Problem :
Five batch jobs A through E, arrive at a computer center at almost the same time. They have estimated running times 10, 6, 2, 4, and 8 minutes. Their (externally determined) priorities are 3, 5, 2, 1, and 4, respectively, with 5 being the highest priority. Determine the mean process turn around time. Ignore process switching overhead. For Round Robin Scheduling, assume that the system is multiprogramming, and that each job gets it fair share of the CPU.All jobs are completely CPU bound.
Solution #1 The following solution comes from this page :
For round robin, during the first 10 minutes, each job gets 1/5 of the
CPU. At the end of the 10 minutes, C finishes. During the next 8
minutes, each job gets 1/4 of the CPU, after which time D finishes.
Then each of the three remaining jobs get 1/3 of the CPU for 6
minutes, until B finishes and so on. The finishing times for the five
jobs are 10, 18, 24. 28, 30, for an average of 22 minutes.
Solution #2 the following solution comes from Cornell University, can be found here, and is obviously different from the previous one even though the problem is given in exactly the same form (this solution, by the way, makes more sense to me) :
Remember that the turnaround time is the amount of time that elapses
between the job arriving and the job completing. Since we assume that
all jobs arrive at time 0, the turnaround time will simply be the time
that they complete. (a) Round Robin: The table below gives a break
down of which jobs will be processed during each time quantum. A *
indicates that the job completes during that quantum.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
A B C D E A B C* D E A B D E A B D* E A B E A B* E A E A E* A A*
The results are different: In the first one C finishes after 10 minutes, for example, whereas in the second one C finishes after 8 minutes.
Which one is the correct one and why? I’m confused.. Thanks in advance!
The problems are different. The first problem does not specify a time quantum, so you have to assume the quantum is very small compared to a minute. The second problem clearly specifies a one minute scheduler quantum.
The mystery with the second solution is why it assumes the tasks run in letter order. I can only assume that this an assumption made throughout the course and so students would be expected to know to make it here.