CSE 330

Operating Systems

SAMPLE Mid Term Exam

Open book, open notes. Q1 and Q2 are from Exam 1 material, Q3 and Q4 is from Exam 2 material

1]

There are two identical processes being run on a system. Each process does the following:

Executes a CPU burst of 1 sec.
Outputs 1 KByte of data

Executes a CPU burst of 1 sec.
Outputs 1 KByte of data

Executes a CPU burst of 1 sec.
Outputs 1 KByte of data

Executes a CPU burst of 1 sec.
Outputs 1 KByte of data

Executes a CPU burst of 1 sec and terminates.

A]    If the output device has a data rate of 1KByte/sec, find the turnaround time of each process under:
i]  FIFO scheduling (non-preemptive)
ii] Round Robin scheduling (assume the time quanta is small, and the exact value is
not important.)

B]    Now we add a output buffer of 10KBytes to the above system. Find the turnaround time of each process under:
i]  FIFO scheduling (non-preemptive)
ii] Round Robin scheduling (assume the time quanta is small, and the exact value is
not important.) 2]

It is stated that the “Shortest Job First Policy” is optimal.

1. What does optimal mean in this context?
2. What property of the SJF policy is optimal and what property makes it not so desirable?
3. Why do not use SJF?
4. Prove that the SJF is optimal with respect to the property in [B]

3]

The following code fragment is a 2-process “solution” to the critical section problem.

try[i] = true;

while (incs[j]) no-op;

while (turn=j and try[j]) no-op;

incs[i] = true;

critical section

try[i] = false;

incs[i] = false;

turn = j;

[2 - continued] 4]

There are 4 processes, executing concurrently. Process P0 is in an infinite loop, incrementing the value of the variable x (x is initialized to 0). P0 is the only process that changes the value of x.

The rest of the processes Pi (1 <=  i<=3) monitor the value of x. Whenever x reaches a value such that it is divisible by i, Pi prints the value of x. For example, P3 will print the sequence 3 6 9 12 ….. as the value of x reaches 3, 6, 9, 12 and so on.

A]    Write the code for all the 4 processes using semaphores. Note that P1 - P3 should be identical.

B]    Write the code for all the 4 processes using CCRs. Note that P1 - P3 should be identical.