CS439: Principles of Computer Systems

Discussion Section 3 Problem Set

Due in Section on Friday, February 6, 2026

The problem set must be completed before section and brought to section. To ensure that your problem set is turned in correctly and that you receive credit for discussion section, you must follow these guidelines exactly.

Include the following header at the top of your file:

Discussion Section Problem Set #:
Student Name:
EID:

You may either type your answers or write them by hand, but you must have a copy with you in discussion section.

In discussion section, your TA will check that you made a good effort on the problem set, you are participating in the discussions in section, and you are ready and able to answer when called upon. If all of those are true, you will receive credit for discussion section that day. Note that a "good effort" on the problem set includes that all answers are in words unique to you and not copied from anywhere else, including the slide deck.

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.

Most round-robin schedulers use a fixed size quantum. Give an argument in favor of and against a small quantum.

Consider a system using a multilevel feedback queue scheduler. Its scheduler is configured to have four queues, which are, in order of highest priority to lowest priority: Q1, Q2, Q3, and Q4. The queues have quantums sized 5s, 10s, 20s, and 40s, respectively.
For each of the following three processes, determine which queue it is in when it begins its final quantum.
- Process A, runtime 40s, blocks every 2s.
- Process B, runtime 42s, never blocks.
- Process C, runtime 70s, blocks at 7s, 11s, 25s, 42s, 48s, where those times are points in its execution.

In the code below, indicate where each variable is stored (e.g., stack, heap, static data segment), and whether that variable would be shared by the threads in a multi-threaded program. If applicable, indicate where the space that variable points to is allocated.

   int i;      //static data segment, shared
   char * j;   //static data segment, shared, no memory yet!

   void foo(int a){

      int b;            //stack, not shared
      static float c;   //static data segment, shared

      /*do stuff*/
   }

   int main(int argc, char**argv){

      int * m;   //stack, not shared, no memory yet!
      int g;     //stack, not shared
      double z;  //stack, not shared

      j = malloc(MAXBUF*sizeof(char));  //heap, pointer shared, memory shared

      createThread(0, foo(), 2);
      createThread(1, foo(), 4);

      /*do stuff*/
   }

Compare and contrast monitors and semaphores. What is an appropriate use of each?

You need two Hydrogen atoms and one Oxygen atom to make water. Using semaphores, write a pseudocode 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.

Solve the water problem using monitors (pseudocode).