

V du:

Process Burst Time

P1 24

P2 3

P3 3

Gia s thi gian vao cua

cac tien trnh la

P2, P3, P1

Thi gian ch :

P1 = 6; P2 = 0; P3 = 3;

Thi gian ch trung bnh

(6+0+3)/3 = 3 , tot hn..

0 3 6 30

P1P2 P3


Liu c xy ra trng hp tr hon v hn nh (starvation hay indefinte blocking) vi gii thut FCFS?

Nhn xt

18Khoa KTMT

2. Shortest-Job-First(SJF) Scheduling

nh thi bieu cong viec ngan nhat trc

Khi CPU c t do, no se cap phat cho tien trnh yeu cau t

thi gian nhat e ket thuc ( tien trnh ngan nhat)

Lien quan en chieu dai thi gian s dung CPU cho lan tiep

theo cua moi tien trnh. S dung nhng chieu dai nay e lap

lch cho tien trnh vi thi gian ngan nhat.

19Khoa KTMT

2. Shortest-Job-First(SJF) Scheduling

Hai hnh thc (Schemes):

Scheme 1: Non-preemptive( tien trnh oc quyen CPU)

Khi CPU c trao cho qua trnh no khong nhng cho en khi no ket thuc chu ky x ly cua no

Scheme 2: Preemptive( tien trnh khong oc quyen)

Neu mot tien trnh CPU mi c a vao danh sach vi chieu dai s dung CPU cho lan tiep theo nho hn thi gian

con lai cua tien trnh ang x ly no se dng hoat ong tien

trnh hien hanh (hnh thc nay con goi la Shortest-

Remaining-Time-First (SRTF).)

SJF la toi u cho thi gian ch i trung bnh toi thieu vi mot tap tien trnh cho trc

20Khoa KTMT

Non-Preemptive SJF Scheduling

V du :

Process Arrival TimeBurst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4

0 8 16

P1 P2P3


P4

127

Average waiting time = (0+6+3+7)/4 = 4

21Khoa KTMT

Preemptive SJF Scheduling

(hay Sortest Remaining Time First - SRTF)

V du 1:Process Arrival TimeBurst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4

0 7 16

P1 P2P3


P4

115

Average waiting time =

(9+1+0+2)/4 = 3

P2 P1

2 4

Process Arrival TimeBurst Time

P1 0 8

P2 1 4

P3 2 9

P4 3 5

VD2:

Thc hin ch no?

22Khoa KTMT

Nhan xet ve giai thuat SJF

Co the xay ra tnh trang oi (starvation) oi vi cac process co CPU-burst ln khi co nhieu process vi CPU-

burst nho en he thong.

C che non-preemptive khong phu hp cho he thong

time sharing (interactive)

Giai thuat SJF ngam nh ra o u tien theo burst time

Cac CPU-bound process co o u tien thap hn so vi

I/O-bound process, nhng khi mot process khong thc

hien I/O c thc thi th no oc chiem CPU cho en khi

ket thuc

23Khoa KTMT


Tng ng vi mi process cn c di ca CPU burst tip theo

Hm la chn: chn process c di CPU burst nh nht

Chng minh c: SJF ti u trong vic gim thi gian i trung

bnh

Nhc im: Cn phi c lng thi gian cn CPU tip theo ca

process

Gii php cho vn ny?

24Khoa KTMT


(Thi gian s dng CPU chnh l di ca CPU burst)

Trung bnh tt c cc CPU burst o c trong qu kh

Nhng thng thng nhng CPU burst cng mi cng phn

nh ung hnh vi ca process trong tng lai

Mt k thut thng dng l s dng trung bnh ham m

(exponential averaging)

n+1 = a tn + (1 - a) n , 0 a 1

n+1 = a tn + (1- a) a tn-1 ++ (1- a)jan-j ++ (1- a)

n+1a0

Nu chn a = th c ngha l tr o c tn v tr d on nc xem quan trng nh nhau.

25Khoa KTMT

D on thi gian s dng CPU

di CPU bursto c

di CPU burst d on,

vi a = v 0 = 10

26Khoa KTMT

3. Priority Scheduling

Moi process se c gan mot o u tien

CPU se c cap cho process co o u tien cao nhat

nh thi s dung o u tien co the:

Preemptive hoac

Nonpreemptive

27Khoa KTMT

Gan o u tien

SJF la mot giai thuat nh thi s dung o

u tien vi o u tien la thi-gian-s-dung-

CPU-d-oan.

Gan o u tien con da vao:

Yeu cau ve bo nh

So lng file c m

T le thi gian dung cho I/O tren thi gian s dung CPU

Cac yeu cau ben ngoai v du nh: so tien ngi dung tra khi thc thi cong viec

28Khoa KTMT

3. Priority Scheduling

Van e: tr hoan vo han nh process co o u tien thap co the khong bao gi c thc thi

Giai phap: lam mi (aging) o u tien cua process se tang theo thi gian

Vd:

IBM, MIT 1973 process 1967.

0 127. Sau 15 tng u tin 1 ln khong bao lu th process c thc thi.

29Khoa KTMT

V d

Thc hin cho cc gii thut FCFS, SJF, priority.

Non preemptive

Preemptive

Process Arrival time Burst time Priority

P1 0 10 3

P2 4 6 2

P3 8 3 1

P4 12 5 4

30Khoa KTMT

4. nh thi luan phien (Round Robin -RR)

Moi process nhan c mot n v nho thi gian CPU

(time slice, quantum time), thong thng t 10-100 msec

e thc thi. Sau khoang thi gian o, process b oat

quyen va tr ve cuoi hang i ready.

Neu co n process trong hang i ready va quantum time

= q th khong co process nao phai ch i qua (n 1)q n v thi gian.

Hieu suat

Neu q ln: RR FCFS

Neu q nho (q khong c qua nho bi v phai ton chi ph chuyen ng canh)

Thi gian ch i trung bnh cua giai thuat RR thng

kha ln nhng thi gian ap ng nho

31Khoa KTMT

Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time

P1 53 53

P2 8 8

P3 68 68

P4 24 24

The Gantt chart is:

32Khoa KTMT


P1 53 33

P2 8 8

P3 68 68

P4 24 24

The Gantt chart is:

P1

0 20

33Khoa KTMT


P1 53 33

P2 8 0

P3 68 68

P4 24 24

The Gantt chart is:

P1

0 20

P2

28

34Khoa KTMT


P1 53 33

P2 8 0

P3 68 48

P4 24 24

The Gantt chart is:

P1

0 20

P2

28

P3

48

35Khoa KTMT


P1 53 33

P2 8 0

P3 68 48

P4 24 4

The Gantt chart is:

P1

0 20

P2

28

P3

48

P4

68

36Khoa KTMT


P1 53 13

P2 8 0

P3 68 48

P4 24 4

The Gantt chart is:

P1

0 20

P2

28

P3

48

P4

68

P1

88

37Khoa KTMT


P1 53 13

P2 8 0

P3 68 28

P4 24 4

The Gantt chart is:

P1

0 20

P2

28

P3

48

P4

68

P1

88

P3

108

38Khoa KTMT


P1 53 0

P2 8 0

P3 68 0

P4 24 0

The Gantt chart is:

P1

0 20

P2

28

P3

48

P4

68

P1

88

P3

108

P4

112

P1

125

P3

145

P3

153

39Khoa KTMT

V du Round Robin

Time Quantum = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24

0

P1 P4P3


P1P2

20

P3 P3 P3P4 P1

37 57 77 97 117 121 134 154 162

turnaround time trung bnh ln hn SJF, nhng ap ng tot hn

40Khoa KTMT

Vi du

SJF - Preemptive

Round Robin - q = 2

Process Arrival

Time

Service

Time

1 0 3

2 2 6

3 4 4

4 6 5

5 8 2

Quantumn time = 3

41Khoa KTMT

RR vi time quantum = 1

Thi gian turn-around trung bnh cao hn so vi SJF nhng

co thi gian ap ng trung bnh tot hn.

u tien CPU-bound process

I/O-bound process thng s dung rat t thi gian cua

CPU, sau o phai blocked i I/O

CPU-bound process tan dung het quantum time, sau o

quay ve ready queue c xep trc cac process b

blocked

42Khoa KTMT

Time quantum va context switch

Process time = 10quantum

context

switch

0 1 2 3 4 5 6 7 8 9 10

0 106

0 10

12

6

1

0

1

9

43Khoa KTMT

Thi gian hon thnh v quantum time

Thi gian hon thnh trung bnh (average turnaround time) khng

chc s c ci thin khi quantum ln

44Khoa KTMT

Quantum va response time

Quantum time phai ln

hn thi gian dung e

x ly clock interrupt

(timer) va thi gian

dispatching

Nen ln hn thi gian

tng tac trung bnh

(typical interaction)

45Khoa KTMT

Quantum time cho Round Robin*

Khi thc hin process switch th OS s s dng CPU ch khng phi process

ca ngi dng (OS overhead)

Dng thc thi, lu tt c thng tin, np thng tin ca process sp thc thi

Performance ty thuc vo kch thc ca quantum time (cn gi l time slice),

v hm ph thuc ny khng n gin

Time slice ngn th p ng nhanh

Vn : c nhiu chuyn ng cnh. Ph tn s cao.

Time slice di hn th throughput tt hn (do gim ph tn OS overhead) nhng

thi gian p ng ln

Nu time slice qu ln, RR tr thnh FCFS.

46Khoa KTMT

Quantum time cho Round Robin

Quantum time v thi gian cho process switch:

Nu quantum time = 20 ms v thi gian cho process switch = 5 ms, nh vy

ph tn OS overhead chim 5/25 = 20%

Nu quantum = 500 ms, th ph tn ch cn 1%

Nhng nu c nhiu ngi s dng trn h thng v thuc loi interactive th s thy p ng rt chm

Ty thuc vo tp cng vic m la chn quantum time

Quantum time nn ln trong tng quan so snh vi thi gian cho process

switch

V d vi 4.3 BSD UNIX, quantum time l 1 giy

47Khoa KTMT

Round Robin

Nu c n process trong hng i ready, v quantum time l q, nh

vy mi process s ly 1/n thi gian CPU theo tng khi c kch

thc ln nht l q

S khng c process no ch lu hn (n - 1)q n v thi gian

RR s dng mt gi thit ngm l tt c cc process u c tm

quan trng ngang nhau

Khng th s dng RR nu mun cc process khc nhau c u tin khc

nhau

48Khoa KTMT

Round Robin: nhc im

Cc process dng CPU-bound vn cn c u tin

V d:

Mt I/O-bound process s dng CPU trong thi gian ngn hn quantum time v b blocked i

I/O. V

Mt CPU-bound process chy ht time slice v li quay tr v hng i ready queue ( pha trc cc

process b blocked)

49Khoa KTMT

5. Highest Response Ratio Next

Chon process ke tiep co gia tr RR (Response ratio) ln

nhat.

Cac process ngan c u tien hn (v service time nho)

timeservice expected

timeservice expected ingspent wait time RR

50Khoa KTMT

Highest Response Ratio Next

Process Arrival

Time

Service

Time

1 0 3

2 2 6

3 4 4

4 6 5

5 8 2

Quantumn time = 3

51Khoa KTMT

6. Multilevel Queue Scheduling

Hang i ready c chia thanh nhieu hang i rieng

biet theo mot so tieu chuan nh

ac iem va yeu cau nh thi cua process

Foreground (interactive) va background process,

Process c gan co nh vao mot hang i, moi hang

i s dung giai thuat nh thi rieng

He ieu hanh can phai nh thi cho cac hang i.

Fixed priority scheduling: phuc vu t hang i co o u tien cao en thap. Van e: co the co starvation.

Time slice: moi hang i c nhan mot khoang thi gian chiem CPU va phan phoi cho cac process trong hang i khoang thi

gian o. V du:

80% cho hang i foreground nh thi bang RR.

20% cho hang i background nh thi bang giai thuat FCFS.

52Khoa KTMT

Multilevel Queue Scheduling*

V d phn nhm cc qu trnh

System Processes

Interactive Processes

Batch Processes

Student Processes

u tin thp nht

u tin cao nht

53Khoa KTMT

7. Hang i phan hoi a cap

Multilevel Feedback Queue

Van e cua multilevel queue

process khong the chuyen t hang i nay sang hang i khac khac phuc bang c che feedback: cho phep process di chuyen mot cach thch hp gia cac hang i

khac nhau.

Multilevel Feedback Queue

Phan loai processes da tren cac ac tnh ve CPU-burst

S dung decision mode preemptive

Sau mot khoang thi gian nao o, cac I/O-bound process va interactive process se cac hang i co o u tien cao

hn con CPU-bound process se cac queue co o u tien

thap hn.

Mot process a ch qua lau mot hang i co o u tien thap co the c chuyen en hang i co o u tien cao

hn (c che nien han, aging).

54Khoa KTMT

7. Multilevel Feedback Queue

V du: Co 3 hang i

Q0 , dung RR vi quantum 8 ms

Q1 , dung RR vi quantum 16 ms

Q2 , dung FCFS

55Khoa KTMT

V d

iu phi u tin nhiu cp xoay vng, s dng 2 cp: Cp 1 s

dng gii thut robin round vi quantumn = 3ms. Cp 2 s dng

gii thut SRTF. Mt process nu cp I 5ms s c chuyn

xung cp II nu ang trng thi waiting cn nu ang trng

thi running th sau khi ra khi s chuyn. Ngc li mt process

ang cp II sau khong thi gian 10ms s c chuyn ln I.

Khi cc process vo b nh chnh th iu vo hng i cp I

56Khoa KTMT

7. Multilevel Feedback Queue (tt)

nh thi dung multilevel feedback queue oi hoi phai

giai quyet cac van e sau

So lng hang i bao nhieu la thch hp?

Dung giai thuat nh thi nao moi hang i?

Lam sao e xac nh thi iem can chuyen mot

process en hang i cao hn hoac thap hn?

Khi process yeu cau c x ly th a vao hang i

nao la hp ly nhat?

57Khoa KTMT

So sanh cac giai thuat

Giai thuat nh thi nao la tot nhat?

Cau tra li phu thuoc cac yeu to sau:

Tan xuat tai viec (System workload)

S ho tr cua phan cng oi vi dispatcher

S tng quan ve trong so cua cac tieu chuan nh thi nh response time, hieu suat CPU, throughput,

Phng phap nh lng so sanh

58Khoa KTMT

c thm

Policy v Mechanism

nh thi trn h thng multiprocessor

nh gi gii thut nh thi CPU

nh thi trong mt s h iu hnh thng dng

Ngun:

Operating System Concepts. Sixth Edition. John Wiley & Sons, Inc.

2002. Silberschatz, Galvin, Gagne

59Khoa KTMT

Bi tp

S dng cc gii thut FCFS, SJF, SRTF, Priority tnh cc gi tr thi gian i, thi gian p ng v thi gian hon thnh trung bnh

60Khoa KTMT

Bi tp (tt)

S dng cc gii thut FCFS, SJF, SRTF, Priority tnh cc gi tr thi gian i, thi gian p ng v thi gian hon thnh trung bnh

61Khoa KTMT

Bi tp

Xt 1 tp cc process sau c thi gian thc thi CPU tnh bng mili

giy:

Gi s th t n thc thi ca cc process l P1, P2, P3, P4, P5.

(tt c process ny u n ti thi im bng 0).

1. V s Gautt thc thi ca cc process theo gii thut nh thi:

FCFS, SJF, RR (quantumn = 1), Priority (cng nh cng u tin,

cng u tin xt Pi v Pj, Pi u tin hn nu i < j).

2. Thi gian i ca cc process trong tng gii thut nh thi?

Process Burst - time Priority

P1 10 3

P2 1 1

P3 2 3

P4 1 4

p5 5 2

62Khoa KTMT

Bi tp

3. Tnh thi gian hon thnh(turnaround time) ca tng process cho

tng gii thut?

Trong cc gii thut sau, gii thut no c th gy ra trng hp 1

process c th khng bao gi c thc thi:

1. First come, first serve

2. Shortest job first

3. Round robin

4. Priority.

Gii thch l do ti sao xy ra trng hp trn?

63Khoa KTMT

Bi tp

1. RR vi q = 2

2. Preemptive Priority vi s cng ln cng u tin

3. iu phi u tin nhiu cp xoay vng, s dng 2 cp: Cp 1 s

dng gii thut robin round vi quantumn = 3ms. Cp 2 s dng

gii thut SRTF. Mt process nu cp I 5ms s c chuyn

xung cp II nu ang trng thi waiting cn nu ang trng

thi running th sau khi ra khi s chuyn. Ngc li mt process

ang cp II sau khong thi gian 10ms s c chuyn ln I.

Khi cc process vo b nh chnh th iu vo hng i cp I.

64Khoa KTMT

Bi tp

Cho 4 tin trnh A, B, C, D vi thi gian vo ready list v thigian cn CPU cho cc ln th 1, th 2, th 3 v thi gian thc hinI/O tng ng nh bn sau:

V gin Gantt, Tnh thi gian i trung bnh, Thi gian p ngtrung bnh, Thi gian lu li trong h thng trung bnh cho ccgii thut (cu 1, 2 ch dng thi gian thc thi 1st execution time nh l burst time)

1. FCFS

2. RR vi q = 3

3. SRFT cho c 3 ln exec

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Chapter04 CPU Scheduling