Scheduling and Communication Synthesis
for Distributed Real-Time Systems
Paul Pop
Department of Computer and Information Science
Linköpings universitet
1 of 30
June 14, 2000
Outline
 Motivation
 System Model and Architecture
 Scheduling and Communication Synthesis
 Time Driven Systems
 Event Driven Systems
 Real Life Example
 Conclusions
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Embedded Systems
General purpose systems
Microprocessor
market shares
in 1999
Embedded systems
99%
1%
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Distributed Real-Time Systems
 Safety critical applications (e.g. Drive-by-Wire):
 timing constraints,
 data and control dependencies.
 Communication protocols: Time Triggered Protocol (TTP),
Controller Area Network (CAN).
 Scheduling of processes and communication of messages:
guaranteeing timing constraints.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Hardware/Software Codesign
System
Specification
Architecture
Selection
Goals of the thesis:

Scheduling...
Scheduling of processes and messages for
distributed hard real-time applications with
Partitioning
control and data dependencies in the context
of a given communication protocol.
Scheduling

Hardware
Synthesis
Software
Synthesis
Communication synthesis...
Optimization of the parameters of the
communication protocol so that the overall
system performance is increased and the
Integration
imposed timing constraints are satisfied.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Related Work
 Scheduling:
 Static cyclic non-preemptive scheduling:
P. Eles, G. Fohler, D. D. Gajski, H. Kasahara,
H. Kopetz, K. Kuchcinski, J. Madsen, J. Xu.
 Fixed priority preemptive scheduling:
J. Axelsson, S. Baruah, A. Burns, J. W. Layland, C. L. Liu,
K. Tindell, J. A. Stankovic, , W. Wolf , T. Y. Yen.
 Communication synthesis:
G. Borriello, R. Ernst, H. Hansson, J. Madsen, R. B. Ortega, K. Tindell.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Characteristics and Message
 Distributed hard real-time applications.
 Heterogeneous system architectures.
 Systems with data and control dependencies.
 Scheduling of processes:
Time triggered: Static cyclic non-preemptive scheduling,
Event triggered: Fixed priority preemptive scheduling.
 Communications using the time-triggered protocol (TPP).
 The performance of the system can be significantly improved
by considering the communication protocol and the
control dependencies during scheduling.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Outline
 Motivation

System Model and Architecture
 Scheduling and Communication Synthesis
 Time Driven Systems
 Event Driven Systems
 Real Life Example
 Conclusions
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Conditional Process Graph (CPG)
Subgraph corresponding
to DCK
P
P00
P
P11
C
P
P22
P
P66
K
P
P12
12
P5
P7
P
P88
P
P99
P
P14
14
D
P13
K
P15
P
P16
16
P
P17
17
P
P10
10
 First processor
 Second processor
 ASIC
P
P11
11
P
P33
C
C
P4
D
P
P18
18
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Hardware Architecture
 Hard real-time distributed systems.
 Nodes interconnected by a broadcast communication channel.
 Nodes consisting of: TTP controller, CPU, RAM, ROM, I/O interface.
 Communication between nodes is based on the time-triggered protocol.
I/O Interface
RAM
CPU
ROM
ASIC
TTP Controller
Node
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Time Triggered Protocol
I/O Interface
 H. Kopetz, Technical University of Vienna.
RAM
 Intended for distributed real-time control
applications that require high degree of
dependability and predictability.
ROM
CPU
ASIC
 Recommended by the X-by-Wire Consortium
for use in safety critical applications in
vehicles.
TTP Controller
Node
S0 S1
Slot
 Integrates all the services required in the
design of fault-tolerant distributed real-time
systems.
S2
S3
S0 S1
S2
TDMA Round
Cycle of two rounds
S3
 Bus access scheme:
time-division multiple-access (TDMA).
 Schedule table located in each TTP
controller: message descriptor list (MEDL).
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Outline
 Motivation
 System Model and Architecture

Scheduling and Communication
Synthesis
 Time Driven Systems
 Event Driven Systems
 Real Life Example
 Conclusions
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Time Triggered Processes
Input
Problem Formulation
 Safety-critical application with several operating modes.
 Each operating mode is modelled by a CPG.
 The system architecture and mapping of processes to nodes are given.
 The worst case delay of each process is known.
Output
 Local schedule tables for each node and the MEDL for the TTP
controllers.
 Delay on the system execution time for each operating mode,
so that this delay is as small as possible.
Note
 Processes scheduled with static cyclic non-preemptive scheduling,
and messages according to the TTP.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Scheduling Example
P1
P4
P3
P2
24 ms
S1
S0
Round 1
m3
m1
m2
Round 2
Round 3
m4
Round 4
P1
P4
P3
P2
22 ms
S0
S1
Round 1
m1
m2
Round 2
m3
Round 3
Round 4
P4
20 ms
P2
S1
Round 1
m1 m2
Round 2
P1
m1
m2
m4
P1
S0
Round 5
P2
P3
m3
m4
P3
m3 m4
P4
Round 3
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Scheduling Strategy
1. The scheduling algorithm has to take into consideration the TTP.
 Priority function for the list scheduling.
2. The optimization of the TTP parameters is driven by the scheduling.
 Sequence and lengths of the slots in a TDMA round are
determined to reduce the delay.
 Two approaches: Greedy heuristic, Simulated Annealing (SA).
 Two variants: Greedy 1 tries all possible slot lengths,
Greedy 2 uses feedback from the scheduling algorithm.
 SA parameters are set to guarantee near-optimal solutions in
a reasonable time.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Experimental Results
Deviations from the near-optimal schedule lengths obtained by SA:
Average Percentage Deviation [%]
60
50
40
Naive Designer
Greedy 1
Greedy 2
30
20
10
0
80
160
240
320
Number of processes
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
400
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June 14, 2000
Outline
 Motivation
 System Model and Architecture

Scheduling and Communication
Synthesis
 Time Driven Systems
 Event Driven Systems
 Real Life Example
 Conclusions
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
17 of 30
June 14, 2000
Event Triggered Processes
Input
Problem Formulation
 An application modelled using conditional process graphs.
 Each process has an execution time, a period, a deadline, and a priority.
 The system architecture and mapping of processes are given.
Output
 Schedulability analysis for systems modelled using CPGs.
 Schedulability analysis for the time-triggered protocol.
 The MEDL for the TTP controllers so that the process set is schedulable
on an as cheap (slow) as possible processor set.
Note
 Processes scheduled with fixed priority preemptive scheduling,
and messages according to the TTP.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Example
P0
27 P1
C
C
30 P2
P9
30 P6
25 P3
CPG
25 P10
22 P7
19 P5
No conditions Conditions
32 P11
24 P4
P12
Worst Case Delays
G1
120
100
G2
82
82
G2: 150
P8
G1: 200
Deadline: 110
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Average Percentage Deviation [%]
Experimental Results
100
Cost function: degree of schedulability
80
Ignoring Conditions
60
Conditions Separation
40
Relaxed Tightness 1
20
Relaxed Tightness 2
Brute Force
0
80
160
240
320
400
Number of processes
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Experimental Results (Cont.)
Average execution time [s]
450
Brute Force
360
270
180
Ignoring Conditions
Relaxed Tightness 2
Conditions Separation
Relaxed Tightness 1
90
0
80
160
240
320
400
Number of processes
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Scheduling of Messages over TTP
messages are dynamically produced by the processes
frames are statically determined by the MEDL
m1
S0
m2
Round 1
S1
m3
m4
Round 2
m5
Round 3
1. Single message per frame, allocated statically:
Static Single Message Allocation (SM)
2. Several messages per frame, allocated statically:
Static Multiple Message Allocation (MM)
3. Several messages per frame, allocated dynamically:
Dynamic Message Allocation (DM)
4. Several messages per frame, split into packets, allocated dynamically
Dynamic Packets Allocation (DP)
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Experimental Results
Average Percentage Deviation [%]
Cost function: degree of schedulability
Ad-hoc
16
Single Message
12
8
Dynamic Message
4
Dynamic Packets
0
Multiple Messages
80
160
240
320
400
Number of processes
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Optimizing Bus Access (SM and MM)
Process p2 misses its deadline!
p1
p2
p3
m1
m2
p1
p2
p3
Swapping m1 with m2:
all processes meet their deadlines.
m2
p1
p2
p3
m1
Putting m1 and m2 in the same slot:
all processes meet their deadline,
the communication delays are reduced.
m1 m2
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
OptimizeDM
OptimizeDM Find the slot sizes that maximize the “degree of schedulability”
OptimizeDM:
for each node Ni do
MinSizeSi = max(size of messages mj
sent by node Ni)
end for
for each slot Si
BestSizeSi = MinSizeSi
Initialization:
the size of a slot in a TDMA round
has to accommodate the largest
message sent by the corresponding
node.
Greedy heuristic: finds the
local optimum for each slot.
for each SlotSize in [MinSizeSi...MaxSize] do
calculate the CostFunction
if the CostFunction is best so far then
BestSizeSi = SlotSizeSi
end if
Local optimum:
find the best size for this slot,
the size that leads to the
“best so far” cost function.
end for
sizeSi = BestSizeSi
end for
end OptimizeDM
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Experimental Results
Average percentage deviations [%]
The quality of the greedy optimization heuristics:
2
1,5
OptimizeSM
OptimizeMM
OptimizeDM
OptimizeDP
1
0,5
0
80
160
240
320
400
Number of processes
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Outline
 Motivation
 System Model and Architecture
 Scheduling and Communication Synthesis
 Time Driven Systems
 Event Driven System

Real Life Example
 Conclusions
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
27 of 30
June 14, 2000
Real Life Example
 Vehicle cruise controller.
 Modelled with a CPG of
32 processes and two conditions.
 Mapped on 5 nodes:
CEM, ABS, ETM, ECM, TCM.
 Time triggered processes:
(deadline 400 ms)
 Ad-hoc:
 Greedy 1:
 Greedy 2:
 SA:
429 ms
314 ms
323 ms
302 ms
 Event triggered processes:
(no messages, deadline 130 ms)
 Ignoring Conditions:
138 ms
 Conditions Separation:
132 ms
 Relaxed Tightness 1, 2:
124 ms
 Brute Force:
124 ms
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Conclusions and Future Work
Time triggered processes:
 Extension to static scheduling for CPGs to handle TTP.
 Improved schedule quality by using new priority function that
considers the time triggered protocol.
 Significant performance improvements can be obtained by optimizing
the access to the communication channel.
Event triggered processes:
 Schedulability analysis with the TTP: four message scheduling
approaches compared based on the issue of schedulability.
 Significant improvements to the “degree of schedulability” through
the optimization of the bus access scheme.
 The pessimism of the analysis can be drastically reduced by
considering the conditions.
Mapping of processes and architecture selection.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000
Contributions
Time triggered processes:
 Static scheduling strategy for systems with
both control and data dependencies.
 Optimization strategies for the synthesis of the
bus access scheme.
Event triggered processes:
 Less pessimistic schedulability analysis for hard real-time
systems with both control and data dependencies
 Schedulability analysis for the time-triggered protocol.
 Optimization strategies for the synthesis of the
bus access scheme.
Scheduling and Communication Synthesis for Distributed Real-Time Systems
Paul Pop
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June 14, 2000