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CS439: Principles of Computer Systems

Homework 3

Due: 8:45a Friday, February 8, 2013

Homeworks will be submitted electronically. Please refer to the homework turnin instructions.

  1. How can fairness and throughput be competing goals for a scheduler? Give an example where a fair scheduler makes bad use of the CPU and an example where a high-throughput scheduler is unfair.

  2. Consider a uniprocessor kernel that user programs can trap into using system calls. The kernel receive and handles interrupts from I/O devices. Would there be any need for critical sections within that kernel?

  3. Describe the priority inversion problem and give an example situation where it may occur.

  4. Now consider the following program fragment: 
    s1->down();
a++;
s2->down();
v++;
s2->up();
s1->up();
    where s1 and s2 are semaphores. All variables are automatic. Now, consider two threads running this fragment of code simultaneously. Can there be a deadlock? Why or why not?

  5. You need two Hydrogen atoms and one Oxygen atom to make water. Using semaphores, write a solution that generates water whenever the right resources are available. You must have three functions: one to generate Oxygen, one to generate Hydrogen, and one to generate water. Assume the three methods may be executed by any number of threads concurrently.

  6. Solve the water problem using monitors.