1. From HRT-HOOD to C Real-Time Systems Lecture 6
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Contents
Introduction
Mapping to C
Static Scheduling
Summary
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Introduction
HRT-HOOD design
C source code
Execution time estimation
Static schedule
Real-time system
RTL library
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Introduction
Timetable (TMT)
Tasks’ description:
• period
• max. Exec. time
Design
time
Timetable (TMT)
T2 start!
Run-time
0 : 05
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Mapping to C
HRT-HOOD object types :
Cyclic
Sporadic
Protected
Passive
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Mapping to C
HRT-HOOD object types :
Cyclic
Sporadic
Protected
Passive
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Mapping to C
Cyclic object mapping :
void Task_J() {
while (1)
rt_switch(); }
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Mapping to C
HRT-HOOD object types :
Cyclic
Sporadic
Protected
Passive
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Mapping to C
Sporadic object mapping :
void Task_J() {
while (1)
if ( rt_signaled( J ) ) }
rt_switch();
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Mapping to C
HRT-HOOD object types :
Cyclic
Sporadic
Protected
Passive
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Mapping to C
Protected object mapping :
We have non-preemtable tasks
Static scheduling
Protected == Passive
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Mapping to C
Protected object mapping – operation activation constraints :
void Buffer_Get(int *elem) {
if (counter == 0) wait(Empty);
.....
counter--;
if (counter == MAX – 1) signal(Full); }
void Buffer_Put(int elem)
if (counter == MAX) wait(Full);
counter++;
if (counter == 1) signal(Empty);
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Mapping to C
Protected object mapping – operation activation constraints :
int Buffer_Get(int *elem) {
if (counter == 0) return FALSE;
.....
counter--;
return TRUE; }
int Buffer_Put(int elem) {
if (counter == MAX) return FALSE;
.....
counter++;
return TRUE; }
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Mapping to C
Protected object mapping – operation activation constraints :
Such objects are called: Non- blocking Protected Objects (nPr)
Cyclic objects using nPr objects are called: Non-blocking Cyclic Objects (nC)
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Mapping to C
Non-blocking cyclic objects :
void Task_Producer() {
int success=1;
while (1) {
if (success) {
success = 0; }
if (Buffer_Put(elem)) success=1;
rt_switch();
void Task_Consumer() {
while (1)
if (Buffer_Get(&elem))
..... }
rt_switch();
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Mapping to C
Exceptions mapping :
Exceptions are mapped into sporadic objects executed after the given object. 
Task
Exception handler
Task set with precedence constraints !!!
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Mapping to C
Implementation steps :
Write the code
Transform the blocking objects into non-blocking ones
Calculate the maximum task execution times
Build the stacic schedule
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Static scheduling
We have the following scheduling problem : 
How can we schedule nonpreemtable cyclic tasks (with precedence constraints) on 1 CPU ?
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Static scheduling
Simple answer : !
We can use well-known (i.e. dynamic) scheduling algorithms !!! 
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Static scheduling
Problem : !
Not always dynamic scheduling algorithms give the feasible static schedule !!! 
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Static scheduling
Problem - example :
Tasks = [ T1, T2, T3 ]
Periods = [ 10, 40, 40 ]
Exec. Times = [ 1, 17, 16 ]
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Static scheduling
Problem - example :
EDF algorithm T1 T2 T1 T3 T1 T1
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Static scheduling
Problem - example :
EDF algorithm T1 T2 T1 T3 T1 T1
Dynamic EDF scheduling –> non-feasible schedule
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Static scheduling
Problem - example :
VG algorithm T1 T2 T1 T1 T3 T1
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Static scheduling
Problem - example :
VG algorithm T1 T2 T1 T1 T3 T1
Static scheduling –> feasible schedule
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Static scheduling
Advantages:
More powerful scheduling algorithms
We can check the system before the run- time phase (we don’t need additional data)
Debug & test phase is easier
The system is fully predictable
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Static scheduling
Disadvantages :
Horizon H = LCM(1, 2, .., n) H
TMT
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Static scheduling
Disadvantages (example) :
H = LCM (17, 37, 131, 271) =
4 tasks  H = 22 mln
Hundreds of tasks  H = ???
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Summary
Introduction
Mapping to C
Static Scheduling
Summary
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