1 Swiss Federal Institute of Technology Computer Engineering and Networks Laboratory Embedded...
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1 Swiss Federal Institute of Technology Computer Engineering and Networks Laboratory Embedded Systems Exercise 2: Scheduling Real-Time Aperiodic Tasks Alexander Maxiaguine 4. May 2005
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1Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Embedded Systems
Exercise 2:Scheduling Real-Time Aperiodic
Tasks
Alexander Maxiaguine
4. May 2005
2Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Definitions: RT Tasks
Tt
Arrival Time
Execution Time
t
Response Time
Deadline
Lateness
3Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Overview
Scheduling aperiodic tasks with real-time constraints
Some known algorithms:
4Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Earliest Due Date (EDD)
Optimization goal:– minimize the maximum lateness
Assumptions on the task set:– independent tasks– synchronous arrival times
EDD is non preemptive
5Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Earliest Due Date (EDD)
T1t
T2t
T3t
t
Jackson’s rule: execute tasks in order of non decreasing
deadlines
T2T1
T3
6Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Earliest Deadline First (EDF)
Optimization goal:– minimize the maximum lateness
Assumptions on the task set:– independent tasks– arbitrary arrival times (dynamic
arrivals)
EDF is preemptive
7Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Earliest Deadline First (EDF)
T1t
T2t
T3t
t
Horn’s rule: at any instant execute the task with the earliest
absolute deadline
T2T1
T3
T1
8Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Latest Deadline First (LDF)
Optimization goal:– minimize the maximum lateness
Assumptions on the task set:– tasks with precedence relations – synchronous arrival times
LDF is non preemptive
9Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Latest Deadline First (LDF)
Main Idea [Lawler]: build the scheduling queue, at run time execute tasks from that queue
•Building the scheduling queue:
– proceed from tail to head
– among the tasks without successors or whose successors have been all scheduled, select the task with the latest deadline to be scheduled last
– repeat the procedure until all tasks in the set are selected
T2
T3T1
10Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Latest Deadline First (LDF)Building the scheduling queue (example):
A
B
C
D
E
A B C D E
di 4 4 8 12
15
deadlines DAG
Scheduling Queue:
A
E
D
C
B
runt-time scheduler
11Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Ealiest Deadline First (EDF*)
Optimization goal:– minimize the maximum lateness
Assumptions on the task set:– tasks with precedence relations – arbitrary arrival times
EDF* is preemptive
12Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Earliest Deadline First (EDF*)Main idea:
modify deadlines and arrival times to satisfy the precedence constraints and then use EDF
T2
T3T1
Modifying deadlines:Task must finish the execution within its deadlineTask must not finish the execution later than the maximum start time of its successor
T1t
T2t
T3t
13Swiss FederalInstitute of Technology
Computer Engineeringand Networks Laboratory
Earliest Deadline First (EDF*)Main idea:
modify deadlines and arrival times to satisfy the precedence constraints and then use EDF
T2
T3T1
Modifying arrival times:Task must start the execution not earlier than its release time. Task must not start the execution earlier than the minimum finishing time of its predecessors.
T1t
T2t
tT3
