chapter06 deadlocks

Khoa KTMT 1

Chng 6 : Tac nghen(Deadlock)

Mo hnh he thong

nh ngha

ieu kien can cua deadlock

Resource Allocation Graph (RAG)

Phng phap giai quyet deadlock

Deadlock prevention

Deadlock avoidance

Deadlock detection

Deadlock recovery

Phng phap ket hp e giai quyet Deadlock

Khoa KTMT 2

Van e deadlock trong he thong

Tnh huong: mot tap cac process b blocked, moi process gi tai

nguyen va ang ch tai nguyen ma process khac trong tap ang gi.

V du 1

Gia s he thong co 2 file tren a.

P1 va P2 moi process ang m mot file va yeu cau m file kia.

V du 2

Semaphore A va B, khi tao bang 1

P0 P1

wait(A); wait(B);

wait(B); wait(A);

Khoa KTMT 3

Mo hnh hoa he thong

He thong gom cac loai tai nguyen, k hieu R1, R

2,, R

m , bao gom:

CPU cycle, khong gian bo nh, thiet b I/O, file, semaphore,

Moi loai tai nguyen Rico W

ithc the (instance).

Gia s tai nguyen tai s dung theo ky (Serially Reusable

Resources)

Yeu cau (request): process phai ch neu yeu cau khong c ap ng ngay

S dung (use): process s dung tai nguyen

Hoan tra (release): process hoan tra tai nguyen

Cac tac vu yeu cau (request) va hoan tra (release) eu la system

call. V du

request/release device

open/close file

allocate/free memory

wait/signal

Khoa KTMT 4

nh ngha

Mot tien trnh goi la deadlocked neu no ang i mot

s kien ma se khong bao gi xay ra.

Thong thng, co nhieu hn mot tien trnh b lien quan trong

mot deadlock.

Mot tien trnh goi la tr hoan vo han nh (indefinitely

postponed) neu no b tr hoan mot khoang thi gian dai

lap i, lap lai trong khi he thong ap ng cho nhng

tien trnh khac .

i.e. Mot tien trnh san sang e x ly nhng no khong bao gi nhan c CPU.

Khoa KTMT 5

ieu kien can e xay ra deadlock

Bon ieu kien can (necessary condition) e xay ra

deadlock

1. Loai tr ho tng (Mutual exclusion): t nhat mot tai

nguyen c gi theo nonsharable mode (v du: printer;

v du sharable resource: read-only files).

2. Gi va ch cap them tai nguyen (Hold and wait): mot

process ang gi t nhat mot tai nguyen va i them tai

nguyen do qua trnh khac ang gi.

Khoa KTMT 6

ieu kien can e xay ra deadlock (tt)

3. Khong trng dung (No preemption): (= no resource

preemption) tai nguyen khong the b lay lai, ma ch co

the c tra lai t process ang gi tai nguyen o khi

no muon.

4. Chu trnh i (Circular wait): ton tai mot tap {P0,,P

n} cac

qua trnh ang i sao cho

P0

i mot tai nguyen ma P1

ang gi

P1


ang gi

Pn


ang gi

Khoa KTMT 7

Resource Allocation Graph (tt)

Ky hieu

Process:

Loai tai nguyen vi 4 thc the:

Piyeu cau mot thc the cua R

j:

Piang gi mot thc the cua R

j:

Pi

Pi

Pi

Rj

Rj

Rj

Khoa KTMT 8

o th cap phat tai nguyen

Resource Allocation Graph

Resource allocation graph (RAG) la o th co hng, vi

tap nh V va tap canh E

Tap nh V gom 2 loai:

P = {P1, P

2,, P

n } (Tat ca process trong he thong)

R = {R1, R

2,, R

m } (Tat ca cac loai tai nguyen trong he thong)

Tap canh E gom 2 loai:

Canh yeu cau (Request edge): PiR

j

Canh cap phat (Assignment edge): RjP

i

Khoa KTMT 9

V du ve RAG

R1 R3

P1 P2 P3

R2 R4

Khoa KTMT 10

V du ve RAG (tt)

R1 R3

P1 P2 P3

R2 R4

Deadlock xay ra!

Khoa KTMT 11

RAG va deadlock

V du mot RAG cha chu trnh nhng khong xay ra

deadlock: P4

co the tra lai instance cua R2.

R1

P1

P2

P3R2

P4

L do v sao khng

xy ra deadlock ?

Khoa KTMT 12

V d

R1 R3

P1 P2 P3

R2R4

Deadlock ?

s lng P, R?

Xc nh Pi dang nm gi ti nguyn

no? Yu cu ti nguyn no?

Xt iu kin cn deadlock? K 1, 2, 4

Khoa KTMT 13

RAG va deadlock (tt)

RAG khong cha chu trnh (cycle) khong co deadlock

RAG cha mot (hay nhieu) chu trnh

Neu moi loai tai nguyen ch co mot thc the deadlock

Neu moi loai tai nguyen co nhieu thc the co the xay ra deadlock

Khoa KTMT 14

Cac phng phap giai quyet deadlock (1)

Ba phng phap

1) Bao am rang he thong khong ri vao tnh trang

deadlock bang cach ngan (preventing) hoac tranh

(avoiding) deadlock.

Khac biet

Ngan deadlock: khong cho phep (t nhat) mot trong 4 ieu kien can cho deadlock

Tranh deadlock: cac qua trnh can cung cap thong tin ve tai nguyen no can e he thong cap phat tai nguyen mot

cach thch hp

Khoa KTMT 15

Cac phng phap giai quyet deadlock (2)

2) Cho phep he thong vao trang thai deadlock,

nhng sau o phat hien deadlock va phuc hoi he

thong.

3) Bo qua moi van e, xem nh deadlock khong

bao gi xay ra trong he thong.

Kha nhieu he ieu hanh s dung phng phap nay.

Deadlock khong c phat hien, dan en viec giam hieu suat cua he thong. Cuoi cung, he thong co the

ngng hoat ong va phai c khi ong lai.

Khoa KTMT 16

1. Ngan deadlock (deadlock prevention)

Ngan deadlock bang cach ngan mot trong 4 ieu kien

can cua deadlock

1. Ngan mutual exclusion

oi vi nonsharable resource (vd: printer): khong lam c

oi vi sharable resource (vd: read-only file): khong can thiet

Khoa KTMT 17

Ngan deadlock (tt)

2. Ngan Hold and Wait

Cach 1: moi process yeu cau toan bo tai nguyen can thiet mot lan. Neu co u tai nguyen th he thong se cap phat, neu khong

u tai nguyen th process phai b blocked.

Cach 2: khi yeu cau tai nguyen, process khong c gi bat ky tai nguyen nao. Neu ang co th phai tra lai trc khi yeu cau.

V du e so sanh hai cach tren: mot qua trnh copy d lieu t tape drive sang disk file, sap xep disk file, roi in ket qua ra

printer.

Khuyet iem cua cac cach tren:

Hieu suat s dung tai nguyen (resource utilization) thap

Qua trnh co the b starvation

Khoa KTMT 18

Ngan deadlock (tt)

3. Ngan No Preemption: neu process A co gi tai nguyen va ang

yeu cau tai nguyen khac nhng tai nguyen nay cha cap phat ngay

c th

Cach 1: He thong lay lai moi tai nguyen ma A ang gi

A ch bat au lai c khi co c cac tai nguyen a b lay lai cung vi tai nguyen ang yeu cau

Cach 2: He thong se xem tai nguyen ma A yeu cau

Neu tai nguyen c gi bi mot process khac ang i them tai nguyen, tai nguyen nay c he thong lay lai va

cap phat cho A.

Neu tai nguyen c gi bi process khong i tai nguyen, A phai i va tai nguyen cua A b lay lai. Tuy nhien he

thong ch lay lai cac tai nguyen ma process khac yeu cau

Khoa KTMT 19

Ngan deadlock (tt)

4. Ngan Circular Wait: gan mot th t cho tat ca cac tai nguyen trong

he thong.

Tap hp loai tai nguyen: R={R1, R

2,,R

m }

Ham anh xa: F: R->N

V du: F(tape drive) = 1, F(disk drive) = 5, F(printer) = 12

F la ham nh ngha th t tren tap cac loai tai nguyen.

Khoa KTMT 20

Ngan deadlock (tt)

4. Ngan Circular Wait (tt)

Moi process ch co the yeu cau thc the cua mot loai tai nguyen theo th t tang dan (nh ngha bi ham F) cua loai tai nguyen. V du

Chuoi yeu cau thc the hp le: tape drive disk drive printer

Chuoi yeu cau thc the khong hp le: disk drive tape drive

Khi mot process yeu cau mot thc the cua loai tai nguyen Rjth no phai

tra lai cac tai nguyen Rivi F(R

i) > F(R

j).

Chng minh gia s ton tai mot chu trnh deadlock

F(R4) < F(R

1)

F(R1) < F(R

2)

F(R2) < F(R

3)

F(R3) < F(R

4)

Vay F(R4) < F(R

4), mau thuan!

P1

R1

P2

P4 P3

R3

R2R4

Khoa KTMT 21

2. Tranh tac nghen

Deadlock avoidance

Deadlock prevention s dung tai nguyen khong hieu qua.

Deadlock avoidance van am bao hieu suat s dung tai

nguyen toi a en mc co the.

Yeu cau moi process khai bao so lng tai nguyen toi a can e thchien cong viec

Giai thuat deadlock-avoidance se kiem tra trang thai cap phat tai nguyen(resource-allocation state) e bao am he thong khong ri vao deadlock.

Trang thai cap phat tai nguyen c nh ngha da tren so tai nguyencon lai, so tai nguyen a c cap phat va yeu cau toi a cua cac

process.

Khoa KTMT 22

Trang thai safe va unsafe

Mot trang thai cua he thong c goi la an toan (safe)

neu ton tai mot chuoi (th t) an toan (safe sequence).

Mot chuoi qua trnh la mot chuoi an toan

neu

Vi moi i = 1,,n, yeu cau toi a ve tai nguyen cua Pico the

c thoa bi

tai nguyen ma he thong ang co san sang (available)

cung vi tai nguyen ma tat ca Pj, j < i, ang gi.

Mot trang thai cua he thong c goi la khong an toan

(unsafe) neu khong ton tai mot chuoi an toan.

Khoa KTMT 23

Chuoi an toan (tt)

V du: He thong co 12 tape drives va 3 qua trnh P0, P

1, P

2

Tai thi iem t0

Con 3 tape drive san sang.

Chuoi la chuoi an toan he thong la an toan

Maximum

needs

Current

needs

P0

10 5

P1

4 2

P2

9 2

Khoa KTMT 24

Chuoi an toan (tt)

Gia s tai thi iem t1, P

2yeu cau va c cap phat 1

tape drive

con 2 tape drive san sang

He thong con an toan khong?

P0

10 5

P1

4 2

P2

9 3

can toi a ang gi

Khoa KTMT 25

Trang thai safe/unsafe va deadlock

Neu he thong ang trang thai safe khong deadlock.

Neu he thong ang trang thai unsafe co the dan en deadlock.

Tranh deadlock bang cach bao am he thong khong i en trang

thai unsafe.

safe

deadlock unsafe

Khoa KTMT 26

Giai thuat o th cap phat tai nguyen

Khai niem canh thnh cau

P1

P2

P1

P2

R1

R2

R1

R2

Khoa KTMT 27

Giai thuat banker

Ap dung cho he thong cap phat tai nguyen trong o moi

loai tai nguyen co the co nhieu instance.

Bat chc nghiep vu ngan hang (banking)

ieu kien

Moi process phai khai bao so lng thc the (instance) toi a

cua moi loai tai nguyen ma no can

Khi process yeu cau tai nguyen th co the phai i mac du tai

nguyen c yeu cau ang co san

Khi process a co c ay u tai nguyen th phai hoan tra trong

mot khoang thi gian hu han nao o.

Khoa KTMT 28

Giai thuat banker (tt)

n: so process, m: so loai tai nguyen

Cac cau truc d lieu

Available: vector o dai m

Available[ j ] = k loai tai nguyen Rjco k instance san sang

Max: ma tran n m

Max[ i, j ] = k qua trnh Piyeu cau toi a k instance cua loai

tai nguyen Rj

Allocation: ma tran n m

Allocation[i, j] = k Pi a c cap phat k instance cua Rj

Need: ma tran n m

Need[i, j] = k Pi can them k instance cua Rj

Nhan xet: Need[i, j] = Max[i, j] Allocation[i, j]

Ky hieu Y X Y[i] X[i], v du (0, 3, 2, 1) (1, 7, 3, 2)

Khoa KTMT 29


1.Giai thuat an toan

Tm mot chuoi an toan

1. Goi Work va Finish la hai vector o dai la m va n. Khi tao

Work := Available

Finish[ i ] := false, i = 1,, n

2. Tm i thoa

(a) Finish[ i ] = false

(b) Needi

Work (hang th i cua Need)

Neu khong ton tai i nh vay, en bc 4.

3. Work := Work + Allocationi

Finish[ i ] := true

quay ve bc 2.

4. Neu Finish[ i ] = true, i = 1,, n, th he thong ang trang thai safe

Thi gian chay cua giai thuat la O(mn2)

Khoa KTMT 30


2. Giai thuat yeu cau (cap phat) tai nguyen

Goi Requestila request vector cua process P

i.

Requesti[ j ] = k P

i can k instance cua tai nguyen R

j.

1. Neu Requesti Need

ith en bc 2. Neu khong, bao

loi v process a vt yeu cau toi a.

2. Neu Requesti Available th qua bc 3. Neu khong, P

i

phai ch v tai nguyen khong con u e cap phat.

3. Gia nh cap phat tai nguyen ap ng yeu cau cua Pi

bang cach cap nhat trang thai he thong nh sau:

Available := Available Requesti

Allocationi:= Allocation

i+ Request

i

Needi

:= Needi Request

i

Khoa KTMT 31


2.Giai thuat yeu cau tai nguyen

Ap dung giai thuat kiem tra trang thai an toan len trang thai tren

Neu trang thai la safe th tai nguyen c cap thc s cho Pi.

Neu trang thai la unsafe th Piphai i, va

phuc hoi trang thai:

Available := Available + Requesti

Allocationi:= Allocation

i Request

i

Needi

:= Needi+ Request

i

Khoa KTMT 32

Giai thuat kiem tra trang thai an toan V du

Co 5 process P0

,, P4

Co 3 loai tai nguyen: A (co 10 instance), B (5 instance) va C (7

instance).

S o cap phat trong he thong tai thi iem T0

Allocation Max Available Need

A B C A B C A B C A B C

P0

0 1 0 7 5 3 3 3 2 7 4 3

P1

2 0 0 3 2 2 1 2 2

P2

3 0 2 9 0 2 6 0 0

P3

2 1 1 2 2 2 0 1 1

P4

0 0 2 4 3 3 4 3 1

Khoa KTMT 33

GT (kiem tra trang thai)an toan Vd (tt)

Allocation Need Work

A B C A B C A B C

P0 0 1 0 7 4 3 3 3 2

P1 2 0 0 1 2 2

P2 3 0 2 6 0 0

P3 2 1 1 0 1 1

P4 0 0 2 4 3 1

Chuoi an toan

7 4 3

7 4 5

10 4 7 10 5 7

5 3 2

P1

P3

P4

P2

Khoa KTMT 34

GT cap phat tai nguyen V du

Yeu cau (1, 0, 2) cua P1

co thoa c khong?

Kiem tra ieu kien Request1 Available:

(1, 0, 2) (3, 3, 2) la ung

Gia nh thoa yeu cau, kiem tra trang thai mi co phai la safe hay khong.

Trang thai mi la safe (chuoi an toan la ), vay co

the cap phat tai nguyen cho P1.

Allocation Need Available

A B C A B C A B C

P0

0 1 0 7 4 3 2 3 0

P1

3 0 2 0 2 0

P2

3 0 2 6 0 0

P3

2 1 1 0 1 1

P4

0 0 2 4 3 1

P4 (3, 3, 0) ?

P0 (0, 2, 0) ?

P3 (0, 2, 1)?

Khoa KTMT 35

3. Phat hien deadlock (Deadlock detection)

Chap nhan xay ra deadlock trong he thong, kiem tra

trang thai he thong bang giai thuat phat hien deadlock.

Neu co deadlock th tien hanh phuc hoi he thong

Cac giai thuat phat hien deadlock thng s dung mo

hnh RAG.

He thong cap phat tai nguyen c khao sat trong moi

trng hp sau

1. Moi loai tai nguyen ch co mot thc the (instance)

2. Moi loai tai nguyen co the co nhieu thc the

Khoa KTMT 36

Moi loai tai nguyen ch co mot thc the

S dung wait-for graph

Wait-for graph c dan xuat t RAG bang cach bo cac node bieu dien tai nguyen va ghep cac canh tng ng.

Co canh t Pien P

j P

iang ch tai nguyen t P

j

Mot giai thuat kiem tra co ton tai chu trnh trong wait-for graph hay

khong se c goi nh ky. Giai thuat phat hien chu trnh co thi

gian chay la O(n2), vi n la so nh cua graph.

R1 R3 R4

P2P1 P3

P5

R2 R5P4

P2P1 P3

P5

P4

Khoa KTMT 37

Moi loai tai nguyen co nhieu thc the

Phng phap dung wait-for graph khong ap dung c cho trng

hp moi loai tai nguyen co nhieu instance.

Cac cau truc d lieu dung trong giai thuat phat hien deadlock

Available: vector o dai m

so instance san sang cua moi loai tai nguyen

Allocation: ma tran n m

so instance cua moi loai tai nguyen a cap phat cho moi process

Request: ma tran n m

yeu cau hien tai cua moi process.

Request [i, j ] = k Pi ang yeu cau them k instance cua Rj

Khoa KTMT 38

Giai thuat phat hien deadlock

1. Goi Work va Finish la vector kch thc m va n. Khi tao:

Work := Available

i = 1, 2,, n, neu Allocationi 0 th Finish[ i ] := false

con khong th Finish[ i ] := true

2. Tm i thoa man:

Finish[ i ] := false va

Requesti Work

Neu khong ton tai i nh the, en bc 4.

3. Work := Work + Allocationi

Finish[ i ] := true

quay ve bc 2.

4. Neu Finish[ i ] = false, vi mot i = 1,, n, th he thong ang trang

thai deadlock. Hn the na, Finish[ i ] = false th Pib deadlocked.

thi gian chay

cua giai thuat

O(mn2)

Khoa KTMT 39

Giai thuat phat hien deadlock V du

He thong co 5 qua trnh P0

,, P4

3 loai tai nguyen: A (7 instance), B (2 instance), C (6 instance).

Allocation Request Available

A B C A B C A B C

P0

0 1 0 0 0 0 0 0 0

P1

2 0 0 2 0 2

P2

3 0 3 0 0 0

P3

2 1 1 1 0 0

P4

0 0 2 0 0 2

Chay giai thuat, tm c chuoi vi Finish[ i ]

= true, i = 1,, n, vay he thong khong b deadlocked.

Khoa KTMT 40

Giai thuat phat hien deadlock V du (tt)

P2

yeu cau them mot instance cua C. Ma tran Request nh sau:

Request

A B C

P0

0 0 0

P1

2 0 2

P2

0 0 1

P3

1 0 0

P4

0 0 2

Trang thai cua he thong la g?

Co the thu hoi tai nguyen ang s hu bi process P0

nhng van

khong u ap ng yeu cau cua cac process khac.

Vay ton tai deadlock, bao gom cac process P1, P

2, P

3, va P

4 .

Khoa KTMT 41

Phuc hoi deadlock (Deadlock Recovery)

Khi deadlock xay ra, e phuc hoi

bao ngi van hanh (operator)

hoac

he thong t ong phuc hoi bang cach be gay chu trnh deadlock:

cham dt mot hay nhieu qua trnh

lay lai tai nguyen t mot hay nhieu qua trnh

Khoa KTMT 42

Deadlock Recovery: Cham dt qua trnh

Phuc hoi he thong b deadlock bang cach cham dt qua

trnh

Cham dt tat ca process b deadlocked, hoac

Cham dt lan lt tng process cho en khi khong con deadlock

S dung giai thuat phat hien deadlock e xac nh con deadlock hay khong

Da tren yeu to nao e chon process can c cham

dt?

o u tien cua process

Thi gian a thc thi cua process va thi gian con lai

Loai tai nguyen ma process a s dung

Tai nguyen ma process can them e hoan tat cong viec

So lng process can c cham dt

Process la interactive process hay batch process

Khoa KTMT 43

Deadlock recovery: Lay lai tai nguyen

Lay lai tai nguyen t mot process, cap phat cho process

khac cho en khi khong con deadlock na.

Cac van e trong chien lc thu hoi tai nguyen:

Chon nan nhan e toi thieu chi ph (co the da tren so tai nguyen s hu, thi gian CPU a tieu ton,...)

Tr lai trang thai trc deadlock (Rollback): rollback process b lay lai tai nguyen tr ve trang thai safe, tiep tuc process t trang

thai o. He thong can lu gi mot so thong tin ve trang thai cac

process ang thc thi.

oi tai nguyen (Starvation): e tranh starvation, phai bao am khong co process se luon luon b lay lai tai nguyen moi khi

deadlock xay ra.

Khoa KTMT 44

Phng phap ket hp e giai quyet Deadlock

Ket hp 3 phng phap c ban

Ngan chan (Prevention)

Tranh (Avoidance)

Phat hien (Detection)

Cho phep s dung cach giai quyet toi u cho moi lp tai

nguyen trong he thong.

Phan chia tai nguyen thanh cac lp theo th bac.

S dung ky thuat thch hp nhat cho viec quan ly deadlock trong moi lp nay.

Khoa KTMT 45

Bai tap

Bai 01: Liet ke 3 trng hp xay ra deadlock trong i

song

Bai 02:

R1 R3

P1 P2 P3

R2R4

Deadlock ?

Khoa KTMT 46

Bai tap

Bai 03:

A) Tm Need

B) He thong co an toan khong

C)Neu P1

yeu cau (0,4,2,0) th co the cap phat cho no

ngay khong?

chapter06 deadlocks

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