Quiz 4 Solutions

Wed Nov 14

You have up to 25 minutes. You may use a standard calculator if necessary, but no text book, notes, or computer.

1. This question is about scheduling tasks on the CPU in an operating system. We’ll use batch processing, which means that once we start a task, we will run it to completion without interruption. There are two ways to select the next task to run: First-come, first-served (FCFS) and Shortest job first (SJF).

   Calculate the average turnaround time for scheduling the following jobs using both FCFS and SJF.

     Job    Time
      J1      3
      J2      8
      J3      2
      J4      4

Solution:

The timeline for FCFS looks like this:

    J1    J2         J3   J4
  |-----|----------|----|------|
  0     3         11   13     17

So the turnaround time for each job is:

  J1   3 - 0 =  3
  J2  11 - 0 = 11
  J3  13 - 0 = 13
  J4  17 - 0 = 17

The average is (3+11+13+17)/4 = 44/4 = 11.0.

Now for SJF, the timeline is:

    J3   J1    J4     J2
  |----|-----|------|----------|
  0    2     5      9         17

So the turnaround times are:

  J1   5 - 0 =  5
  J2  17 - 0 = 17
  J3   2 - 0 =  2
  J4   9 - 0 =  9

And that average is (5+17+2+9)/4 = 33/4 = 8.25.

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