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ser334_exam3_practice.lyx
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SER 334 Practice Exam 3
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(SOLN)
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Updated 12/2/2021
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Exam Instructions
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The exam is open one note card (3x5 inches).
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No electronic items are allowed.
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Scenario questions:
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Topic
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Possible
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MC: CPU Scheduling
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15
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SA: CPU Scheduling
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MC: Main Memory
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15
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MC: Mass-Storage Structure
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Total:
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90
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CPU Scheduling [25 points]
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Would it make sense to use a periodic scheduler for a system where new jobs
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[5 points]
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Yes.
Periodic schedulers use preemption to ensure that no matter how many jobs
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\begin_layout Enumerate
Yes.
Periodic schedulers are meant to receive new jobs periodically.
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No.
Periodic schedulers need to know exactly what will run in order to schedule
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ANS
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No.
Periodic schedulers only support threads, not processes.
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Staff Notes:
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No.
Periodic schedulers are about regularity: we simply can't do reliable schedulin
g if we don't know exactly what will run.
We can't check if a set of periodic tasks can be scheduled before execution
since introducing arbitrary new jobs at run-time may cause processes to
miss their deadlines.
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Consider the following set of processes (tuples look like process, burst
size): (P0, 10), (P1, 30), (P2, 15), (P3, 5).
Assume the processes arrive in this order as well.
Compute the average waiting time for First-Come First-Served.
[5 points]
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60/4
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155/4
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105/4
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ANS
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165/4
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Staff Notes:
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Order: P0, P1, P2, P3
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Seq: P0[10], P1[30], P2[15], P3[5]
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0 10 40 55 60
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AWT: (0+10+40+55) / 4 = 105/4
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ATT: (10+40+55+60) / 4 = 165/4
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Consider the following set of processes (tuples look like process, burst
size): (P0, 10), (P1, 30), (P2, 15), (P3, 5).
Assume the processes arrive in this order as well.
Compute the average turnaround time for Round Robin Scheduling (use time
quantum 10).
[5 points]
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95/4
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60/4
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120/4
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155/4
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ANS
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Staff Notes:
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Order: P0, P1, P2, P3, P1, P2, P1
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Seq: P0[10], P1[10], P2[10], P3[5], P1[10], P2[5], P1[10]
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0 10 20 30 35 45 50 60
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AWT: (0+30+35+30) / 4 = 95/4
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ATT: (10+60+50+35) / 4 = 155/4
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In a multi-level feedback queue, why does the first queue typically use
a smaller time quantum than the later ones? (That is, what does that design
choice maximize.) [10 points]
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Staff Ans:
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The idea is to let smaller jobs complete first without interference from
long jobs.
Longer jobs need to be split up into many different bursts and effectively
saturate the CPU because the proportion of computation time is longer than
for short jobs.
Longer jobs filter down into longer time quantum queues, where they will
be naturally completed after short running jobs, and where fewer context
switches will be required.
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This optimizes for throughput.
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