02/2015 Duy Phan

Chng 5: ng b - 1

2 ng b Duy Phan

n tp chng 4

3 ng b Duy Phan

Bi tp chng 4

4 ng b Duy Phan

Mc tiu

Hiu c vn tranh chp gia cc tin trnh

trong h iu hnh

Bit c cc gii php gii quyt tranh chp

Hiu c cc vn trong gii quyt tranh chp

Bit c cc yu cu ca cc gii php trong

vic gii quyt tranh chp v phn nhm cc gii

php

5 ng b Duy Phan

Ni dung

Gii thiu v race condition

Gii thiu cc gii php tng qut gii quyt tranh chp

Phn tch cc chi tit cc vn trong vic gii quyt tranh chp

Yu cu ca gii php trong vic gii quyt tranh chp

Phn nhm cc gii php

6 ng b Duy Phan

t vn

Kho st cc process/thread thc thi ng thi v chia se d liu (qua shared memory, file).

Nu khng c s kim sot khi truy cp cc d liu chia se th c th a n ra trng hp khng nht qun d liu (data inconsistency).

duy tr s nht qun d liu, h thng cn c c ch bo m s thc thi c trt t ca cc process ng thi. Q

L p

R

7 ng b Duy Phan

Bi ton 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

C

Buffer (N)

Gii hn, khng gii hn

???

8 ng b Duy Phan

Bounded buffer

Qu trnh Producer

item nextProduce;

while(1){

while(count == BUFFER_SIZE); /*ko lam gi*/

buffer[in] = nextProducer;

count++;

in = (in+1)%BUFFER_SIZE;

Qu trnh Consumer

item nextConsumer;

while(1){

while(count == 0); /*ko lam gi*/

nextConsumer = buffer[out];

count--;

out = (out+1)%BUFFER_SIZE;

bin count c chia se

gia producer va consumer

9 ng b Duy Phan

Bounded buffer (tt)

Cc lnh tng, gim bin count tng ng trong ngn ng my l:

Producer (count++)

register1 = count

register1 = register1 + 1

count = register1

Consumer (count--)

register2 = count

register2 = register2 - 1

count = register2

Trong , cc register l cc thanh ghi ca CPU

10 ng b Duy Phan

Bounded buffer (tt)

M my ca cc lnh tng v gim bin count c th b thc thi xen k

Gi s count ang bng 5. Chui thc thi c th xy 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}

Cn phi c gii php cc lnh count++, count-- phi l n nguyn (atomic), nghia l thc hin nh mt lnh n, khng b ngt na chng.

11 ng b Duy Phan

Bounded buffer (tt)

Race condition: nhiu process truy xut va thao tac ng thi

ln d liu chia se (nh bin count)

Kt qua cui cung cua vic truy xut ng thi nay phu

thuc th t thc thi cua cac lnh thao tac d liu.

d liu chia se c nht quan, cn bao am sao cho tai mi

thi im chi co mt process c thao tac ln d liu chia se.

Do o, cn co c ch ng b hoat ng cua cac process nay.

12 ng b Duy Phan

Vn Critical Section

Gi s c n process truy xut ng thi d liu chia se

Cu trc ca mi process Pi c on code nh sau: Do { entry section /* vao critical section */

critical section /* truy xut d liu chia xe */ exit section /* ri critical section */

remainder section /* lam nhng vic khac */ } While (1)

Trong mi process c nhng on code c cha cc thao tc ln d liu chia se. on code ny c gi l vng tranh chp (critical section, CS).

13 ng b Duy Phan

Vn Critical Section (tt)

Vn Critical Section: phai bao am s loai tr

tng h (mutual exclusion, mutex), tc la khi mt

process ang thc thi trong vung tranh chp, khng co

process nao khac ng thi thc thi cac lnh trong

vung tranh chp.

14 ng b Duy Phan

Yu cu ca li gii cho CS Problem

Li gii phi tha ba tnh cht:

(1) Loi tr tng h (Mutual exclusion): Khi mt process P ang thc thi trong vng tranh chp (CS) ca n th khng c process Q no khc ang thc thi trong CS ca Q.

(2) Progress: Mt tin trnh tm dng bn ngoi min gng khng c ngn cn cc tin trnh khc vo min gng

(3) Ch i gii hn (Bounded waiting): Mi process chi phi ch c vo vng tranh chp trong mt khong thi gian c hn nh no . Khng xy ra tnh trng i ti nguyn (starvation).Khng c gi thit no t ra cho s lin h v tc ca cc tin trnh, cung nh v s lng b x ly trong h thng

15 ng b Duy Phan

Phn loi gii php

Nhm gii php Busy Waiting

S dng cc bin c hiu

S dng vic kim tra lun phin

Gii php ca Peterson

Cm ngt

Chi th TSL

Nhm gii php Sleep & Wakeup

Semaphore

Monitor

Message

16 ng b Duy Phan

Cc gii php Busy waiting

Tip tc tiu th CPU trong khi ch i vo min gng

Khng i hi s tr gip ca H iu hnh

While (cha c quyn) do_nothing() ;

CS;

T b quyn s dng CS

17 ng b Duy Phan

Cc gii php Sleep & Wake up

T b CPU khi cha c vo min gng

Cn H iu hnh h tr

if (cha c quyn) Sleep() ;

CS;

Wakeup (somebody);

18 ng b Duy Phan

Tng kt

Race condition

Cc gii php tng qut gii quyt tranh chp

Cc chi tit cc vn trong vic gii quyt tranh chp

Yu cu ca gii php trong vic gii quyt tranh chp

Cc nhm cc gii php

19 ng b Duy Phan

Cu hi n tp

02/2015 Duy Phan

Kt thc chng 5-1

01/2015 Duy Phan

Chng 5: ng b - 2

2 ng b Duy Phan

Cu hi n tp 5 - 1

3 ng b Duy Phan

Mc tiu

Hiu c nhm gii php Busy waiting bao gm:

Cc gii php phn mm

Cc gii php phn cng

4 ng b Duy Phan

Ni dung

Cc gii php phn mm

S dng gii thut kim tra lun phin

S dng cc bin c hiu

Gii php ca Peterson

Gii php Bakery

Cc gii php phn cng

Cp ngt

Ch th TSL

5 ng b Duy Phan

Gii thut 1

Bin chia se

int turn; /* khi u turn = 0 */

nu turn = i thi 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);

Tho man mutual exclusion (1)

Nhng khng tho man yu cu v progress (2) va bounded waiting (3) vi tinh cht strict alternation ca gii thut

6 ng b Duy Phan

Gii thut 1 (tt)

V d:

P0 c RS (remainder section) rt ln cn P1 c RS nh ???

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);

7 ng b Duy Phan

Gii thut 2

Bin chia se

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

Nu flag[ i ] = true thi Pi sn sang vao critical section.

Process Pi

do {

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

while ( flag[ j ] ); /* Pi nhng Pj */

critical section

flag[ i ] = false;

remainder section

} while (1);

Bo m c mutual exclusion. Chng minh?

Khng tha man progress. Vi sao?

8 ng b Duy Phan

Gii thut 3 (Peterson)

Bin chia se

Kt hp c gii thut 1 v 2.

Process Pi, vi I = 0 hay 1

do {

flag[ i ] = true; /* Process i sn sang */ turn = j; /* Nhng process j */ while (flag[ j ] and turn == j);

critical section

flag[ i ] = false;

remainder section

} while (1);

Tho man c c 3 yu cu (chng minh?)

gii quyt bai toan critical section cho 2 process

9 ng b Duy Phan

Gii thut Peterson 2 process

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);

10 ng b Duy Phan

Gii thut 3: Tnh ng n

Gii thut 3 tha mutual exclusion, progress, v bounded waiting

Mutual exclusion c m bo bi v

P0 v P1 u trong CS nu v ch nu flag[0] = flag[1] = true v turn = I cho mi Pi (khng th xy ra)

Chng minh tha yu cu v progress v bounded waiting

Pi khng th vo CS nu v ch nu b kt ti vng lp while() vi iu kin flag[j]=true v turn = j

Nu Pj khng mun vo CS th flag[j] = false v do Pi c th vo CS

11 ng b Duy Phan

Gii thut 3: Tnh ng n (tt)

Nu Pj bt flag[j]=true v ang ch ti while() th

c ch hai trng hp l turn = i hoc turn = j

Nu turn = i v Pi vo CS. Nu turn = j th Pj vo CS

nhng s bt flag[j]=false khi thot ra -> cho php Pi

v CS

Nhng nu Pj c thi gian bt flag[j]=true th Pj

cng phi gn turn = i

V Pi khng thay i tri cua bin turn khi ang ket

trong vong lp while(), Pi se ch va o CS nhiu

nht la sau mt ln Pj vao CS (bounded waiting)

12 ng b Duy Phan

Gii thut bakery: n process

Trc khi vao CS, process Pi nhn mt con s. Process nao gi con s nho nht thi c vao CS

Trng hp Pi va Pj cung nhn c mt chi s:

N u i < j thi Pi c vao trc. (i xng)

Khi ra khoi CS, Pi t lai s cua minh bng 0

C ch cp s cho cac process thng tao cac s theo c ch tng dn, vi du 1, 2, 3, 3, 3, 3, 4, 5,

Ki hiu

(a,b) < (c,d) nu a < c hoc if a = c va b < d

max(a0,,ak) la con s b sao cho b ai vi moi i = 0,, k

13 ng b Duy Phan

Gii thut 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;

remainder section

} while (1);

14 ng b Duy Phan

T software n hardware

Khuyt im ca cc gii php software:

Cac process khi yu cu c vao vung tranh chp u phai lin tuc kim tra iu kin (busy waiting), t n nhiu thi gian x ly cua CPU

N u thi gian x ly trong vung tranh chp ln, mt giai phap hiu qua nn co c ch block cac process c n i.

Cc gii php phn cng:

Cm ngt (disable interrupts)

Du ng cac lnh c bit

15 ng b Duy Phan

Cm ngt

Trong h thng uniprocessor:

mutual exclusion c m bo

Nhng nu system clock c

cp nht do interrupt th

Trong h thng multiprocessor:

mutual exclusion khng c

m bo

Ch cm ngt ti CPU thc thi

lnh disable_interrupts

Cc CPU khc vn c th truy

cp b nh chia s

Process Pi: do { disable_interrupts(); critical section enable_interrupts(); remainder section } while (1);

16 ng b Duy Phan

Lnh TestAndSet

c v ghi mt bin trong mt thao tc atomic

(khng chia ct 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);

17 ng b Duy Phan

Lnh TestAndSet (tt)

Mutual exclusion c bo m: nu Pi vo CS, cc

process Pj khc u ang busy waiting

Khi Pi ra khoi CS, qu trinh chn la process Pj vo

CS k tip l tuy y khng bo m iu kin bounded waiting. Do o co th xy ra starvation (bi

bo oi)

Cc processor (vi du Pentium) thng thng cung cp

mt lnh n l Swap(a, b) co tc dung hon chuyn

ni dung cua a v b.

Swap(a, b) cung co u nhc im nh

TestAndSet

18 ng b Duy Phan

Swap v mutual exclusion

Bin chia se lock c khi to gia tr false

Mi process Pi co bin cc b key

Process Pi nao thy gia tr lock = false thi c vao CS.

Process Pi s loi tr cac process Pj khac khi thit lp lock = true

void Swap(boolean *a,

boolean *b) {

boolean temp = *a;

*a = *b;

*b = temp;

}

Bi n chia se (khi ta o 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)

Khng thoa man bounded waiting

19 ng b Duy Phan

Gii thut dung TestAndSet tho man 3 yu cu

C u tru c d li u du ng chung (khi ta o la false)

bool waiting[ n ];

bool lock;

Mutual exclusion: Pi chi co th va o CS n u va chi n u ho c waiting[ i ] = false, ho c key = false

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

Process u tin thc thi TestAndSet mi co key == false; ca c process kha c u pha i i

waiting[ i ] = false chi khi process kha c ri kho i CS

Chi co m t waiting[ i ] co gia tri false

Progress: chng minh tng t nh mutual exclusion

Bounded waiting: waiting in the cyclic order

20 ng b Duy Phan

Gii thut dung TestAndSet tho man 3 yu cu (tt)

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)

21 ng b Duy Phan

n tp

Cc gii php phn mm

S dng gii thut kim tra lun phin

S dng cc bin c hiu

Gii php ca Peterson

Gii php Bakery

Cc gii php phn cng

Cp ngt

Ch th TSL

01/2015 Duy Phan

Kt thc chng 5-2

01/2015Duy Phan

Chng 5: ng b - 3

2 ng bDuy Phan

Cu hi n tp 5 - 2

3 ng bDuy Phan

Mc tiu

Bit c cc gii php ng b tin trnh theo kiu

Sleep & Wake up bao gm:

Semaphore

Critical Region

Monitor

p dng cc gii php ny vo cc bi ton ng b

kinh in

4 ng bDuy Phan

Ni dung

Cc gii php Sleep & Wake up

Semaphore

Cc bi ton ng b kinh in

Critical Region

Monitor

5 ng bDuy Phan

Cc gii php Sleep & Wake up

int busy; // =1 nu CS ang bi chimint blocked; // s P ang bi khoado{

if (busy){blocked = blocked +1;sleep();

}else busy =1;

CS;busy = 0;if (blocked !=0){

wakeup (process);blocked = blocked -1;

}RS;

} while (1);

6 ng bDuy Phan

Semaphore

L cng cu ng b cung cp bi OS m khng i hi busy

waiting

Semaphore S l mt bin s nguyn.

Ngoi thao tc khi ng bin thi chi c th c truy xut qua

hai tc vu c tinh n nguyn (atomic) v loai tr (mutual

exclusive)

wait(S) hay cn goi la P(S): giam gia tri semaphore (S=S-1) . K o

nu gia tri nay m thi process thc hin lnh wait() bi blocked.

signal(S) hay cn goi la V(S): tng gia tri semaphore (S=S+1) . K

o nu gia tri nay khng dng, mt process ang blocked bi mt

lnh wait() se c hi phuc thc thi.

Tranh busy waiting: khi phai i thi process se c t vao mt

blocked queue, trong o cha cac process ang ch i cung mt

s kin.

7 ng bDuy Phan

Semaphore (tt)

P(S) hay wait(S) s dung gianh tai nguyn va

giam bin m S=S-1

V(S) hay signal(S) se giai phong tai nguyn va tng

bin m S= S+1

Nu P c thc hin trn bin m

8 ng bDuy Phan

Hin thc semaphore

inh nghia semaphore la mt record

typedef struct {

int value;

struct process *L; /* process queue */

} semaphore;

Gia s h iu hanh cung cp hai tac vu (system

call):

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

wakeup(P): hi phuc qua trinh thc thi cua process P

ang blocked

9 ng bDuy Phan

Hin thc semaphore (tt)

Cac tac vu semaphore c hin 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

10 ng bDuy Phan

Hin thc semaphore (tt)

Khi mt process phai ch trn semaphore S, no se

bi blocked va c t trong hang i semaphore

Hang i nay la danh sach lin kt cac PCB

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

chon mt process t hang i va a vao hang i

ready

block() va wakeup() thay i trang thai cua process

block: chuyn t running sang waiting

wakeup: chuyn t waiting sang ready

11 ng bDuy Phan

V d s dng semaphore 1

Shared data:

semaphore mutex;

/* initially mutex.value = 1 */

Process Pi:

do {

wait(mutex);

critical section

signal(mutex);

remainder section

} while (1);

Dung cho n process

Khi tao S.value = 1

Chi duy nht mt

process c vao CS

(mutual exclusion)

cho phep k process

vao CS, khi tao

S.value = k

12 ng bDuy Phan

V d s dng semaphore 2

ng b hoat ng

theo yu cu, P1 phai inh

nghia nh sau:

S1;

signal(synch);

Va P2 inh nghia nh sau:

wait(synch);

S2;

Hai process: P1 va P2

Yu cu: lnh S1 trong

P1 cn c thc thi

trc lnh S2 trong P2

inh nghia semaphore

synch ng b

Khi ng semaphore:

synch.value = 0

13 ng bDuy Phan

Nhn xt

Khi S.value 0: s process co th thc thi wait(S)

ma khng bi blocked = S.value

Khi S.value < 0: s process ang i trn S la

S.value

Atomic va mutual exclusion: khng c xay ra

trng hp 2 process cung ang trong thn lnh

wait(S) va signal(S) (cung semaphore S) tai mt

thi im (ngay ca vi h thng multiprocessor)

do o, oan ma inh nghia cac lnh wait(S) va signal(S) cung chinh la vung tranh chp

14 ng bDuy Phan

Nhn xt (tt)

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

thng thng rt nh: khoang 10 lnh.

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

Uniprocessor: co th dung c ch cm ngt (disable

interrupt). Nhng phng phap nay khng lam vic

trn h thng multiprocessor.

Multiprocessor: co th dung cac giai phap software

(nh giai thut Dekker, Peterson) hoc giai phap

hardware (TestAndSet, Swap).

Vi CS rt nh nn chi phi cho busy waiting se rt thp.

15 ng bDuy Phan

Deadlock v starvation

Deadlock: hai hay nhiu process ang ch i v han inh mt s

kin khng bao gi xay ra (vd: s kin do mt trong cac process

ang i tao ra).

Goi S va Q la hai bin 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), ri P1 thc thi wait(Q), ri P0 thc thi wait(Q)

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

Starvation (indefinite blocking) Mt tin trinh co th khng bao

gi c ly ra khi hang i ma no bi treo trong hang i o.

16 ng bDuy Phan

Cc loi semaphore

Counting semaphore: mt s nguyn co gia tri

khng han ch.

Binary semaphore: co tri la 0 hay 1. Binary

semaphore rt d hin thc.

Co th hin thc counting semaphore bng binary

semaphore.

17 ng bDuy Phan

Cc bi ton ng b kinh in

Bounded Buffer Problem

Readers and Writers Problem

Dining-Philosophers Problem

18 ng bDuy Phan

Bi ton bounder buffer

D liu chia s:

Semaphore full, empty, mutex;

Khi tao:

full = 0; /* s buffers y */

empty = n; /* s buffers trng */

mutex = 1;

out

n buffers

19 ng bDuy Phan

Bi ton bounder buffer

do {

wait(full)

wait(mutex);

nextc = get_buffer_item(out);

signal(mutex);

signal(empty);

consume_item(nextc);

} while (1);

do {

nextp = new_item();

wait(empty);

wait(mutex);

insert_to_buffer(nextp);

signal(mutex);

signal(full);

} while (1);

producer consumer

20 ng bDuy Phan

Bi ton Dining Philosophers

5 trit gia ngi n va

suy nghi

Mi ngi cn 2 chic

ua (chopstick) n

Trn ban chi co 5 ua

Bai toan nay minh hoa

s kho khn trong vic

phn phi tai nguyn

gia cac process sao

cho khng xay ra

deadlock va starvation

0

23

4 0 1

4 2

3

D liu chia s:

Semaphore chopstick[5];

Khi u cac bin u la 1

21 ng bDuy Phan

Bi ton Dining Philosophers (tt)

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);

22 ng bDuy Phan

Bi ton Dining Philosophers (tt)

Giai phap trn co th gy ra deadlock Khi tt ca trit gia oi bung cung luc va ng thi cm

chic ua bn tay trai deadlock

Mt s giai phap khac giai quyt c deadlock Cho phep nhiu nht 4 trit gia ngi vao cung mt luc

Cho phep trit gia cm cac ua chi khi ca hai chic ua u

sn sang (nghia la tac vu cm cac ua phai xay ra trong

CS)

Trit gia ngi vi tri l cm ua bn trai trc, sau o mi

n ua bn phai, trong khi o trit gia vi tri chn cm

ua bn phai trc, sau o mi n ua bn trai

Starvation?

23 ng bDuy Phan

Bi ton Reader-Writers

Writer khng c cp nht d liu khi co mt

Reader ang truy xut CSDL

Tai mt thi im, chi cho phep mt Writer c

sa i ni dung CSDL

Database

R1

R2

R3

W1 W2

24 ng bDuy Phan

Bi ton Reader-Writers (tt)

B oc trc b ghi (first

reader-writer)

D liu chia s

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);

25 ng bDuy Phan

Bi ton Reader-Writers (tt)

mutex: bao v bin readcount

wrt

Bao am mutual exclusion i vi cac writer

c s dung bi reader u tin hoc cui cung vao

hay ra khi vung tranh chp.

Nu mt writer ang trong CS va co n reader ang i

thi mt reader c xp trong hang i cua wrt va n 1

reader kia trong hang i cua mutex

Khi writer thc thi signal(wrt), h thng co th phuc hi

thc thi cua mt trong cac reader ang i hoc writer

ang i.

26 ng bDuy Phan

Cc vn vi semaphore

Semaphore cung cp mt cng cu manh me bao am

mutual exclusion va phi hp ng b cac process

Tuy nhin, nu cac tac vu wait(S) va signal(S) nm rai rac

rt nhiu processes kho nm bt c hiu ng cua cac tac vu nay. Nu khng s dung ung co th xay ra tinh trang deadlock hoc starvation.

Mt process bi die co th keo theo cac process khac

cung s dung bin semaphore.

signal(mutex)

critical section

wait(mutex)

wait(mutex)

critical section

wait(mutex)

signal(mutex)

critical section

signal(mutex)

27 ng bDuy Phan

Critical Region (CR)

La mt cu truc ngn ng cp cao (high-level language

construct, c dich sang ma may bi mt compiler),

thun tin hn cho ngi lp trinh.

Mt bin chia s v kiu d liu T, khai bao nh sau

v: shared T;

Bin chia s v chi co th c truy xut qua phat biu sau

region v when B do S; /* B la mt biu thc Boolean */

Y nghia: trong khi S c thc thi, khng co qua trinh

khac co th truy xut bin v.

28 ng bDuy Phan

CR v bi ton bounded buffer

D liu chia se:

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

29 ng bDuy Phan

Monitor

Cung la mt cu truc ngn ng cp cao tng t

CR, co chc nng nh semaphore nhng d iu

khin hn

Xut hin trong nhiu ngn ng lp trinh ng thi

nh

Concurrent Pascal, Modula-3, Java,

Co th hin thc bng semaphore

30 ng bDuy Phan

Monitor (tt)

La mt module phn mm,

bao gm

Mt hoc nhiu thu tuc

(procedure)

Mt oan code khi tao

(initialization code)

Cac bin d liu cuc b

(local data variable)

c tinh cua monitor

Local variable chi co

th truy xut bi cac

thu tuc cua monitor

Process vao monitor

bng cach goi mt

trong cac thu tuc o

Chi co mt process co

th vao monitor tai mt

thi im mutual exclusion c bao

am

shared data

entry queue

operations

initialization

code

M hinh cua mt monitor

n gian

31 ng bDuy Phan

Cu trc ca monitor

monitor monitor-name{

shared variable declarations

procedure body P1 () {. . .

}

procedure body P2 () {. . .

}

procedure body Pn () {. . .

}

{

initialization code

}

}

32 ng bDuy Phan

Condition variable

Nhm cho phep mt process i trong monitor, phai khai

bao bin iu kin (condition variable)

condition a, b;

Cac bin iu kin u cuc b va chi c truy cp bn

trong monitor.

Chi co th thao tac ln bin iu kin bng hai thu tuc:

a.wait: process goi tac vu nay se bi block trn bin iu kin a

process nay chi co th tip tuc thc thi khi co process khac thc

hin tac vu a.signal

a.signal: phuc hi qua trinh thc thi cua process bi block trn bin

iu kin a.

Nu co nhiu process: chi chon mt

Nu khng co process: khng co tac dung

33 ng bDuy Phan

Monitor c condition variable

Cac process co th i entry

queue hoc i cac condition

queue (a, b,)

Khi thc hin lnh a.wait,

process se c chuyn vao

condition queue a

Lnh a.signal chuyn mt

process t condition queue a

vao monitor

Khi o, bao am mutual

exclusion, process goi a.signal

se bi blocked va c a vao

urgent queue

entry queueshared data

...

operations

initialization

code

a

b

34 ng bDuy Phan

Monitor c condition variable (tt)

local data

condition variables

procedure 1

procedure k

initialization code

...

monitor waiting area entrance

entry queue

c1.wait

condition c1

condition cn

cn.wait

urgent queue

cx.signal

...

MONITOR

exit

35 ng bDuy Phan

Monitor v dining philosophers

monitor dp {

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

condition self[5];

0

1

23

4

36 ng bDuy Phan

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

}

37 ng bDuy Phan

Dining philosophers (tt)

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;

}

}

38 ng bDuy Phan

Dining philosophers (tt)

Trc khi n, mi trit gia phai goi ham pickup(),

n xong ri thi phai goi ham putdown()

dp.pickup(i);

n

dp.putdown(i);

Giai thut khng deadlock nhng co th gy

starvation.

39 ng bDuy Phan

Cu hi n tp

Semaphore l g? Nu cch hoat ng cua semaphore

v ng dung vo mt bi ton ng b?

Monitor l g? Nu cch hoat ng cua monitor v

ng dung vo mt bi ton ng b?

40 ng bDuy Phan

Bi tp

S dng

01/2015Duy Phan

Kt thc chng 5-3

ch5-1.pdfch5-2.pdfch5-3.pdf