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|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Exercise 4:
Scheduling Periodic and Mixed Task Sets
25. April 2018
presented by
Stefan Drašković [email protected] slides
by
Roman Trüb [email protected]
1
Embedded Systems
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Clicker Quiz
Lecture Summary
The Exercise
Solution Discussion
2
Overview
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and Networks Laboratory Stefan Drašković | 25.04.2018
Clicker Quiz
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Clicker Quiz – Question 1
Schedulable with EDF
Test failed
Task set is not schedulable with EDF
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018 5
Clicker Quiz – Question 2
Schedulable with RM
Test failed
With this test result, no statement can be made since
schdulablility test condition is not necessary.
Task set could still be schedulable with RM.
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and Networks Laboratory Stefan Drašković | 25.04.2018 6
Clicker Quiz – Question 3
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Clicker Quiz – Question 4
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Lecture Summary
Exercise 4
Institut für Technische Informatikund
KommunikationsnetzeComputer Engineering andNetworks Laboratory
Stefan Draskovic 3.4.2019 Page 1 of 5
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|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Periodic with
D = T
Periodic with
D < TMixed Tasks
Static
PriorityRM DM Polling Server
Dynamic
PriorityEDF EDF
Total
Bandwidth
Server
9
Scheduling Algorithms
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Periodic with
D = T
Periodic with
D < TMixed Tasks
Static
PriorityRM DM Polling Server
Dynamic
PriorityEDF EDF
Total
Bandwidth
Server
10
Scheduling Algorithms
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Static priority assignment:
Tasks with shorter period get higher priority
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Rate Monotonic (RM)
t
tT1
tT3
tT2
High
Low
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RM – Schedulability Test
Sufficient
(but not necessary)
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Periodic with
D = T
Periodic with
D < TMixed Tasks
Static
PriorityRM DM Polling Server
Dynamic
PriorityEDF EDF
Total
Bandwidth
Server
14
Scheduling Algorithms
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Static priority assignment:
Tasks with smaller relative deadline get higher priority
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Deadline Monotonic (DM)
t
tT3
tT1
tT2
High
Low
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and Networks Laboratory Stefan Drašković | 25.04.2018 16
DM – Schedulability Test
Sufficient
(but not necessary)
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RM & DM – Schedulability Test
Necessary &
Sufficient
Ri Ri+CiWorst-case Interference:
Longest Response Time Ri(computed iteratively)
Exercise 4
Institut für Technische Informatikund
KommunikationsnetzeComputer Engineering andNetworks Laboratory
Stefan Draskovic 3.4.2019 Page 2 of 5
-
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Periodic with
D = T
Periodic with
D < TMixed Tasks
Static
PriorityRM DM Polling Server
Dynamic
PriorityEDF EDF
Total
Bandwidth
Server
18
Scheduling Algorithms
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Dynamic Priority Assignment:
Always execute the task with the currently closest deadline
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Earliest Deadline First (EDF)
t
tT1
tT3
tT2
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Di= Ti Di< Ti
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EDF – Schedulability Tests
Sufficient
(but not necessary)
Necessary &
Sufficient
Utilization:
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Mixed (or hybrid) task set contains both periodic and
aperiodic tasks
Basic idea for scheduling:
Schedule the periodic tasks as usual
Serve the aperiodic tasks via a "server" that behaves like a
periodic
task but serves the aperiodic tasks
21
Mixed Task Scheduling
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Periodic with
D = T
Periodic with
D < TMixed Tasks
Static
PriorityRM DM Polling Server
Dynamic
PriorityEDF EDF
Total
Bandwidth
Server
22
Scheduling Algorithms
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Idea: Introduce an artificial periodic task (Cs, Ts) which
serves the aperiodic requests
23
RM – Polling Server
Schedulability test for mixed task set:
Sufficient
(but not necessary)
Sufficient
(but not necessary)
Aperiodic guarantee:Assumption: aperiodic task finishes
before new aperiodic request arrives
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Periodic with
D = T
Periodic with
D < TMixed Tasks
Static
PriorityRM DM Polling Server
Dynamic
PriorityEDF EDF
Total
Bandwidth
Server
24
Scheduling Algorithms
|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
Idea: For every aperiodic request a deadline which is
based on the server’s parameters (Cs, Ts) is assigned.
The aperiodic requests are then scheduled with EDF as
any other periodic instance.
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EDF – Total Bandwidth Server
Assignment of the deadlines to the aperiodic requests:
NOTE: recursive
computation!
Aperiodic requests
are assumed to be
ordered by increasing
release time ri
Exercise 4
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KommunikationsnetzeComputer Engineering andNetworks Laboratory
Stefan Draskovic 3.4.2019 Page 3 of 5
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|Computer Engineering
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EDF – Total Bandwidth Server
Schedulability test for mixed task set:
Necessary &
Sufficient
Set of periodic tasks with
utilization Up and a Total
Bandwidth Server with utilization
Us is schedulable with EDF
Utilization of Total Bandwidth Server:
Utilization of the periodic tasks:
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Execise 4
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Computer Engineering
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Execise 4 - Solution
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1. Maximum utilization of the server:
2. Order tasks by increasing release time: J4, J6, J5
Then, calculate new deadlines recursively:
33
Task 1 – Solution
Exercise 4
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Stefan Draskovic 3.4.2019 Page 4 of 5
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|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
2. EDF schedule with aperiodic tasks:
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Task 1 – Solution
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1. Sufficient test:
Test failed! Since it is not necessary, we still don’t know
if
the task set is schedulable with RM or not.
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Task 2 – Solution
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2. Necessary test: Iterative algorithm
1) Order tasks by their priority (already ok in this case)
2) Start with task which has lowest priority:
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Task 2 – Solution
Necessary test succeeds task set is schedulable with RM
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3. RM schedule
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Task 2 – Solution
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|Computer Engineering
and Networks Laboratory Stefan Drašković | 25.04.2018
1. Aperiodic guarantee:
2. Sufficient schedulability test for RM Polling Server:
40
Task 3 – Solution
Test succeeded!
Substituting values & considering minimum
Task set is schedulable with RM
Exercise 4
Institut für Technische Informatikund
KommunikationsnetzeComputer Engineering andNetworks Laboratory
Stefan Draskovic 3.4.2019 Page 5 of 5
