Process Synchronization[2]

7/25/2019 Process Synchronization[2]

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PROCESS

SYNCHRONIZATION

OPERATING SYSTEM


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TOPICS COVERED:

BackgroundThe Critical-Section Problem

Peteron! SolutionS"nchroni#ation $ard%areSema&hore'onitorClaic Problem o( S"nchroni#ationS"nchroni#ation E)am&le*lternati+e *&&roache


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B*C,RO./DPROD.CER-CO/S.'ER PROB0

R*CE CO/DIT


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PROD.CER-CO/S.'ERPROB0E'The &roducer1conumer &roblem i a claic e)am&

multi-&roce "nchroni#ation &roblem2 The &roblemdecribe t%o &rocee3 the &roducer and the con%ho hare a common3 4)ed-i#e bu5er2

The &roducer6 7ob i to generate a &iece o( data3 &uinto the bu5er and tart again2 *t the ame time3 th

conumer i conuming the one &iece at a time2


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R*CE CO/DITIO/

%here e+eral &rocee acce and mani&uthe ame data concurrentl"

the outcome o( the e)ecution de&end on th&articular order in %hich the acce take &l


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CRITIC*0-SECTIO/PROB0E'


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General structureof a typical processEntry SectionCode re8ueting entr" into the

critical ection2

Critical Section Code in %hich onl" one&roce can e)ecute at an" onetime2

Exit Section The end o( the critical ection3releaing or allo%ing other in2

Reain!er Section Ret o( the code *9TER

the critical ection2


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Critical-Section Problem

Conider "tem o( n&rocee P0, P1, , Pn-1

Each &roce ha a critical section egment o( c

1 Process may be changing common variables, uptable, writing fle, etc.

1 When one process is in its critical section, no otexecuting in its critical section

Critical-section problem i to deign a &rotocolthi2


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Critical-Section ProblemRe8uirement1.Mutual Exclusion - If process P

i is executing in its critical section, the

processes can be executing in their critical sections

2. Progress - If no process is executing in its critical section and t

processes that wish to enter their critical section, then

processes not in their remainder section can participate

of the process that will enter its critical section next; the be postponed indefinitely

3. Bounded Waiting - A bound must exist on the number of times that o

are allowed to enter their critical sections after a process

request to enter its critical section and before that reque


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PETERSO/!SO0.TIO/

Click icon to add &icture


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Structure o( &roce Piin Peteron! olutionSoftware solution; no hardware support

Two process solutionThe two processes share two variables:

int turn;

Boolean flag[2];

turnindicates whose turn it is to enter thecritical section

Theflagarray indicates whether aprocess is ready to enter the criticalsection

flag[i]= true ==> processPiis ready

Click icon to add &icture


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PETERSO/!S SO0.TIO/


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S=/C$RO/I>*TIO/$*RD


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S"nchroni#ation $ard%are*OC+ING( &rotecting critical ection through the

lock

.ni&roceor 1 could diable interru&t1 Currently running code would execute without 1 Generally too inecient on multiprocessor syst

'an" "tem &ro+ide hard%are u&&ort (or criticcode more e?cient and eaier (or &rogrammer

'odern machine &ro+ide &ecial atoic ,ar!)ainstructions

*tomic @ non-interru&table1 Either test memory word and set value1 r swap contents o! two memory words


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tetAandAet intruction

De4nition o( thetetAandAet intruction

Boolean test_and_set(boolean *target)

{

boolean rv = *target;

*target = TRUE;

return rv:

}

'utual-e)cluionim&lementation

do { while(test_and

; #* do nothi#* !riti!al se!t

lo!" = $alse;

#* re%ainder se!

} while (true);

T%o test_and_set()cannot be e)ecutedimultaneoul"

Shared boolean +ariablelo!"

3 initiali#ed to9*0SE


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com&are AandA%a& intruct

De4nition o( thecom&areAandA %a&intruction

int !o%&are_and_swa&(int*value' int e&e!ted' int

new value){int te%& = *value;

i$ (*value == e&e!ted)*value = new value;

return te%&;

}

'utual-e)cluionim&lementationdo {while (!o%&are_and_s) += ) ##value'e

; #* do nothing

#* !riti!al se!t

lo!" = ;

#* re%ainder se!

} while (true);

The com&are and %a& intruction3 in contrat to theet intruction3 o&erate on three o&erand


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'.TE 0OC,


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'.TE 0OC,S

OS deigner build o(t%are tool to ol+e critical ect

Sim&let "nchroni#ation i done %ith utex -utuaexclusion. loc/

Protect critical region b" 4rt a!,uire()a lock then&rocee attem&ting to get the lock are blocked3 andrelease()it

a!,uire() { while (+available)

; #* bus- wait *#

available = $alse;

}

Sol tion to critical


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Solution to critical(section pro'le

usin% utex loc/s Calls toa!,uire()andrelease()must beatomic Usually implemented via hardware atomicinstructions

But this solution requiresbusy waiting

All other processes trying to get the lock

must continuously loop This lock is therefore called aspinlock

Very wasteful of CPU cycles

They are often employed on multiprocessor

systems


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SE'*P$ORE


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Sema&hore

S"nchroni#ation tool that doe not re8uire bu" %ai

Sema&hore S1 integer +ariable

T%o tandard o&eration modi(" S :wait()and sig

0e com&licated

Can onl" be acceed +ia t%o indi+iible atomico&eration

wait (.) {while (. /= )

; ## bus- wait .00;}


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Sema&hore .age

Countin% seap,ore 1 integer +alue can range o+er an u

domain0inary seap,ore 1 integer +alue can range onl" bet%ee

1 "hen e#uivalent to a mutex lock

Can ol+e +ariou "nchroni#ation &roblem

Conider P1 and P2 that re8uireS1 to ha&&en be(ore S2

P1: S1;

signal(synch);

P2:

wait(synch); 0

S2;


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Deadlock and Star+ation

1ea!loc/ 1 t%o or more &rocee are %aiting inde4nitee+ent that can be caued b" onl" one o( the %aiting &roc

0et S andQbe t%o ema&hore initiali#ed to F P2 P"

wait(.); //exec 1st wait(2);

wait(2); //exec 3rd wait(.);

333 333

signal(.); signal(2); signal(2); signal(.);

Star3ation1 in!e4nite 'loc/in%1 $ process may never be removed !rom the semaphor

which it is suspended


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C*ASSIC PRO0*EMO5 SYNCHRONIZATIO


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The Bounded-Bu5er Problem

Thi i a generali#ation o( the &roducer-conumer &ro

%herein acce i controlled to a hared grou& o( bu5o( a limited i#e2

In our &roblem3 the &roducer and conumer &roceehare the (ollo%ing data tructure:


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The Bounded-Bu5er Problem

tructure o( the&roducer &roce

do {

333 #* &rodu!e an ite% in net_&rodu!ed *#

333

wait(e%&t-);

wait(%ute);

333 #* add net_&rodu!ed to the bu$$er *#

333

signal(%ute);

signal($ull);

} while (true);

structure of theconsumer pr

do {

wait($ull);

wait(%ute);

333#* re%ove an ite% $ro% bu$$er to

333

signal(%ute);

signal(e%&t-);

333#* !onsu%e the ite% in net_!onsu

333

} while (true);


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The Reader1


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The Reader1


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The Dining-Philoo&her Probl Conider 4+e &hiloo&her itting around a

table3 in %hich there are 4+e inglecho&tick e+enl" ditributed and an

endle bo%l o( rice in the center3 aho%n in the diagram2

Thee &hiloo&her &end their li+ealternating bet%een t%o acti+itie: eatingand thinking2


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The Dining-Philoo&her Probl

One im&le olution i to re&reent each cho&tick %ith a ema&hiloo&her trie to grab a cho&tick b" e)ecuting a )ait-. o&ema&hore2 She releae her cho&tick b" e)ecuting the si%n

Some &otential olution to the &roblem include:

Onl" allo% (our &hiloo&her to dine at the ame time2 0imultaneou &rocee2

*llo% &hiloo&her to &ick u& cho&tick onl" %hen both aa critical ection2

.e an a"mmetric olution3 in %hich odd &hiloo&her &cho&tick 4rt and e+en &hiloo&her &ick u& their right c


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'O/ITOR

' i


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'onitor

* high-le+el abtraction that&ro+ide a con+enient ande5ecti+e mechanim (or&roce "nchroni#ation

$bstract data type3 internal+ariable onl" acceible b"code %ithin the &rocedure

Onl" one &roce ma" beacti+e %ithin the monitor at atime

Sc,eatic 3ie)

C diti V i bl


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Condition Variable

* con!ition 3aria'lei baicall" a container o( threthat are %aiting (or a certain condition2

* +ariable o( t"&e condition ha onl" t%o legalo&eration3 )aitand si%nal2

1 "he wait operation bloc)s a process until some otprocess calls signal, and adds the bloc)ed procesa list associated with that condition.

1 "he signal process does nothing i! there are noprocesses waiting on that condition.


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Condition Variable


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S=/C$RO/I>*TIO/E*'P0E

S nchroni ation in


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S"nchroni#ation in


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'ute) di&atcher ob7ect


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S"nchroni#ation in 0inu)

The im&let "nchroni#ation techni8ue %ithin the 0

kernel i an atomic integer3 %hich i re&reented uo&a8ue data t"&e atoic8t#

The (ollo%ing code illutrate declaring an atomic incounterand then &er(orming +ariou atomic o&era

S"nchroni#ation in 0inu)


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S"nchroni#ation in 0inu) *tomic integer are &articularl" e?cient in ituation

%here an integer +ariable uch a a counternebe u&dated3 ince atomic o&eration do not re8uire

o+erhead o( locking mechanim2 0inu) alo &ro+ide &inlock and ema&hore a %

reader1%riter +erion o( thee t%o lock (or lockinthe kernel2

S h i ti i S l i


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S"nchroni#ation in Solari Im&lement a +ariet" o( lock to u&&ort multitaking3

multithreading including real-time thread3 and multi&r

.e a!apti3e utex loc/s(or e?cienc" %hen &rotect(rom hort code egment

1 +tarts as a standard semaphore spin0loc)

.e con!ition 3aria'les

.e rea!ers()riterslock %hen longer ection o( codacce to data

.e turnstilesto order the lit o( thread %aiting to acan ada&ti+e mute) or reader-%riter lock


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Pthread S"nchroni#ation

Pthread *PI i OS-inde&endent

It &ro+ide:

1 mutex loc)s

1 condition variables

1 read0write loc)s

Pthread ue the pt,rea!8utex8t data t"&e (or m


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*0TER/*TIV*PPRO*C$E

Tranactional 'emor"


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Tranactional 'emor"

*ure that o&eration ha&&en a a ingle logical uin it entiret"3 or not at all

Related to the 4eld o( databae "tem

Challenge i auring atomicit"3 de&ite com&uter (ailure

Tranaction - collection o( intruction or o&eration&er(orm ingle logical (unction

1 1ere we are concerned with changes to stable s1 "ransaction is series o! readand writeoperation

Soft)are transactional eory ST'3 a the nugget3 im&lement tranactional memor" e)cluo(t%are

Har!)are transactional eory $T' re8uire


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9unctional Programming0anguage9unctional language do not maintain tate

1 nce a variable is assigned a value, it canchange anymore

/o uch &roblem a race condition and

deadlockThere(ore uitable (or concurrentJ&arallel

&rogramming on multicore

E2g2: Erlang3 Scala

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Process Synchronization[2]