synchronization

Question

the following five concurrent processes operating on counting semaphore variable (s) which is initialized to 0

P1: wait (s); cs; signal (s);

P2: wait (s); cs; signal (s);

P3: wait (s); cs; signal (s);

P4: signal s); cs; wait (s);

P5: signal (s); cs; wait (s);

what is the maximum possible value of s?

A) 1 B) 2 C) 3 D) 0

Answer 1

P4 and P5 can do signal operation without any intermediate wait and thus s can go up to value 2.

Comments

Sir, please explain it in more detail..?

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first p4 execute making s=1 and got prempt !  p1 excutes->p2 execute->p3 execute    atlast value of s=1 now p5 execute making s=2 ..hence possible maximum value of s=2 at any particular stage of execution of 5 process

Answer 2

Execute first 3 fully and last 2 only till critical section. So value of S becomes 2 which is the maximum possible.