presentation oss 07-05-2013

8/13/2019 Presentation OSS 07-05-2013

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Operating System Server

Dining Philosophers Problem using

semaphores

Emily Branagan, B00055105

Ciara Walsh, B00050867

Jacek Klos, B00053402Jose ernan!e" B00065734

#an$el %anche" B00065732


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New and previous solution

Semaphore Solution is an evolution from

the Mutual Exclusion Locks Solution.

The main problem with the previous

solution was that If all five philosophers

pick up their left chopsticks at the same

time, they will not be able to take the

other one and a deadlockwill occur.

&o 'i( )ha) *ro+lem )his sema*hore

sol$)ion only allos )ha) 'o$r

*hiloso*hers si) !on a) )he )a+le-&o achie.e )ha) a sema*hore

ini)iali"e! )o 'o$r is $se!, each

*hiloso*her ai) on )his

sema*hore +e'ore he can *ick $*

cho*s)icks, an! e ha.e 'inishe!

ea)ing, he has )o signal )he

sema*hore )o release )he chair-


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Semaphore is a synchronization tool.

Extension of the mutual locks solution but there aredifferences.

It involves the execution of two atomic solutions,

Wait! and Si"nal!.

Semaphore Solution


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Execution

Wait!The counter is initialized to #$

Wait be"ins and the counter decreases, execution continues.

The counter is at %$

The thread is suspended and placed in a &ueue Si"nal!

'o waitin" threads$

The counter that was on % is now increased to #, the thread

starts the wait process a"ainThreads waitin"$

The counter must be on %, one thread is chosen to leave the

&ueue.


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Advantages of Semaphores

There is no wastin" of resources, due to no busy waitin"

When a process is de(&ueued, it is done in the semaphoreimplementation They allow flexible resource mana"ement

They are machine independent


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Disadvantages of Semaphores

Impractical when it comes to lar"e scale use

Semaphores use &ueues in its implementation, in thecase of )I)* if a hi"h priority process comes into the

&ueue it must wait while other low priority process

finish.


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Philosopher code

/i'n!e' %E

/!e'ine %E

/incl$!e &hrea!Class-h

macro !e'ines

/!e'ine 9:#%E% 5

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

hiloso*her class !e'ini)ion;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

class hiloso*her< *$+lic &hrea!

=

public:

Philosopher(int Number, int iter); // builder

private:

int No; // position of the philosopher

int Iteration;

void ThreadFunc();//where the code of each philosopher will be

>?

/en!i'


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Initialiing mutex loc!s and

semaphore

locks an! sema*hores

s)a)ic %ema*hore

o$rChairs@o$rChairs,9:#%E% ; 1A?

s)a)ic #$)e( Cho*s)ick1@Cho*s)ick1A?s)a)ic #$)e( Cho*s)ick2@Cho*s)ick2A?

s)a)ic #$)e( Cho*s)ick3@Cho*s)ick3A?



s)a)ic #$)e( Cho*s)ick9:#%E%D

!"hopstic#$, !"hopstic#%, !"hopstic#&, !"hopstic#',

!"hopstic# ;


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"hread#unc$% &ode

Four"hairs*+ait(); // allows ' to sit down onl if s-. (semaphore00)

"hopstic#1No2034oc#();

"hopstic#1(No 5 $) 6 N789F9PI4


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'ain Program

hiloso*her hiloso*hers9:#%E%D?

int i, Iteration;

.

-

// create all the philosopher threads and fire them up

for (i . ; i ? N789F9PI4


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JN- join thread.SS- semaphore signal.SW- semaphore wait.

SE- semaphore enter.MW- Mutex wait.ML- Mutex lock.MU- Mutex unlock.

(egend about "hread'entor


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'utex Solution &ode

Bela();




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&ircular waiting example


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&ircular waiting example


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Solution to circular waiting

Four"hairs*+ait(); // allows ' to sit down onl if s-. (semaphore00)




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Solution to &ircular )aiting


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Solution to &ircular )aiting


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Deadloc! and Starvation solution

*ow does it come to a deadloc! sytuation+

If two philosophers are really fast eatin" and thinkin", they

always be in control of the chopsticks, avoidin" the

others philosophers.

When all philosopers take one chopstick, and a waitin" for

the other one.


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+ere is a typical eadlock -roblem

q ll thread are redq ll philosopher blockedq ll philospher waitin"


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Solution to avoid the deadloc! problem

F /reatin" a semaphore waitin"! that allows four

philosophers at a table while blockin" the fifth. Thismake sure that one philosopher will have both

chopsticks.

F /reatin" the )I)* )irst In )irst *ut! &ueue with limitedeatin" time. This prevents from someone eatin" all the

time.

presentation oss 07-05-2013

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