Tentamen vid Institutionen för Datavetenskap, Linköpings universitet
TENTAMEN
TDDB47 Realtidssystem (Real-time Systems)
Date:
13th of December 2007
Time:
14-18
Room:
TER1
Contact
Teachers:
Mikael Asplund (Tel: 282668, 0707-481462)
Material:
English-Swedish-English dictionary
Calculator
Total:
40 points
Assessment:
F, 3, 4 and 5:
Fail:
3:
4:
5:
20 27 34 -
19 p
26p
33 p
40 p
INSTRUCTIONS:
Write your name, class and personal number (personnummer) on each sheet of paper that you
hand in. Further, pages should only contain one answer per page (answers to sub-questions
can be on the same page). You are asked to only answer on the front page of the paper. Sort
all the sheets that you hand in with respect to the number of the question.
Make sure your answers are presented clearly and precisely. Points will not be given to
answers that cannot be read due to bad handwriting. Your answers may be in Swedish or
English.
Your answers shall clearly show the method for the solution. A correct answer without any
explanation will not be given any credits (figures can be of help when describing but should
be accompanied by a text description). In those cases where you are in doubt about the
question, write down your interpretation and assumptions, and answer the question based on
the interpretation. Note, a correct answer embedded in errors may give reduction in points, so
make sure your answers are to the point.
Good luck!!!
Mikael Asplund
Examiner for TDDB47
Tentamen vid Institutionen för Datavetenskap, Linköpings universitet
Q1:
Scheduling (15p)
Suppose that you are given the task of designing the
software for an unmanned aerial vehicle (UAV)
which is to be controlled from the ground. There are
four periodic tasks in your particular part of the
system: 1) a process to collect radar data, 2) a
control loop to regulate the airplane, 3) a
communication process to interact with the rest of
the system 3) a sensor process to collect data such as
air speed, altitude etc. The tasks have the following
timeliness specifications:
Task
Radar (R)
Flight control (F)
Communication (C)
Sensor (S)
a)
Period (ms)
40
60
120
60
WCET (ms)
5
20
30
5
Assuming you want to construct a cyclic scheduler, compute the minor and major
cycle of this task set. What are the major and minor cycles and why are they
computed like this?
(3 points)
b) Assuming rate monotonic (RM) scheduling, will this task set be schedulable
according to the utilization schedulability test? What are the assumptions on the
task set that the schedulability analysis for RM requires?
(3 points)
c)
Take the same task set and do a response time analysis. Will the set be
schedulable according to this test?
(3 points)
d) During the design of the system it is realized that the system must be able to
respond to requests from other airplanes regarding its position. These requests are
sporadic, they must be replied to within 30ms and it takes 3ms to complete the
request. Provide a solution and find the minimum arrival time that can be
tolerated using your solution.
(3 points)
e)
Compare the benefits and drawbacks of using priority inheritance, OCPP and
ICPP respectively to avoid priority inversion.
(3 points)
Q2: Dependability and fault tolerance (6p)
a) Explain the difference between availability and reliability.
(2 points)
b) Explain what a fault model is and why it is needed. State three different fault
models and describe them shortly.
(3 points)
c) Explain the concept of triple modular redundancy (TMR).
(1 point)
Tentamen vid Institutionen för Datavetenskap, Linköpings universitet
Q3: Design (5p)
a) Name three techniques that can be done during the design phase to ensure that the
non-functional properties of a system are according to specification?
(2 points)
b) Argue (shortly) for or against the following statements:
1. The best way to get the worst case execution time of a task is to actually
measure it.
2. Design of real-time systems is no different from normal system design.
3. Formal verification can be very time-consuming.
(3 points)
Q4: Distributed systems & Real-time communication (8p)
a)
Compare the synchronous and asynchronous system models. What are their
respective definitions and what are their benefits and drawbacks?
(3 points)
b) How many clocks are needed to maintain internal synchronization provided that
there are 2 malicious clocks?
(1 point)
c)
Is it a good idea to use time-stamps from physical clocks to order messages in a
distributed system? Is there an alternative?
(2 points)
d) What is the approach that TTA uses to cope with byzantine node failures?
(2 points)
Q5: Operating systems and real-time databases (6p)
a)
What are the two main ways of interacting with hardware devices? What are the
consequences for real-time systems?
(2 points)
b) In what way is memory management different in a real-time OS compared to a
standard general purpose OS?
(2 points)
c)
Compare optimistic and pessimistic concurrency control in databases (don't
forget the real-time perspective).
(2 points)
Tentamen vid Institutionen för Datavetenskap, Linköpings universitet
Notation for Tasks/Processes
• C
- Worst-case-execution time of the task
• B
- Worst-case blocking time
• n
- Number of tasks in the system
• T
- Minimum inter-arrival time (period) of task
• R
- Worst-case response time of the task
Utilisation-based schedulability test for Rate Monotonic:
n
∑
i=1
n
1
2
3
4
5
 
Ci
Ti
≤n 21/n−1 .
Utilisation bound
100%
82.8%
78.0%
75.7%
74.3%
Response time analysis
Ri =C i Bi
•
•
w
n1
i
∑
∀ j ∈hp  i
⌈ ⌉
Ri
Cj
Tj
Where hp(i) contains the tasks that have a higher priority than task i
Solved using recurring equations:
=C i Bi
∑
∀ j ∈hp i
⌈ ⌉
w ni
Cj
Tj