# hw5 - CSE 422S(Fall 2009 Operating Systems Organization...

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Unformatted text preview: CSE 422S (Fall 2009) Operating Systems Organization Homework 5 Reading: Tannenbaum, Sections 2.3, 2.5 Due: Wed, Nov. 4, 2009 Problem 1 (2 Points) Consider the following parallel program: int X = 0; // Global (shared) int want[10] = {0, ... , 0}; // Global (shared) void sum() { for (int n=0; n < 10; n++) if ( ! want[n]) { X += n; want[n] = 1; } } void main() { X=0; parbegin sum(); sum(); parend; print(X); } The construct parbegin S1; S2; ... parend with statements S1, S2, ... means that the statements can execute in parallel subject to any synchronization. Determine the smallest value of the shared variable X that will be printed and explain how you arrived at this answer. Assume: 1) processes can execute at any relative speed; 2) a value can be incremented/decremented after it has been loaded into a register; and 3) the values of X and want[n] are loaded only once for each execution of the loop. Problem 2 (0 Points) This problem considers Peterson’s 2-process algorithm given in class. a) Explain how the algorithm prevents one process from monopolizing the critical section; i.e., prevent starvation? b) Explain how the algorithm guarantees freedom from deadlock? Problem 3 (6 Points) The following is a software-only mutual exclusion algorithm for two processes (id = 0 and 1): 1 boolean blocked[2] = { 0, 0 }; 2 int who = 0; 3 void P (int id) { 4 while (TRUE) { 5 blocked[id] = TRUE; 6 while (who != id) { while (blocked[1-id]) who = id; } 7 // ... some critical section is here ... 8 blocked[id] = FALSE; // exit section 9 } 10 } 11 void main () { parbegin P(0); P(1); parend; } 1 a) If the processes execute at the same rate as much as possible, in what order will the processes enter the critical section....
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hw5 - CSE 422S(Fall 2009 Operating Systems Organization...

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