CS 520: Introduction to Operating Systems 
Homework Assignment #3 
 
This assignment is somewhat open-ended—start working on it as soon as you can! 
 
A reminder: You may work in groups; however, you may not show anyone your 
code or copy of any part of anyone else’s code or report. 
 Those who submit isomorphic programs or same reports, get 0 points each as a 
minimum. (For the rest of the penalties please check the Syllabus.) 
 
I recommend that you write everything in small—no longer than one-page-long—
methods and test each method separately. Start by testing your pseudo-random 
variable package and check the distributions to ensure that everything works. 
Include this test in a separate method. 
 
Then prepare and test the event data structure. You must keep events in a heap—this 
is exactly why this structure was developed. The good news is that you may use the 
Java library. 
 
 After that, writing and debugging the rest of the program will be straight-forward. I 
recommend that you finish the program in the first week and spend the second week 
on experimenting and writing the report. 
 
As a minimum, a program must be written and a report prepared with the specific 
questions answered, but there is much room for using your creativity—please use it! The 
most important thing that you will have learned from this assignment is the discreet 
simulation technique, which you will apply (by re-using most of the code you develop to 
deal with this particular problem) later. 
Write the bus simulation, as explained in the class and described in the notes. Feel free to 
look on the web for and then re-use any suitable code for the random number generator or 
for linked list algorithms. With that, 1) don’t forget to include a reference to the source 
of any code you re-use and 3) remember to test all re-used code as you are the only one 
responsible for its performance.. 
The purpose of the simulation is to observe the behavior of the system, and answer the 
following questions: 
1. Does the distance between the adjacent buses remain the same? If not, what 
should be done to ensure that it be the same? 
2. What is the average size of a waiting queue at each stop (and what are its 
maximum and minimum)? (You may provide this information on an hourly [simulation 
time] base.)  2 
Plot the positions of buses as a function of time (you will need to generate periodic 
snapshots of the system for that). Feel free to change parameters; then observe and 
document the results. 
What you hand in must include 
1. Your working Java program (it must be well-commented) as well as all required 
input files in the txt format. 
2. A README txt file explaining what you are submitting. 
3. Your report, in the PDF format, including plots, observations, and—especially—
your recommendations for change in the scheduling policies. Needless, to say you 
will need runs that demonstrate that your recommendations solve the problems 
you observe. Remember that preparing a report is an essential part of this 
assignment; this is not merely a programming exercise. The program must 
be working (or the assignment will get a grade of 0), but the quality of your 
report will determine your grade. 
 
Feel free to change the simulation parameters. My initial suggestion is that there are 
 
• 15 bus stops 
• 5 buses 
• The time to drive between any two contiguous stops is 5 minutes 
• The passenger’s mean arrival rate at each stop is 5 persons/min 
• The boarding time is 2 seconds for each passenger 
• The total simulation time is 8 hours. 
 
Make sure you start this project as early as possible so that you have enough time to run 
experiments and prepare a comprehensive report. (100 points) 
 
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