List of top Operating System Questions

Three processes arrive at time zero with CPU bursts of 16, 20, and 10 milliseconds. If the scheduler has prior knowledge about the length of the CPU bursts, the minimum achievable average waiting time for these three processes in a non-preemptive scheduler (rounded to nearest integer) is \(\underline{\hspace{2cm}}\) milliseconds.

Consider the following pseudocode, where S is a semaphore initialized to 5 in line#2 and counter is a shared variable initialized to 0 in line #1. Assume that the increment operation in line#7 is not atomic. 

1. int counter = 0; 
2. Semaphore S = init(5); 
3. void parop(void) 
4. { 
5.     wait(S); 
6.     wait(S); 
7.     counter++; 
8.     signal(S); 
9.     signal(S); 
10. }

If five threads execute the function parop concurrently, which of the following program behavior(s) is/are possible?