COMP 346 Winter 2000: TA-2

1. Assume that we have a computer with 7 identical CPUs. Show how we can run a given process P on this machine if the process consists of the following piece of code. For each case, indicate how many threads there are in the process. Describe those threads!
   1. [Points: 5]

   	for ( i = 1 ; i <= 100 ; i++ )
   		X[i] = X[i] + S ;		// X is an array and S is a 
   						// scalar 
   

   2. [Points: 5]

   for ( i = 1 ; i <= 100 ; i++ )
   		Y[i] = Y[i] + Y[i+1];	// Y is an array
   

   3. [Points: 15]

   for ( i = 1 ; i <= 100 ; i++ ) {	// Hint: You can make this 
   						// loop parallel
   		A[i]	= A[i] + B[i];
   		B[i+1]	= C[i] + D[i]; 	// A, B, C and D are arrays
   	    }
   
   

   
   
2. Answer Question 6.3 in the textbook (Silberschatz & Galvin, 5th ed.) [Points: 20]


3. Answer Question 6.6 in the textbook (Silberschatz & Galvin, 5th ed.) [Points: 15]


4. Answer Question 7.13 in the textbook (Silberschatz & Galvin, 5th ed.) [Points: 25]


5. Consider the following situation with the synchronization of two processes.

            ---------------------------------------------------------------
            Process A            			    Process B
            ---------------------------------------------------------------
            while True do         		while True do
               na = na + 1 ;       		    nb = nb + 1 
            end                      		end



1. Synchronize the execution of the two processes using two general semaphores such that at any time nb < na <= nb + 10. Assume that initially na = nb = 0. [Points: 5]
2. How would you modify (simplify) your solution if the only constraint is na <= nb + 10? [Points: 5]
3. Would your solutions for (1) and (2) be still valid if there are several A and B type processes executing the same code? If your answer is YES, explain why; if it is NO give the correct solutions. [Points: 15]
6. A variable X is shared by two processes executing the following program fragment:

               repeat
               X := X + 1  ; 
               if (X = 20) then X := 0 ; 
               forever; 
   

   Starting from an initial value of 0, show that it is possible for the value of X to increase beyond 20. What can you do to ensure that the value of X does not go beyond 20? [Points: 15]