Chapter05 2 Process Synchronization

1Chng V - Phan II

ong Bo va Giai Quyet Tranh Chap

(Process Synchronization)

2Khoa KTMT

Noi dung

at van e (tai sao phai ong bo va giai

quyet tranh chap ?)

Van e Critical section

Cac giai phap phan mem

Giai thuat Peterson, va giai thuat bakery

ong bo bang hardware

Semaphore

Cac bai toan ong bo

Critical region

Monitor

3Khoa KTMT

at van e

Khao sat cac process/thread thc thi ong thi va chia

se d lieu (qua shared memory, file).

Neu khong co s kiem soat khi truy cap cac d lieu chia

se th co the a en ra trng hp khong nhat quan d

lieu (data inconsistency).

e duy tr s nhat quan d lieu, he thong can co c che

bao am s thc thi co trat t cua cac process ong thi.

Q

Lp R

4Khoa KTMT

Bai toan Producer-Consumer

Producer-Consumer

P khng c ghi d liu vo buffer y

C khng c c d liu t buffer ang trng

P v C khng c thao tc trn buffer cng lc

P

CBuffer (N)

Gii han, khong gii

han ???

5Khoa KTMT

at van e

Xet bai toan Producer-Consumer vi bounded buffer

Bounded buffer, them bien em count

#define BUFFER_SIZE 10 /* 10 buffers */

typedef struct {

. . .

} item;

item buffer[BUFFER_SIZE];

int in = 0, out = 0, count = 0;

6Khoa KTMT

Bounded buffer (tt)

Qua trnh Producer

item nextProduced;

while(1) {

while (count == BUFFER_SIZE); /* do nothing */

buffer[in] = nextProduced;

count++;

in = (in + 1) % BUFFER_SIZE;

}

Qua trnh Consumer

item nextConsumed;

while(1) {

while (count == 0); /* do nothing */

nextConsumed = buffer[out] ;

count--;

out = (out + 1) % BUFFER_SIZE;

}

bien count c chia se

gia producer va consumer

7Khoa KTMT

Bounded buffer (tt)

Cac lenh tang, giam bien count tng ng trong ngon ng may la:

(Producer) count++:

register1

= count

register1

= register1

+ 1

count = register1

(Consumer) count--:

register2

= count

register2

= register2

- 1

count = register2

Trong o, cac registerila cac thanh ghi cua CPU.

8Khoa KTMT

Bounded buffer (tt)

Ma may cua cac lenh tang va giam bien count co the b thc thi xen

ke

Gia s count ang bang 5. Chuoi thc thi sau co the xay ra:

0: producer register1 := count {register1 = 5}

1: producer register1 := register1 + 1 {register1 = 6}

2: consumer register2 := count {register2 = 5}

3: consumer register2 := register2 - 1 {register2 = 4}

4: producer count := register1 {count = 6}

5: consumer count := register2 {count = 4}

Cac lenh count++, count-- phai la n nguyen

(atomic), ngha la thc hien nh mot lenh n, khong

b ngat na chng.

9Khoa KTMT

Bounded buffer (tt)

Race condition: nhieu process truy xuat va thao tac

ong thi len d lieu chia se (nh bien count) Ket qua cuoi cung cua viec truy xuat ong thi nay phu thuoc

th t thc thi cua cac lenh thao tac d lieu.

e d lieu chia se c nhat quan, can bao am

sao cho tai moi thi iem ch co mot process c

thao tac len d lieu chia se. Do o, can co c che

ong bo hoat ong cua cac process nay.

10Khoa KTMT

Van e Critical Section

Gia s co n process cung truy xuat ong thi d lieu

chia se

Cau truc cua moi process Pi - Moi process co oan

code nh sau :

Do {

entry section /* vao critical section */

critical section /* truy xuat d lieu chia xe */

exit section /* ri critical section */

remainder section /* lam nhng viec khac */

} While (1)

Trong moi process co nhng oan code co cha cac

thao tac len d lieu chia se. oan code nay c

goi la vung tranh chap (critical section, CS).

11Khoa KTMT

Van e Critical Section

Van e Critical Section: phai bao am s loai tr

tng ho (MUTual EXclusion, mutex), tc la khi mot

process ang thc thi trong vung tranh chap, khong

co process nao khac ong thi thc thi cac lenh

trong vung tranh chap.

12Khoa KTMT

Yeu cau cua li giai cho Critical Section Problem

Li giai phai thoa bon tnh chat:

(1) oc quyen truy xuat (Mutual exclusion): Khi mot process P ang

thc thi trong vung tranh chap (CS) cua no th khong co process Q

nao khac ang thc thi trong CS cua Q.

(2) Progress: Mot tien trnh tam dng ben ngoai mien gang (CS)

khong c ngan can cac tien trnh khac vao mien gang (CS) va

viec la chon P nao vao CS phai co han nh

(3) Ch i gii han (Bounded waiting): Moi process ch phai ch

e c vao vung tranh chap trong mot khoang thi gian co han

nh nao o. Khong xay ra tnh trang oi tai nguyen (starvation).

(4)Khong co gia thiet nao at ra cho s lien he ve toc o cua cac

tien trnh, cung nh ve so lng bo x ly trong he thong

13Khoa KTMT

Nhm gii php Busy Waiting

S dng cc bin c hiu

S dng vic kim tra lun phin

Gii php ca Peterson

Cm ngt

Ch th TSL

Nhm gii php Sleep & Wakeup

Semaphore

Monitor

Message

Phan loai giai phap

14Khoa KTMT

Cac giai phap Busy waiting

While (cha c quyn) do nothing() ;

CS;

T b quyn s dng CS

Tip tc tiu th CPU trong khi ch i vo min gngKhng i hi s tr gip ca H iu hnh

15Khoa KTMT

Cac giai phap Sleep & Wake up

if (cha c quyn) Sleep() ;

CS;

Wakeup( somebody);

T b CPU khi cha c vo min gngCn c H iu hnh h tr

16Khoa KTMT

Cac giai phap Busy waiting

Giai thuat 1

Bien chia se

Int turn; /* khi au turn = 0 */

neu turn = i th Pi

c phep vao critical section, vi i = 0 hay 1

Process Pi

do {

while (turn != i);

critical section

turn = j;

remainder section

} while (1);

Thoa man mutual exclusion (1)

Nhng khong thoa man yeu cau ve progress (2) va bounded

waiting (3) v tnh chat strict alternation cua giai thuat

17Khoa KTMT

Process P0:

do

while (turn != 0);

critical section

turn := 1;

remainder section

while (1);

Process P1:

do

while (turn != 1);

critical section

turn := 0;

remainder section

while (1);

V du:

P0 co RS (remainder section) rat ln con P1 co RS nho???

Giai thuat 1 (tt)

P0 P1 P0 ?(I/O)

H Thng b treo

18Khoa KTMT

Giai thuat 2

Bien chia se

boolean flag[ 2 ]; /* khi au flag[ 0 ] = flag[ 1 ] = false */

Neu flag[ i ] = true th Pi

san sang vao critical section.

Process Pi

do {

flag[ i ] = true; /* Pisan sang vao CS */

while ( flag[ j ] ); /* Pinhng P

j*/

critical section

flag[ i ] = false;

remainder section

} while (1);

Bao am c mutual exclusion. Chng minh?

Khong thoa man progress. V sao?

19Khoa KTMT

Giai thuat 3 (Peterson)

Bien chia se: ket hp ca giai thuat 1 va 2

Process Pi, vi i = 0 hay 1

do {

flag[ i ] = true; /* Process i san sang */

turn = j; /* Nhng process j */

while (flag[ j ] and turn == j);

critical section

flag[ i ] = false;

remainder section

} while (1);

Thoa man c ca 3 yeu cau (chng minh?)

giai quyet bai toan critical section cho 2 process.

20Khoa KTMT

Process P0

do {

/* 0 wants in */

flag[0] = true;

/* 0 gives a chance to 1 */

turn = 1;

while (flag[1] && turn == 1);

critical section;

/* 0 no longer wants in */

flag[0] = false;

remainder section;

} while(1);

Process P1

do {

/* 1 wants in */

flag[1] = true;

/* 1 gives a chance to 0 */

turn = 0;

while (flag[0] && turn == 0);

critical section;

/* 1 no longer wants in */

flag[1] = false;

remainder section;

} while(1);

Giai thuat Peterson-2 process

21Khoa KTMT

Giai thuat 3: Tnh ung an

Giai thuat 3 thoa mutual exclusion, progress, va

bounded waiting

Mutual exclusion c bao am bi v

P0 va P1 eu trong CS neu va ch neu flag[0] =

flag[1] = true va turn = i cho moi Pi (khong the xay ra)

Chng minh thoa yeu cau ve progress va

bounded waiting

Pi khong the vao CS neu va ch neu b ket tai vong

lap while() vi ieu kien flag[ j ] = true va turn = j .

Neu Pj khong muon vao CS th flag[ j ] = false va do

o Pi co the vao CS.

22Khoa KTMT

Giai thuat 3: Tnh ung an (tt)

Neu Pj a bat flag[ j ] = true va ang ch tai while() th

co ch hai trng hp la turn = i hoac turn = j

Neu turn = i th Pi vao CS. Neu turn = j th Pj vao CS

nhng se bat flag[ j ] = false khi thoat ra cho phep

Pi vao CS

Nhng neu Pj co u thi gian bat flag[ j ] = true th Pj

cung phai gan turn = i

V Pi khong thay oi tr cua bien turn khi ang ket

trong vong lap while(), Pi se ch e vao CS nhieu

nhat la sau mot lan Pj vao CS (bounded waiting)

23Khoa KTMT

Giai thuat bakery: n process

Trc khi vao CS, process Pi nhan mot con so.

Process nao gi con so nho nhat th c vao CS

Trng hp Pi va Pj cung nhan c mot ch so:

Neu i < j th Pi c vao trc. (oi xng)

Khi ra khoi CS, Pi at lai so cua mnh bang 0

C che cap so cho cac process thng tao cac so

theo c che tang dan, v du 1, 2, 3, 3, 3, 3, 4, 5,

K hieu

(a,b) < (c,d) neu a < c hoac if a = c va b < d

max(a0,,a

k) la con so b sao cho b a

ivi moi i = 0,, k

24Khoa KTMT

Giai thuat bakery: n process (tt)

/* shared variable */

boolean choosing[ n ]; /* initially, choosing[ i ] = false */

int num[ n ]; /* initially, num[ i ] = 0 */

do {

choosing[ i ] = true;

num[ i ] = max(num[0], num[1],, num[n 1]) + 1;choosing[ i ] = false;

for (j = 0; j < n; j++)

{

while (choosing[ j ]);

while ((num[ j ] != 0) && (num[ j ], j) < (num[ i ], i));

}

critical section

num[ i ] = 0; // khng chim quyn vo vng CS

remainder section

} while (1);

25Khoa KTMT

T software en hardware

Khuyet iem cua cac giai phap software

Cac process khi yeu cau c vao vung tranh chap eu phai lien tuc kiem tra ieu kien (busy waiting),

ton nhieu thi gian x ly cua CPU

Neu thi gian x ly trong vung tranh chap ln, mot giai phap hieu qua nen co c che block cac process

can i.

Cac giai phap phan cng (hardware)

Cam ngat (disable interrupts)

Dung cac lenh ac biet

26Khoa KTMT

Cam ngat

Trong he thong uniprocessor:

mutual exclusion c bao

am.

Nhng neu system clockc cap nhat do interrupt

th sao?

Tren he thong multiprocessor:

mutual exclusion khong c

am bao

Ch cam ngat tai CPU thc thi lenh disable_interrupts

Cac CPU khac van co the truy cap bo nh chia se

Process Pi:

do {

disable_interrupts();

critical section

enable_interrupts();

remainder section

} while (1);

27Khoa KTMT

Lenh TestAndSet

oc va ghi mot bien trong mot

thao tac atomic (khong chia cat

c).

boolean TestAndSet(boolean &target)

{

boolean rv = target;

target = true;

return rv;

}

Shared data:

boolean lock = false;

Process Pi :

do {

while (TestAndSet(lock));

critical section

lock = false;

remainder section

} while (1);

28Khoa KTMT

Lenh TestAndSet (tt)

Mutual exclusion c bao am: neu Pivao CS, cac

process Pj

khac eu ang busy waiting

Khi Pira khoi CS, qua trnh chon la process P

jvao CS

ke tiep la tuy y khong bao am ieu kien bounded

waiting. Do o co the xay ra starvation (b bo oi)

Cac processor (v du Pentium) thong thng cung cap

mot lenh n la Swap(a, b) co tac dung hoan chuyen noi

dung cua a va b.

Swap(a, b) cung co u nhc iem nh TestAndSet

29Khoa KTMT

Swap va mutual exclusion

Bien chia se lock c khi

tao gia tr false

Moi process Pico bien cuc bo

key

Process Pinao thay gia tr

lock = false th c vao CS.

Process Pise loai tr cac

process Pjkhac khi thiet lap

lock = true

void Swap(boolean &a,

boolean &b) {

boolean temp = a;

a = b;

b = temp;

}

Bien chia se (khi tao la false)

bool lock;

bool key;

Process Pi

do {

key = true;

while (key == true)

Swap(lock, key);

critical section

lock = false;

remainder section

} while (1)

Khong thoa man bounded waiting

30Khoa KTMT

Giai thuat dung TestAndSet thoa man 3 yeu cau (1)

Cau truc d lieu dung chung (khi tao la false)

bool waiting[ n ];

bool lock;

Pi ch co the vao CS neu va ch neu hoac waiting[ i ] = false, hoac

key = false.

31Khoa KTMT


waiting[ i ] = true;

key = true;

while (waiting[ i ] && key)

key = TestAndSet(lock);

waiting[ i ] = false;

j = (i + 1) % n;

while ( (j != i) && !waiting[ j ] )

j = (j + 1) % n;

if (j == i)

lock = false;

else

waiting[ j ] = false;

critical section

remainder section

do {

} while (1)

32Khoa KTMT


Cau truc d lieu dung chung (khi tao la false)

bool waiting[ n ];

bool lock;

Mutual exclusion: Pi ch co the vao CS neu va ch neu

hoac waiting[ i ] = false, hoac key = false

key = false ch khi TestAndSet (hay Swap) c thc thi

Process au tien thc thi TestAndSet mi co key == false;

cac process khac eu phai i

waiting[ i ] = false ch khi process khac ri khoi CS

Ch co mot waiting[ i ] co gia tr false

Progress: chng minh tng t nh mutual exclusion

Bounded waiting: waiting in the cyclic order

33Khoa KTMT

Cac giai phap Sleep & Wake up

int busy; // =1 neu CS ang b chiem

Int blocked; // so P ang b khoa

do

{

if (busy==1){

blocked = blocked +1;

sleep();

}

else

busy =1;

CS;

busy = 0;

if(blocked !=0){

wakeup(process);

blocked = blocked -1;

}

RS;

} while(1);

Trng hp:

-A vao CS

-B kch hoat va tang blocked

-A kch hoat lai

-B kch hoat lai

-?????

34Khoa KTMT

Semaphore

La cong cu ong bo cung cap bi OS ma khong oi hoi

busy waiting

Semaphore S la mot bien so nguyen.

Ngoai thao tac khi ong bien th ch co the c truy

xuat qua hai tac vu co tnh n nguyen (atomic) va loai

tr (mutual exclusion)

wait(S) hay con goi la P(S): giam gia tr semaphore (S=S-1) . Ke

o neu gia tr nay am th process thc hien lenh wait() b

blocked.

signal(S) hay con goi la V(S): tang gia tr semaphore (S=S+1) .

Ke o neu gia tr nay khong dng, mot process ang blocked

bi mot lenh wait() se c hoi phuc e thc thi.

Tranh busy waiting: khi phai i th process se c at

vao mot blocked queue, trong o cha cac process

ang ch i cung mot s kien.

35Khoa KTMT

Semaphore

P(S) hay wait(S) s dung e gianh tai nguyen va giam

bien em S=S-1

V(S) hay signal(S) se giai phong tai nguyen va tang bien

em S= S+1

Neu P c thc hien tren bien em

36Khoa KTMT

Hien thc semaphore

nh ngha semaphore la mot record

typedef struct {

int value;

struct process *L;/* process queue */

} semaphore;

Gia s he ieu hanh cung cap hai tac vu

(system call):

block(): tam treo process nao thc thi lenh nay

wakeup(P): hoi phuc qua trnh thc thi cua process P

ang blocked

37Khoa KTMT

Hien thc semaphore (tt)

Cac tac vu semaphore c hien thc nh sau

void wait(semaphore S) {

S.value--;

if (S.value < 0) {

add this process to S.L;

block();

}

}

void signal(semaphore S) {

S.value++;

if (S.value

38Khoa KTMT

Hien thc semaphore (tt)

Khi mot process phai ch tren semaphore S, no

se b blocked va c at trong hang i

semaphore

Hang i nay la danh sach lien ket cac PCB

Tac vu signal() thng s dung c che FIFO khi

chon mot process t hang i va a vao hang

i ready

block() va wakeup() thay oi trang thai cua

process

block: chuyen t running sang waiting

wakeup: chuyen t waiting sang ready

39Khoa KTMT

V du s dung semaphore 1 : Hien thc mutex vi semaphore

Dung cho n process

Khi tao S.value = 1

Ch duy nhat mot

process c vao CS

(mutual exclusion)

e cho phep k process

vao CS, khi tao

S.value = k

Shared data:

semaphore mutex;

/* initially mutex.value = 1 */

Process Pi:

do {

wait(mutex);

critical section

signal(mutex);

remainder section

} while (1);

40Khoa KTMT

V du s dung semaphore 2 :ong bo process bang semaphore

Hai process: P1 va P2

Yeu cau: lenh S1 trong

P1 can c thc thi

trc lenh S2 trong P2

nh ngha semaphore

synch e ong bo

Khi ong semaphore:

synch.value = 0

e ong bo hoat ong

theo yeu cau, P1 phai

nh ngha nh sau:

S1;

signal(synch);

Va P2 nh ngha nh sau:

wait(synch);

S2;

41Khoa KTMT

semanphore

Xet hai tien trnh x ly oan chng

trnh sau :

process P1 { A1 ; A2 }

process P2 { B1 ; B2 }

ong bo hoa hoat ong cua hai tien

trnh nay sao cho ca A1 va B1 eu

hoan tat trc khi A2 hay B2 bat

au .

42Khoa KTMT

Nhan xet

Khi S.value 0: so process co the thc thi wait(S) ma

khong b blocked = S.value

Khi S.value < 0: so process ang i tren S la

S.value

Atomic va mutual exclusion: khong c xay ra trng

hp 2 process cung ang trong than lenh wait(S) va

signal(S) (cung semaphore S) tai mot thi iem (ngay

ca vi he thong multiprocessor)

do o, oan ma nh ngha cac lenh wait(S) va

signal(S) cung chnh la vung tranh chap

43Khoa KTMT

Nhan xet (tt)

Vung tranh chap cua cac tac vu wait(S) va signal(S)

thong thng rat nho: khoang 10 lenh.

Giai phap cho vung tranh chap wait(S) va signal(S)

Uniprocessor: co the dung c che cam ngat (disable interrupt). Nhng phng phap nay khong lam viec tren he

thong multiprocessor.

Multiprocessor: co the dung cac giai phap software (nh giai thuat Dekker, Peterson) hoac giai phap hardware

(TestAndSet, Swap).

V CS rat nho nen chi ph cho busy waiting se rat thap.

44Khoa KTMT

Deadlock va starvation

Deadlock: hai hay nhieu process ang ch i vo han nh mot s

kien khong bao gi xay ra (vd: s kien do mot trong cac process

ang i tao ra).

Goi S va Q la hai bien semaphore c khi tao = 1

P0 P1

wait(S); wait(Q);

wait(Q); wait(S);

signal(S); signal(Q);

signal(Q); signal(S);

P0 thc thi wait(S), roi P1 thc thi wait(Q), roi P0 thc thi wait(Q) b

blocked, P1 thc thi wait(S) b blocked.

Starvation (indefinite blocking) Mot tien trnh co the khong bao gi

c lay ra khoi hang i ma no b treo trong hang i o.

45Khoa KTMT

Cac loai semaphore

Counting semaphore: mot so nguyen co gia tr

khong han che.

Binary semaphore: co tr la 0 hay 1. Binary

semaphore rat de hien thc.

Co the hien thc counting semaphore bang

binary semaphore.

46Khoa KTMT

Cac bai toan ong bo (kinh ien)

Bounded Buffer Problem

Readers and Writers Problem

Dining-Philosophers Problem

47Khoa KTMT

Cac bai toan ong bo

Bai toan bounded buffer

D lieu chia se:

semaphore full, empty, mutex;

Khi tao:

full = 0; /* so buffers ay */

empty = n; /* so buffers trong */

mutex = 1;

out

n buffers

48Khoa KTMT

Bounded buffer

do {

wait(full)

wait(mutex);//2

nextc = get_buffer_item(out);

signal(mutex);//4

signal(empty);

consume_item(nextc);

} while (1);

do {

nextp = new_item();

wait(empty);

wait(mutex);//1

insert_to_buffer(nextp);

signal(mutex);//3

signal(full);

} while (1);

producer consumer

49Khoa KTMT

Bai toan Dining Philosophers (1)

5 triet gia ngoi an va suy

ngh

Moi ngi can 2 chiec

ua (chopstick) e an

Tren ban ch co 5 ua

Bai toan nay minh hoa s

kho khan trong viec phan

phoi tai nguyen gia cac

process sao cho khong

xay ra deadlock va

starvation

D lieu chia se:

semaphore chopstick[5];

Khi au cac bien eu la 1

0

1

23

4 0 1

4 2

3

50Khoa KTMT


Triet gia th i:

do {

wait(chopstick [ i ])

wait(chopstick [ (i + 1) % 5 ])

eat

signal(chopstick [ i ]);

signal(chopstick [ (i + 1) % 5 ]);

think

} while (1);

51Khoa KTMT


Giai phap tren co the gay ra deadlock

Khi tat ca triet gia oi bung cung luc va ong thi cam chiec ua ben tay trai deadlock

Mot so giai phap khac giai quyet c deadlock

Cho phep nhieu nhat 4 triet gia ngoi vao cung mot luc

Cho phep triet gia cam cac ua ch khi ca hai chiec ua eu san sang (ngha la tac vu cam cac ua phai

xay ra trong CS)

Triet gia ngoi v tr le cam ua ben trai trc, sau o mi en ua ben phai, trong khi o triet gia v tr

chan cam ua ben phai trc, sau o mi en ua

ben trai

Starvation?

52Khoa KTMT

Bai toan Readers-Writers (1)

Readers - Writers

W khng c cp nht d liu khi c

mt R ang truy xut CSDL .

Ti mt thi im , ch cho php mt W c

sa i ni dung CSDL.

Database

R1

R2R3

W1 W2

53Khoa KTMT


Bo oc trc bo ghi (first

reader-writer)

D lieu chia se

semaphore mutex = 1;

semaphore wrt = 1;

int readcount = 0;

Writer process

wait(wrt);

...

writing is performed

...

signal(wrt);

Reader Process

wait(mutex);

readcount++;

if (readcount == 1)

wait(wrt);

signal(mutex);

...

reading is performed

...

wait(mutex);

readcount--;

if (readcount == 0)

signal(wrt);

signal(mutex);

54Khoa KTMT


mutex: bao ve bien readcount

wrt

Bao am mutual exclusion oi vi cac writer

c s dung bi reader au tien hoac cuoi cung vao hay ra khoi vung tranh chap.

Neu mot writer ang trong CS va co n reader

ang i th mot reader c xep trong hang

i cua wrt va n 1 reader kia trong hang i

cua mutex

Khi writer thc thi signal(wrt), he thong co the

phuc hoi thc thi cua mot trong cac reader ang

i hoac writer ang i.

55Khoa KTMT

Cac van e vi semaphore

Semaphore cung cap mot cong cu manh me e bao

am mutual exclusion va phoi hp ong bo cac process

Tuy nhien, neu cac tac vu wait(S) va signal(S) nam rai

rac rat nhieu processes kho nam bat c hieu ng

cua cac tac vu nay. Neu khong s dung ung co the

xay ra tnh trang deadlock hoac starvation.

Mot process b die co the keo theo cac process khac

cung s dung bien semaphore.

signal(mutex)

critical section

wait(mutex)

wait(mutex)

critical section

wait(mutex)

signal(mutex)

critical section

signal(mutex)

56Khoa KTMT

Critical Region (CR)

La mot cau truc ngon ng cap cao (high-level language construct,

c dch sang ma may bi mot compiler), thuan tien hn cho ngi

lap trnh.

Mot bien chia se v kieu d lieu T, khai bao nh sau

v: shared T;

Bien chia se v ch co the c truy xuat qua phat bieu sau

region v when B do S; /* B la mot bieu thc Boolean */

Y ngha: trong khi S c thc thi, khong co qua trnh khac co the

truy xuat bien v.

57Khoa KTMT

CR va bai toan bounded buffer

D liu chia s:

struct buffer

{

int pool[n];

int count,

in,

out;

}

region buffer when (count < n) {

pool[in] = nextp;

in = (in + 1) % n;

count++;

}

Producer

region buffer when (count > 0){

nextc = pool[out];

out = (out + 1) % n;

count--;

}

Consumer

58Khoa KTMT

Monitor (1)

Cung la mot cau truc ngon ng cap cao tng t CR, co

chc nang nh semaphore nhng de ieu khien hn

Xuat hien trong nhieu ngon ng lap trnh ong thi nh

Concurrent Pascal, Modula-3, Java,

Co the hien thc bang semaphore

59Khoa KTMT

Monitor (2)

La mot module phan

mem, bao gom

Mot hoac nhieu thu tuc(procedure)

Mot oan code khi tao(initialization code)

Cac bien d lieu cuc bo(local data variable)

ac tnh cua monitor

Local variable ch co the truy xuat bi cac thu tuc

cua monitor

Process vao monitor bang cach goi mot trong cac thu

tuc o

Ch co mot process co the vao monitor tai mot thi

iem mutual exclusion

c bao amshared data

entry queue

operations

initializationcode

Mo hnh cua mot monitor

n gian

60Khoa KTMT

Cau truc cua monitor

monitor monitor-name{

shared variable declarationsprocedure body P1 () {

. . .}procedure body P2 () {

. . .} procedure body Pn () {

. . .} {

initialization code}

}

61Khoa KTMT

Condition variable

Nham cho phep mot process i trong monitor, phai khai bao bien ieu kien (condition variable)

condition a, b;

Cac bien ieu kien eu cuc bo va ch c truy cap ben

trong monitor.

Ch co the thao tac len bien ieu kien bang hai thu tuc:

a.wait: process goi tac vu nay se b block tren bien ieu kien a

process nay ch co the tiep tuc thc thi khi co process khac thc hien tac vu a.signal

a.signal: phuc hoi qua trnh thc thi cua process b block tren bien ieu kien a.

Neu co nhieu process: ch chon mot

Neu khong co process: khong co tac dung

62Khoa KTMT

Monitor co condition variable

Cac process co the i entry queue hoac i cac condition

queue (a, b,)

Khi thc hien lenh a.wait, process se c chuyen vao

condition queue a

Lenh a.signal chuyen mot process t condition queue a

vao monitor

Khi o, e bao am mutual

exclusion, process goi a.signal

se b blocked va c a vao

urgent queue

entry queueshared data

...

operations

initializationcode

a

b

63Khoa KTMT

Monitor co condition variable (tt)

local data

condition variables

procedure 1

procedure k

initialization code

...

monitor waiting area entranceentry queue

c1.wait

condition c1

condition cn

cn.wait

urgent queue

cx.signal

...MONITOR

exit

64Khoa KTMT

Monitor va dining philosophers

monitor dp

{

enum {thinking, hungry, eating} state[5];

condition self[5];

0

1

23

4

65Khoa KTMT

Dining philosophers (tt)

void pickup(int i) {

state[ i ] = hungry;

test( i );

if (state[ i ] != eating)

self[ i ].wait();

}

void putdown(int i) {

state[ i ] = thinking;

// test left and right neighbors

test((i + 4) % 5); // left neighbor

test((i + 1) % 5); // right

}

66Khoa KTMT


void test(int i) {

if ( (state[(i + 4) % 5] != eating) &&

(state[ i ] == hungry) &&

(state[(i + 1) % 5] != eating) ) {

state[ i ] = eating;

self[ i ].signal();

}

void init() {

for (int i = 0; i < 5; i++)

state[ i ] = thinking;

}

}

67Khoa KTMT


Trc khi an, moi triet gia phai goi ham pickup(), an

xong roi th phai goi ham putdown()

dp.pickup(i);

an

dp.putdown(i);

Giai thuat khong deadlock nhng co the gay

starvation.

68Khoa KTMT

Bi Tp

Xet giai phap ong bo hoa sau :

Pi

while (TRUE) {

int j = 1-i;

flag[i]= TRUE; turn = j;

while (turn == j && flag[j]==TRUE);

critical-section ();

flag[j] = FALSE;

Noncritical-section ();

}

ay co phai la mot giai phap bao am 3 iu kin khong ?

69Khoa KTMT

Bi tp

Xet giai phap phan mem do Dekker e ngh e to chc truy xat oc quyen

cho hai tien trnh . Hai tien trnh P0, P1 chia se cac bien sau :

var flag : array [0..1] of boolean; (khi ong la false)

turn : 0..1;

Cau truc mot tien trnh Pi ( i =0 hay 1, va j la tien trnh con lai ) nh

sau :

repeat

flag[i] := true;

while flag[j] do

if turn = j then

begin flag[i]:= false;

while turn = j do ;

flag[i]:= true;

end;

critical_section();

turn:= j;

flag[i]:= false;

non_critical_section();

until false;

Giai phap nay co phai la mot giai phap ung thoa man 4 yeu cau

khong ?

70Khoa KTMT

Bi tp

Gia s mot may tnh khong co ch th TSL, nhng co ch th Swap co kha nang

hoan oi noi dung cua hai t nh ch bang mot thao tac khong the phan

chia :

procedure Swap() var a,b: boolean);

var temp : boolean;

begin

temp := a;

a:= b;

b:= temp;

end;

S dung ch th nay co the to chc truy xuat oc quyen khong ? Neu co, xay

dng cau truc chng trnh tng ng

71Khoa KTMT

Bi tp semanphore

Xet hai tien trnh x ly oan chng trnh sau :

process P1 { A1 ; A2 } process P2 { B1 ; B2 }

ong bo hoa hoat ong cua hai tien trnh nay sao cho ca A1 va

B1 eu hoan tat trc khi A2 hay B2 bat au .

72Khoa KTMT

Bi tp semanphore

S dng semaphore vit li chng trnh sau theo m hnh x l

ng hnh:

A = x1 + x2; B = A*x3; C= A + x4; D= B + C; E = D*x5 + C;

Gi s c 5 process mi process s thc hin 1 biu thc.

73Khoa KTMT

Bi tp semanphore

Xet hai tien trnh sau :

process A { while (TRUE) na = na +1; }

process B { while (TRUE) nb = nb +1; }

ong bo hoa x ly cua hai tien trnh tren, s dung hai semaphore tong quat, sao cho tai bat ky thi iem nao cung co

nb < na

74Khoa KTMT

Bi tp semanphore

Mot bien X c chia se bi hai tien trnh cung thc hien oan

code sau :

do

X = X +1;

if ( X == 20) X = 0;

while ( TRUE );

Bat au vi gia tr X = 0, chng to rang gia tr X co the vt qua

20. Can sa cha oan chng trnh tren nh the nao e bao

am X khong vt qua 20 ?

75Khoa KTMT

Bi tp semanphore

Tong quat hoa cau hoi 8) cho cac tien trnh x ly oan chng

trnh sau :

process P1 { for ( i = 1; i

76Khoa KTMT

1. Trng value trong cu trc semanphore dng th hin tnh cht g:

S process b kha

S process c nh thc

S process c th ng thi truy xut ti nguyn

C 3 cu trn iu sai.

77Khoa KTMT

2. Trong gii php Sleep & Wakeup th ngha ca thng s Busy l:

S process b kha

S process c nh thc

S process c th ng thi truy xut ti nguyn

C 3 cu trn iu sai.

78Khoa KTMT

Thi gian ch di nht ca 1 process nu dng k thut

semanphore cho n process:

Khng ch

Ch 1 ln process thc thi

Ch N-1 process thc thi

Ch N process thc thi

79Khoa KTMT

Trong cc gii php sau, gii php no khng s dng c trn

h thng multiprocessor:

Dekker

Peterson

C ch cm ngt (disable interrupt)

TestandSet

Documents

Chapter05 2 Process Synchronization