CS 3733 Operating Systems, Fall 2007 Assignment 3 Comments

• This assignment was graded on the basis of 30 points.
• Both of the statements in this assignment are asserting that something is always true.
• Such a statement can be proved false by finding a single counterexample.
• You cannot prove such a statement true by doing an experiment unless the experiment covers all possible cases (usually not the case).
• Part of this assignment was to make you think about the meaning of the words large and small.
• Is 100, a large or small quantum? Would it make a difference if I told you that the units were in attoseconds. (An attosecond is 10^-18 seconds.)
• Is 1 a small quantum? Small, compared to what?
• For each of the two statements, you were first asked to explain what you think about it and then do an experiment. The first explanation should not be based on the result of the experiment.

About Statement 1

• It is true that if the quantum is larger than the largest CPU burst, RR will behave like FCFS.
• Whether statement 1 is true depends on what it means.
• Tell me what a large quantum is, and I will show you an experiment in which RR with this quantum does not behave like FCFS. In this sense, Statement 1 is false.
• Tell me what the processes are like, and I will find a quantum for which RR behaves like FCFS. In this sense Statement 1 is true.
• You cannot prove that Statement 1 is true by doing a single experiment, or even many experiments.

About Statement 2

• This statement is false because it is sometimes, in fact maybe even usually, not true.
• You can prove it false by doing an experiment that contradicts it in one case.
• However, you cannot really do an experiment that shows what happens for all small values of the quantum. You need to stop with some smallest quantum that you use. It could be argued that the results would change if you just used a smaller quantum.
• You should also consider that running a simulator cannot prove anything unless you also prove that the simulator is running correctly.