NPTEL : Synchronisation (semaphore)

Question

In a certain application, the initial value of a counting semaphore s is 10. The following operations were completed on the semaphore in the given order 5P, 20P, 2V, 18P, 5V, 10P, 1V. The new value of counting semaphore is  _____________ ?

Comments

p() means wait means decrement

v() means signal means increment

initial value is 10

10-5-20+2-18+5-10+1 = -35

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shouldn’t it be 1 ?

Given the initial value of semaphore s: 10

1. 5P (decrement by 5):
   • Current value of semaphore s: 10 - 5 = 5
2. 20P (decrement by 20):
   • Current value of semaphore s: 5 - 20 = -15 (constrained to 0, as semaphores cannot go below 0)
3. 2V (increment by 2):
   • Current value of semaphore s: 0 + 2 = 2
4. 18P (decrement by 18):
   • Current value of semaphore s: 2 - 18 = -16 (constrained to 0)
5. 5V (increment by 5):
   • Current value of semaphore s: 0 + 5 = 5
6. 10P (decrement by 10):
   • Current value of semaphore s: 5 - 10 = -5 (constrained to 0)
7. 1V (increment by 1):
   • Current value of semaphore s: 0 + 1 = 1

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roopkathaaa No, it is not a binary semaphore.

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can you please help me with this ?

Let the base priority of a system be 130 and the nice value provided is -10. What is the time slice that the process gets in the O(1) scheduling algorithm?

if it should be 300 or 400.

Answer 1

Here operation $P$ decreases the value of counting semaphores, and operation $V$ increments the value of counting semaphores.

initial value of counting semaphore =$10$,

new value of CS after performing $5P,20P,2V,18P,5V10P,1V$ operations: $10-5-20+2-18+5-10+1=-35$

Comments

can you please help me with this ?

Let the base priority of a system be 130 and the nice value provided is -10. What is the time slice that the process gets in the O(1) scheduling algorithm?

if it should be 300 or 400.