Slide Bài Giảng Hệ Diều Hành Chương 4 - UIT

01/2015 Duy Phan

Chng 4: nh thi CPU - 1

2 nh thi CPU Duy Phan

Mc tiu

Bit c cc khi nim c bn v nh thi

Bit c cc tiu chun nh thi CPU

Hiu c cc gii thut nh thi

Vn dng cc gii thut nh thi lm bi tp v

m phng


Ni dung

Cc khi nim c bn v nh thi

Cc b nh thi

Cc tiu chun nh thi CPU

Cc gii thut nh thi

First-Come, First-Served (FCFS)

Shortest Job First (SJF)

Shortest Remaining Time First (SRTF)

Round-Robin (RR)

Priority Scheduling


Khi nim c bn

Trong cc h thng multitasking

Thc thi nhiu chng trnh ng thi lm tng hiu sut h thng

Ti mi thi im, ch c mt process c thc thi

= > cn phi gii quyt vn phn chia, la chn process thc thi sao cho c hiu qu nht

-> chin lc nh thi CPU

nh thi CPU

Chn mt process (t ready queue) thc thi

Vi mt multithreaded kernel, vic nh thi CPU l do OS chn kernel thread c chim CPU


Cc b nh thi

ready

running

suspended

ready

suspended

blocked

new

terminated blocked

Long-term

scheduling

Long-term

scheduling

Medium-term

scheduling

Medium-term

scheduling

Short-term

scheduling


Cc b nh thi (tt)

Long-term scheduling

Xc nh chng trnh no c chp nhn np vo h thng thc thi

iu khin mc multiprogramming ca h thng

Long term scheduler thng c gng duy tr xen ln CPU-bound v I/O-bound process

Medium-term scheduling

Process no c a vo (swap in), a ra khi (swap out) b nh chnh

c thc hin bi phn qun l b nh v c tho lun phn qun l b nh


Cc b nh thi (tt)

Short-term scheduling

Xc nh process no trong ready queue s c chim CPU thc thi k tip (cn c gi l nh thi CPU, CPU scheduling)

Short term scheduler cn c gi vi tn khc l dispatcher

B nh thi short-term c gi mi khi c mt trong cc s kin/interrupt sau xy ra:

Ngt thi gian (clock interrupt)

Ngt ngoi vi (I/O interrupt)

Li gi h thng (operating system call)

Signal


Dispatcher

Dispatcher s chuyn quyn iu khin CPU v cho process c chn bi b nh thi ngn hn

Bao gm:

Chuyn ng cnh (s dng thng tin ng cnh trong PCB)

Chuyn ch ngi dng

Nhy n v tr thch hp trong chng trnh ng dng khi ng li chng trnh (chnh l program counter trong PCB)

Cng vic ny gy ra ph tn

Dispatch latency: thi gian m dispatcher dng mt process v khi ng mt process khc



Hng ngi dng (User-oriented)

Thi gian p ng (Response time): khong thi gian process nhn yu cu n khi yu cu u tin c p ng (time-sharing, interactive system) cc tiu

Thi gian quay vng (hon thnh) (Turnaround time): khong thi gian t lc mt process c np vo h thng n khi process kt thc cc tiu

Thi gian ch (Waiting time): tng thi gian mt process i trong ready queue cc tiu


Cc tiu chun nh thi CPU (tt)

Hng h thng (System-oriented)

S dng CPU (processor utilization): nh thi sao cho CPU cng bn cng tt cc i

Cng bng (fairness): tt c process phi c i x nh nhau

Thng lng (throughput): s process hon tt cng vic trong mt n v thi gian cc i


Hai yu t ca gii thut nh thi

Hm chn la (selection function): dng chn process

no trong ready queue c thc thi (thng da trn

u tin, yu cu v ti nguyn, c im thc thi ca

process,)

V d:

w = tng thi gian i trong h thng

e = thi gian c phc v

s = tng thi gian thc thi ca process (bao gm c e)


Hai yu t ca gii thut nh thi (tt)

Ch quyt nh (decision mode): chn thi im thc hin hm chn la nh thi

C hai ch :

Khng trng dng (Non-preemptive)

Khi trng thi running, process s thc thi cho n khi kt thc hoc b blocked do yu cu I/O

Trng dng (Preemptive)

Process ang thc thi (trng thi running) c th b ngt na chng v chuyn v trng thi ready

Chi ph cao hn non-preemptive nhng nh i li bng thi gian p ng tt hn v khng c trng hp mt process c chim CPU qu lu


Preemptive v Non-preemptive

Hm nh thi c thc hin khi

(1)Chuy n t tra ng tha i running sang waiting

(2) Chuy n t tra ng tha i running sang ready

(3) Chuy n t tra ng tha i waiting, new sang ready

(4) K t thu c thc thi

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

Trng hp 1, 4 c go i la i nh thi nonpreemptive

Trng hp 2, 3 c go i la i nh thi preemptive

Thc hin theo c ch no kh hn? Ti sao?


Cc gii thut nh thi




Round-Robin (RR)

Priority Scheduling

Highest Response Ratio Next (HRRN)

Multilevel Queue

Multilevel Feedback Queue


Kho st gii thut nh thi

Service time = thi gian process cn CPU trong mt chu k CPU-I/O

Process c service time ln l cc CPU-bound process

Process Arrival

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

mt chu k CPU-I/O



Hm la chn:

Tin trnh no yu cu CPU trc s c cp pht CPU trc

Process s thc thi n khi kt thc hoc b blocked do I/O

Ch quyt nh: non-preemptive algorithm

Hin thc: s dng hng i FIFO (FIFO queues)

Tin trnh i vo c thm vo cui hng i

Tin trnh c la chn x l c ly t u ca queues

0 5 10 15 20

P1

P2

P3 P4 P5

add run


First-Come, First-Served (FCFS) (tt)

Gi s th t vo ca cc tin trnh l

P1, P2, P3

Thi gian ch

P1 = 0

P2 = 24

P3 = 27

Thi gian ch trung bnh?

(0+24+27)/3 = 17

V d :

Process Burst Time

P1 24

P2 3

P3 3

0 24 27 30

P1 P2 P3

Gantt Chart for Schedule


First-Come, First-Served (FCFS) (tt)

Gi s th t vo ca cc tin trnh l

P2, P3, P1

Thi gian ch

P1 = 6

P2 = 0

P3 = 3

Thi gian ch trung bnh?

(6+0+3)/3 = 3

=> tt hn

V d :

Process Burst Time

P1 24

P2 3

P3 3

0 3 6 30

P2 P3 P1



Shortest-Job-First (SJF)

nh thi biu cng vic ngn nht trc

Khi CPU c t do, n s cp pht cho tin trnh yu

cu t thi gian nht kt thc ( tin trnh ngn nht)

Lin quan n chiu di thi gian s dng CPU cho

ln tip theo ca mi tin trnh

S dng nhng chiu di ny lp lch cho tin trnh

vi thi gian ngn nht


Shortest-Job-First (SJF) (tt)

Scheme 1: Non-preemptive

Khi CPU c trao cho qu trnh n khng nhng cho n khi n kt thc chu k x l ca n

Scheme 2: Preemptive

Nu mt tin trnh mi c a vo danh sch vi chiu di s dng CPU cho ln tip theo nh hn thi gian cn li ca tin trnh ang x l, n s dng hot ng tin trnh hin hnh Shortest-Remaining-Time-First (SRTF)

SJF l ti u cho thi gian ch i trung bnh ti thiu vi mt tp tin trnh cho trc


Non-Preemptive SJF

V d:

Process Arrival TimeBurst Time

P1 0 7

P2 2 4

P3 4 1

P4 5 4

0 8 16

P1 P2 P3


P4

12 7

Average waiting time =

(0+6+3+7)/4 = 4


Preemptive SJF (SRTF)

V d 1:


P1 0 7

P2 2 4

P3 4 1

P4 5 4

0 7 16

P1 P2 P3


P4

11 5

Average waiting time =

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

P2 P1

2 4


P1 0 8

P2 1 4

P3 2 9

P4 3 5

V d 2:


Nhn xt v gii thut SJF

C th xy ra tnh trng i (starvation) i vi cc process c CPU-burst ln khi c nhiu process vi CPU-burst nh n h thng.

C ch non-preemptive khng ph hp cho h thng time sharing (interactive)

Gii thut SJF ngm nh ra u tin theo burst time

Cc CPU-bound process c u tin thp hn so vi I/O-bound process, nhng khi mt process khng thc hin I/O c thc thi th n c chim CPU cho n khi kt thc


Nhn xt v gii thut SJF (tt)

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?


Nhn xt v gii thut SJF (tt)

(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 ng hnh vi ca process trong tng lai

Mt k thut thng dng l s dng trung bnh hm 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 n c xem quan trng nh nhau.


D on thi gian s dng CPU

di CPU burst o c

di CPU burst d on, vi a = v 0 = 10


Priority Scheduling

Mi process s c gn mt u tin

CPU s c cp cho process c u tin cao

nht

nh thi s dng u tin c th:

Preemptive hoc

Nonpreemptive


Priority Scheduling (tt)

SJF l mt gii thut nh thi s dng u tin vi u tin l thi-gian-s-dng-CPU-d-on

Gn u tin cn da vo:

Yu cu v b nh

S lng file c m

T l thi gian dng cho I/O trn thi gian s dng CPU

Cc yu cu bn ngoi v d nh: s tin ngi dng tr khi thc thi cng vic


Priority Scheduling (tt)

Vn : tr hon v hn nh process c u tin thp c th khng bao gi c thc thi

Gii php: lm mi (aging) u tin ca process s tng theo thi gian


Cu hi n tp

C bao nhiu b nh thi? K tn?

Nu cc tiu ch nh thi?

Nu u im v nhc im ca cc gii thut

nh thi FCFS, SJF, SRTF, Priority?


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


Bi tp (tt)


01/2015 Duy Phan

Kt thc chng 4-1

01/2015 Duy Phan

Chng 4: nh thi CPU - 2


Mc tiu

Bit c cc khi nim c bn v nh thi

Bit c cc tiu chun nh thi CPU

Hiu c cc gii thut nh thi

Vn dng cc gii thut nh thi lm bi tp v

m phng


n tp chng 4 - 1


Cc b nh thi


Cc gii thut nh thi




Priority Scheduling


Bi tp chng 4 - 1



Ni dung


Cc b nh thi


Cc gii thut nh thi First-Come, First-Served (FCFS)



Priority Scheduling

Round-Robin (RR)


Multilevel Queue



Round Robin (RR)

Mi process nhn c mt n v nh thi gian CPU (time slice,

quantum time), thng thng t 10-100 msec thc thi

Sau khong thi gian , process b ot quyn v tr v cui

hng i ready

Nu c n process trong hng i ready v quantum time = q th

khng c process no phi ch i qu (n -1)q n v thi gian


Round Robin (RR) (tt)

Hiu sut:

Nu q ln: RR => FCFS

Nu q nh: q khng c qu nh bi v phi tn

chi ph chuyn ng cnh

Thi gian ch i trung bnh ca gii thut RR

thng kh ln nhng thi gian p ng nh



V d: Quantum time = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24

0

P1 P4 P3


P1 P2

20

P3 P3 P3 P4 P1

37 57 77 97 117 121 134 154 162

turn-around time trung bnh ln hn SJF, nhng p ng tt hn



Quantum time = 1:

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

gian p ng trung bnh tt hn

u tin CPU-bound process

I/O-bound

CPU-bound



Quantum time v context switch:

Process time = 10

quantum context

switch

1 2 3 4 5 6 7 8 9 10

10 6

10

12

6

1

0

1

9



Thi gian hon thnh trung bnh (average turnaround time)

khng chc s c ci thin khi quantum ln



Quantum time v response time


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


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

Time slice nn ln trong tng quan so snh vi thi

gian cho process switch

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


Quantum time cho Round Robin (tt)

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


Nhc im ca Round Robin

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.

Mt CPU-bound process chy ht time slice

v li quay tr v hng i ready queue (

pha trc cc process b blocked)


Highest Response Ratio Next

Chn process k tip c gi tr RR (Response ratio) ln nht

Cc process ngn c u tin hn (v service time nh)

timeservice expected

timeservice expected ingspent wait time RR


Multilevel Queue Scheduling

Hng i ready c chia thnh nhiu hng

i ring bit theo mt s tiu chun nh

c im v yu cu nh thi ca process

Foreground (interactive) v background process,

Process c gn c nh vo mt hng i,

mi hng i s dng gii thut nh thi

ring


Multilevel Queue Scheduling (tt)

H iu hnh cn phi nh thi cho cc hng

i.

Fixed priority scheduling: phc v t hng i c u tin

cao n thp. Vn : c th c starvation.

Time slice: mi hng i c nhn mt khong thi gian

chim CPU v phn phi cho cc process trong hng i

khong thi gian . V d: 80% cho hng i foreground

nh thi bng RR v 20% cho hng i background nh thi

bng gii thut FCFS


Multilevel Queue Scheduling (tt)

V d phn nhm cc qu trnh

System Processes

Interactive Processes

Batch Processes

Student Processes

u tin thp nht

u tin cao nht



Vn ca multilevel queue

process khng th chuyn t hng i ny sang hng i khc

khc phc bng c ch feedback: cho php process di chuyn mt cch thch hp gia cc hng i khc nhau


Multilevel Feedback Queue (tt)


Phn loi processes da trn cc c tnh v CPU-burst

S dng decision mode preemptive

Sau mt khong thi gian no , cc I/O-bound process v interactive process s cc hng i c u tin cao hn cn CPU-bound process s cc queue c u tin thp hn

Mt process ch qu lu mt hng i c u tin thp c th c chuyn n hng i c u tin cao hn (c ch nin hn, aging)



V d: C 3 hng i

Q0 , dng RR vi quantum 8 ms

Q1 , dng RR vi quantum 16 ms

Q2 , dng FCFS



nh thi dng multilevel feedback queue i hi phi

gii quyt cc vn sau

S lng hng i bao nhiu l thch hp?

Dng gii thut nh thi no mi hng i?

Lm sao xc nh thi im cn chuyn mt

process n hng i cao hn hoc thp hn?

Khi process yu cu c x l th a vo hng

i no l hp l nht?


So snh cc gii thut

Gii thut nh thi no l tt nht?

Cu tr li ph thuc cc yu t sau:

Tn xut ti vic (System workload)

S h tr ca phn cng i vi dispatcher

S tng quan v trng s ca cc tiu chun nh

thi nh response time, hiu sut CPU,

throughput,

Phng php nh lng so snh


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

(c trong ti liu tham kho sch gc ting Anh)


Tng kt


Cc b nh thi


Hai yu t ca gii thut nh thi


Tng kt (tt)

Cc gii thut nh thi




Round-Robin (RR)

Priority Scheduling


Multilevel Queue



Bi tp

01/2015 Duy Phan

Kt thc chng 4

ch4-1.pdfch4-2.pdf

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Slide Bài Giảng Hệ Diều Hành Chương 4 - UIT