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Given below is a program which when executed spawns two concurrent processes :
semaphore X : = 0 ;
/* Process now forks into concurrent processes P1 & P2 */

P1 P2
repeat forever      
V (X) ;                      
Compute ;                  
P(X) ;                        
 repeat forever
 P(X) ;
 Compute ;
 V(X) ;


Consider the following statements about processes P1 and P2:

  1. It is possible for process P1 to starve.
  2. It is possible for process P2 to starve.

Which of the following holds?

  1. Both I and II are true.
  2. I is true but II is false.
  3. II is true but I is false
  4. Both I and II are false
asked in Operating System by Veteran (21.9k points) | 3.1k views
how?

@reena+kandari Question says It is possible, it doesn't says that it is always right.

@reena Before 2011 there were no official GATE keys :)
here a possibility haas been asked, its like there exists,(_OR_ OR..) so we only need to proove 1 possibility in which the statements turns out to be true(A)... if cant find any such case then all false (D)
in worst case both can starve foreever, if possible..
@Bikram Sir and @Arjun Sir pls explain this

The main function of up operation is to wake up the blocked process from queue and put into the critical section .In this question if we start from p1 it can go into the critical section becoz for up operation even the x=0 it not matters and new value of x is still 0, after going into cs if P2 wants to enter or perform down operation it will be blocked ,it still wait for p1 to come out from cs .My doubt ,is after this how p1 will perform up operation? is this unambiguous?

P1 will starve for come out from cs and P2 will starve to go into the critical section .Is this fine ?

The main function of down operation is to wake up the blocked process from queue and put into the critical section

what you have written is a function of V(S) i.e is up operation.. 

joshi_nitish  thnq for correcting me ,now my question is event value of x=0,process p1 can enter in cs then in this case the value of x is o still .if process p2 comes it will wait for p1 infinitely long time ?Am I right ?

Down operation means P ( wait) , it means if the value of semaphore variable is not negative, decrement the value by 1. If the semaphore variable is now negative, the process executing wait is blocked (i.e., added to the semaphore's queue) until the value is greater or equal to 1.

And Up operation (Signal ) V means : First increments the value of semaphore variable by 1. After the increment, if the pre-increment value was negative (meaning there are processes waiting for a resource), it transfers a blocked process from the semaphore's waiting queue to the ready queue.

Here P1 is performing up and P2 performing down operation.

if we start from P1, initially x ==0 , so after V(x) it's new value would be x==1

and for P2 ,  x==0 so it need to wait until x become 1.
@bikram sir if the value of x==1 .after up operation for p1 it will do nothing or according to question it will go into critical section ?and if this time  p2 wants to enter in cs it will go becoz value of x is 1 for p2 ?

 set2018 

 if the value of x==1 .after up operation for p1 , according to question it will go into critical section .

if this time  p2 wants to enter in cs it will go becoz value of x is 1 for p2 

It may be possible p1 keep looping so that makes the possibility of starvation for p2.

Another possibility is p2 can enter and keep looping so p1 can starve .

So both are possible.

@Habibkhan @Hemant Parihar @junaid ahmad   Please check

Mutual exclusion is not satisfied.

Suppose P1 executes V(x) and enter compute and gets preempted. Now P2 can execute P(x) and enter Compute.

Progress is not satisfied.

Suppose initially only P2 wants to enter the CS..not P1.Then P2 cant enter the CS.It has to wait for P1 to enter the CS as X=0. The decision of who will enter critical section is dependent on p1 who is not interested to enter CS.

Bounded Waiting is not satisfied.

Suppose p1 executes completely and gets preempted. P2 can't enter CS since X = 0. Again P1 executes completely and gets preempted. Again P2 can't enter CS.

@shivam chauhan

Yes, Mutual exclusion and progress are not satisfied here. But bounding waiting is satisfied.

When P2 show the desire to go into CS and got sleep. Then, when P1 do V(X) then it will wake up the P2 to go into CS.
Wake up operation is done only if we have a blocked list of processes maintained. And by default, there is no queue associated with counting semaphore. So P1 after executing V(x) does not necessarily preempt and can move into compute.

See the answer both P1 and P2 can starve.

Hi @Shivam Chauhan ji 

And by default, there is no queue associated with counting semaphore

Could you please mention some reference for this ?

I think, after V(S) of $P_{2}$, $P_{1}$ will also come in ready state (If in case it was waiting for V(S) operation) but $P_{2}$ could be selected for execution again. Due to which $P_{1}$ could be moved back to blocked list of respective semaphore.

 

@Hemant Parihar and @Shivam Chauhan ji What is your opinion ?

Doubt regarding bounded waiting

Considering the case in which P1 is executing the CS, P2 enters the CS (as mutual exclusion not satisfied here), suppose P1 wants to exit first, it will be blocked at P(X). It will only be unblocked when P2 comes out of the CS and finishes the remaining code i.e V(X). So value of X will be 1 again. Thus, either P1 can finish the remaining code by using X value or P2 can use X value to again enter CS. Here, we cannot say when will that happen as there might be a case in which P2 keeps on coming. So, bounded waiting not satisfied. Is this approach correct?

Also, is there a relation between starvation and bounded waiting?
there is no bounded waiting  otherwise process executes infinitely long.

9 Answers

+39 votes
Best answer

Check : What is Starvation?
 

Here $P_2$ can go in infinite waiting while process $P_1$ executes infinitely long.

 

Also, it can be the case that the Process $P_1$ starves for $\infty$ long time on the semaphore S, after it has successfully executed its critical section once, while $P_2$ executes infinitely long.

 

Both $P_1$ and $P_2$ can starve for $\infty$ long period of time.

answer = option A

answered by Veteran (30.5k points)
selected by
@Bikram Sir,
A process executing up operation never gets blocked. Is it correct statement ??

AnilGoudar 

Up operation ( Signal V )  transfers a blocked process from the semaphore's waiting queue to the ready queue.

So yes , executing up operation never gets blocked.  It is correct statement .

but is it guaranteed to have a queue?
@ Bikram sir

By default, semaphores do not have queue implementation right??
@Arjun
I went through all the comments, and i am unable to understand onething, when both processes can wake to each other up, then why will even they starve?
we are considering worst case ...

Hi @sushmita ji,

By default, semaphores do not have queue implementation right?

Could you please mention some reference ? 

Hi @Manu Thakur ji,

I went through all the comments, and i am unable to understand onething, when both processes can wake to each other up, then why will even they starve?

Please read my top comment. I hope it answers your doubt.

Later, i understood that part. Anyway,Thanks @Chhotu ji,

@Manu Thakur ji

Later, i understood that part

If your reasoning is different then mine then please mention.

+26 votes
A is the answer.

Case 1:Here P1 is performing signal operation so it is first one to start...Now it may happen that after executing CS it makes X=0 again tries to enter CS my making X=1 so it is possible that P2 can starve(just a possibility)

Case 2:If P1 executes signal operation and its execution is suspended temporarily, P2 executes wait and enter CS  and then execute signal operation making X=1 now P2 can enter critical section by executing wait operation..this may happen for infinite amount of time..so in this P1 may starve
answered by Veteran (33.9k points)
edited by
actually we are not told to ensure mutual exclusion. The given code does not ensure it.
+6 votes
Answer: A

I. Case: P1 will enter its critical section once at the beginning and then only P2 keeps on entering its critical section again and again. Thus, making P1 starve.

II. Case: P1 will enter its critical section once at the beginning and then only P1 keeps on entering its critical section again and again. Thus, making P2 starve.
answered by Veteran (36k points)
answer should be (D)

after process P1 run for one time  ;

when process P2 into critical section , process P1 can enter into critical section every time ...so it is not possible process P1 to starve ....

when process P1 into critical section , process P2 can enter into critical section every time ...so it is not possible process P2 to starve ....
D) should be answer ...
Please elaborate more on this with the help of examples.

@mithileshupadhyaya
+5 votes

Ans A)

P1 starts , and go infinitely . So P2 starves here.

P2 cannot start process any time.right?

 

Now, P1 starts and makes semaphore value X=1 and executes CS once. Now, P2 takes this semaphore and executes infinitely. So, P1 starves here

 

Now, P1 starts and makes semaphore value X=1 and executes CS once. Now, P2 takes that semaphore and it also executes it's critical section then Pagain takes this semaphore and looks which one executes next V(s) of P1 or P(s) of P1 or P(x) for P2 .

So, more than one process can be in CS here

So, Mutual Exclusion violated , But Progress and Bounded Waiting satisfied here

answered by Veteran (76.9k points)
if bounded waiting has been satisfied then , there will be no starvation...

 

BW is not satisfied here..

@srestha 

Here

ME is satisfied.

BW is satisfied .

Progress is violated.

See this comment

Initially x = 0

p1 ups --- x = 1

p2 downs --- x= 0

--Now both are in critical section----

p2 ups first this time-- ( x = 1 , no one blocked till now)

p2 downs again (x= 0 )

Now p1 downs (x = 0, p1 blocked --- now whenever x ups p1 will wake up first..)

p2 ups (x= 0 , p1 unbloked and resumes so, no starvation here)

Can p1 be down any time?

Can we tell it is like producer consumer problem.

there can be many cases like this

P(X)        V(X)

P(X)         V(X)

or

P(X)       P(X)

V(X)        V(X)

What is the starvation or deadlock for these too. If one could be solved, other can be solve also.rt?
Yes, there are many possibilities.. here starvation possible so progress can not satisfy either for p1 or for p2.

And in 2 process CS problem with strict alternation BW is satisfied.
@Bikram sir, How ME is satisfied? Both processes can be in their CS at the same time.

 srestha  

ME satisfies here

reason:when p1 enter CS we cant preempt  in between Thus P2 will never comes into between 

 set2018 as soon as V(x); P1 enters CS and at the same moment in P2 P(X); happens so both are in CS
explain how ME is satisfied?

+2 votes
I think no process will starve but mutual exclusion may not be possible.

And option D is correct.
As x is 0 process p1 will compute first. While p1 is in critical section(compute) x=1 then process p2 can execute p(x) and be in compute too.

Right or not??
answered by Boss (8.2k points)
It is clear that P1 will go to CS first. Now what is starvation? If P1 goes to CS a finite number of times and then just wait indefinitely, is this starvation?

Yes talking about possibility then yes it is possible for both P1 and P2 to starve.

Case 1:

Intially P1 executes V(x) and makes  x=1. P1 is in CS.

then P2 executes P(x) and makes x=0 and  P2 is also in CS. P2 executes several times and not letting P1 complete even a single times. (P1 starves)

Case 2:

Intially P1 executes V(x) and makes  x=1. and P1 is in CS. Now instead of P2, P1 completes its execution  with P(x) of P1. P1 goes into CS several times and not letting P2 into CS even a single times. (P2 starves)

@Arjun Suresh Sir, is it necessary for P1 to enter CS first? The wait condition blocks for a semaphore value less than 0, not equal to 0..

Equal to 0 holds in bin semaphores or mutexes. Normal semaphores have a condition of <0 for wait and <=0 for signal.

Excerpt from Galvin et al; Operating Systems Concepts : ed. 7 Pg 202-203

wait(semaphore *S) {
S->value—;
if (S->value < 0) {
add this process to S->list;
block();

signal(semaphore *S) {
S->value++;
if (S->value <= 0) {
remove a process P from S->list;
wakeup(P);

 The semaphore has a value of 0 in the beginning, either process can enter.

Now, if P1 enters and P2 tries to execute, it enters directly, because there is no way P1 can mutually exclude P2, because it increases the semaphore whenever it enters .

If P2 enters , and is interrupted , and P2 wishes to enter while the semaphore value is less than 0, it doesn't matter because signal doesn't block a process.

If P2 enters after P1, it faces no problems at all.

Even if mutex is taken into account, I can't see how P1 starves. P2 can starve because P1 might take infinite time to set mutex to 1, but for P1, it never has a lock condition because mutex signal does not have a lock condition. P1 can never starve either way, because there is no busy waiting or block in Signal, in any definition.
–1 vote
Ans is A.
answered by (371 points)
edited by
–3 votes
Answer D

As  semaphore x , value is 0

So , if p2 executes then deadlock occurs. So, P1 executes first , the value of x increments and p1 entries in the critical section ,suppose P2 preempts the execution of P1 . After the complete execution of P2 , the rest code of P1 executes. Therefore , P1 and P2 both will get a chance to enter its critical section at once . They both will not starve for infinite time . So answer D is correct
answered by (99 points)
–4 votes

C

Consider a situation such that process P2 never preempt process P1 then process P2 will starve.

If semaphore was a binary semaphore the process P1 could also starve but here nothing is give so we'll take it to be a general semaphore.

answered by Active (2.1k points)
why cant p1 starve ?
p1:v(x)   x=0 to 1

p1:compute

p2:p(x)    x=1 to 0

p1:p(x)    wait

p2:compute

p2:v(x) x=0 to 1

p2:p(x) x=1 to 0

and so on

in this case p1 will starve isnt it ?
–5 votes
Here mutual exclusion, progress and bounded wait, all 3 are not guaranteed.

However process p2 can starve if P1 keeps on executing forever. Note that p2  cannot execute forever in any case so P1 can never starve. So ans is C.
answered by Loyal (4.3k points)
progress is guaranteed rt?

@mojo: p2 can also execute forever.

I think progress not guaranteed..suppose initially only P2 wants to enter the CS..not P1.Then P2 cant enter the CS.It has to wait for P1 to enter the CS as X=0.

So decision of who will enter into CS is depended on P1 who is not interested to enter(in this situation).

So its against progress statement.

correct me if i m wrong
@kishalay here we can't maked any assumption as p1,2:repeat forever is used
what does it prove??..progress satisfied?
progress defination says, one process should not stop other process from entering the CS, & that forever,

 

here its temporary due to starvation,

here a possibility haas been asked, its like there exists,(_OR_ OR..) so we only need to proove 1 possibility in which the statements turns out to be true(A)... if cant find any such case then all false (D)
Sorry Nitin i did not get what are u trying to say
check now?
Answer:

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