Real Time Systems Design The RTS design is investigated using common component based model 1.
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Transcript of Real Time Systems Design The RTS design is investigated using common component based model 1.
Real Time Systems Design
The RTS design is investigated using common component based model
1
Embedded vs. Real Time Systems
• Real Time System: Correctness of the system depends not only on the logical results, but also on the time in which the results are produced.
• Embedded system: is a computer system that performs a limited set of specific functions. It often interacts with its environment.
EmbeddedSystems
Real TimeSystems
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Examples• Real Time Embedded:
– Nuclear reactor control– Flight control– Basically any safety critical system– GPS– MP3 player– Mobile phone
• Real Time, but not Embedded:– Stock trading system– Skype– Pandora
• Embedded, but not Real Time:– Home temperature control– Sprinkler system– Washing machine, refrigerator, etc.– Blood pressure meter
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Characteristics of RTS
• Event-driven, reactive.• High cost of failure.• Concurrency/multiprogramming.• Stand-alone/continuous operation.• Reliability/fault-tolerance requirements.• Predictable behavior.
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Time
digital clock
tick granule
now
instantinstantpast future
durationevent event
time line
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Definitions
• Hard real-time — systems where it is absolutely imperative that responses occur within the required deadline. E.g. Flight control systems.
• Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. E.g. Data acquisition system.
• Real real-time — systems which are hard real-time and which the response times are very short. E.g. Missile guidance system.
A single system may have all hard, soft and real real-time subsystems.In reality many systems will have a cost function associated with missing each deadline
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Control systems
• Man-machine interface: input devices, e.g. keyboard and output devices, e.g. display
• Instrumentation interface: sensors and actuators that transform between physical signals and digital data
• Most control systems are hard real-time• Deadlines are determined by the controlled object, i.e. the temporal
behavior of the physical phenomenon
Operator Controlled Object
Real-Time Computer
System
Man-Machine Interface
Instrumentation Interface
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Control system example
Example: A simple one-sensor, one-actuator control system.
control-lawcomputation
A/D
A/DD/A
sensor plant actuator
rk
yk
y(t) u(t)
uk
referenceinput r(t)
The systembeing controlled
Outside effects
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Control systems cont’d.
Pseudo-code for this system:
set timer to interrupt periodically with period T;at each timer interrupt do
do analog-to-digital conversion to get y;compute control output u;output u and do digital-to-analog conversion;
end do
set timer to interrupt periodically with period T;at each timer interrupt do
do analog-to-digital conversion to get y;compute control output u;output u and do digital-to-analog conversion;
end do
T is called the sampling period. T is a key design choice. Typicalrange for T: seconds to milliseconds.
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Taxonomy of Real-Time Systems
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Taxonomy of Real-Time Systems
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Taxonomy of Real-Time Systems
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Taxonomy: Static
• Task arrival times can be predicted• Static (compile-time) analysis possible• Allows good resource usage (low idle time for
processors).
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Taxonomy: Dynamic
• Arrival times unpredictable• Static (compile-time) analysis possible only for
simple cases.• Processor utilization decreases dramatically.• In many real systems, this is very difficult to
handle.• Must avoid over-simplifying assumptions
– e.g., assuming that all tasks are independent, when this is unlikely.
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Taxonomy: Soft Real-Time• Allows more slack in the implementation• Timings may be suboptimal without being
incorrect.• Problem formulation can be much more
complicated than hard real-time• Two common and an uncommon way of handling
non-trivial soft real-time system requirements– Set somewhat loose hard timing constraints– Informal design and testing– Formulate as an optimization problem
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Taxonomy: Hard Real-Time
• Creates difficult problems.– Some timing constraints are inflexible
• Simplifies problem formulation.
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Taxonomy: Periodic• Each task (or group of tasks) executes
repeatedly with a particular period.• Allows some static analysis techniques to be
used.• Matches characteristics of many real
problems• It is possible to have tasks with deadlines
smaller, equal to, or greater than their period.– The later are difficult to handle (i.e., multiple
concurrent task instances occur).
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Periodic
• Single rate:– One period in the system– Simple but inflexible– Used in implementing a lot of wireless sensor
networks.
• Multi rate:– Multiple periods– Should be harmonics to simplify system design
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Taxonomy: Aperiodic
• Are also called sporadic, asynchronous, or reactive.
• Creates a dynamic situation• Bounded arrival time interval are easier to
handle • Unbounded arrival time intervals are
impossible to handle with resource-constrained systems.
Example: Adaptive Cruise Control• Demo video
• Control system• Hard Real Time• Multi-rate periodic
• Camera• GPS• Low-speed mode for
rush hour traffic
United States Patent 7096109 20
Data Acquisition and Signal-Processing Systems
• Examples:– Video capture.– Digital filtering.– Video and voice compression/decompression.– Radar signal processing.
• Response times range from a few milliseconds to a few seconds.
• Typically simpler than control systems
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Other Real-Time Applications• Real-time databases.
• Examples: stock market, airline reservations, etc.• Transactions must complete by deadlines.• Main dilemma: Transaction scheduling algorithms and real-time
scheduling algorithms often have conflicting goals.• Data is subject temporal consistency requirements.
• Multimedia.• Want to process audio and video frames at steady rates.
– TV video rate is 30 frames/sec. HDTV is 60 frames/sec.– Telephone audio is 16 Kbits/sec. CD audio is 128 Kbits/sec.
• Other requirements: Lip synchronization, low jitter, low end-to-end response times (if interactive).
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Are All Systems Real-Time Systems?
• Question: Is a payroll processing system a real-time system?– It has a time constraint: Print the pay checks every two weeks.
• Perhaps it is a real-time system in a definitional sense, but it doesn’t pay us to view it as such.
• We are interested in systems for which it is not a priori obvious how to meet timing constraints.
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The “Window of Scarcity”
• Resources may be categorized as:
– Abundant: Virtually any system design methodology can be used to realize the timing requirements of the application.
– Insufficient: The application is ahead of the technology curve; no design methodology can be used to realize the timing requirements of the application.
– Sufficient but scarce: It is possible to realize the timing requirements of the application, but careful resource allocation is required.
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Example: Interactive/Multimedia Applications
sufficientbut scarceresources
abundantresources
insufficientresources
Requirements(performance, scale)
1980 1990 2000Hardware resources in year X
RemoteLogin
NetworkFile Access
High-qualityAudio
InteractiveVideo
The interestingreal-timeapplicationsare here
The interestingreal-timeapplicationsare here
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OS or not?
Hardware
Operating
System
User Programs
Typical OS Configuration
Hardware
Including Operating
System Components
User Program
Typical Embedded Configuration
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Foreground/Background Systems• Task-level, interrupt level• Critical operations must
be performed at the interrupt level (not good)
• Response time/timing depends on the entire loop
• Code change affects timing
• Simple, low-cost systems
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RTS Programming• Because of the need to respond to timing demands made by different stimuli/responses,
the system architecture must allow for fast switching between stimulus handlers.• Because of different priorities, unknown ordering and different timing requirements of
different stimuli, a simple sequential loop is not usually adequate.• Real-time systems are therefore usually designed as cooperating processes with a real-time
kernel controlling these processes.
Concurrent programming
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Real Time Java?
• Java supports lightweight concurrency (threads and synchronized methods) and can be used for some soft real-time systems.
• Java is not suitable for hard RT programming but real-time versions of Java are now available that address problems such as– Not possible to specify thread execution time;– Uncontrollable garbage collection;– Not possible to access system hardware;– Etc.
– Real-Time Specification for Java– Sun Java Real-Time System
Requires a Real Time OS underneath (e.g., no Windows support)
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Classification of Scheduling Algorithms
All scheduling algorithms
static scheduling(or offline, or clock driven)
dynamic scheduling(or online, or priority driven)
static-priorityscheduling
dynamic-priorityscheduling
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Scheduling strategies
• Non pre-emptive scheduling– Once a process has been scheduled for execution, it runs to
completion or until it is blocked for some reason (e.g. waiting for I/O).
• Pre-emptive scheduling– The execution of an executing processes may be stopped if a higher
priority process requires service.
• Scheduling algorithms– Round-robin;– Rate monotonic;– Shortest deadline first;– Etc.
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Real-time operating systems
• Real-time operating systems are specialised operating systems which manage the processes in the RTS.
• Responsible for process management and resource (processor and memory) allocation.
• Do not normally include facilities such as file management.
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Operating system components
• Real-time clock– Provides information for process scheduling.
• Interrupt handler– Manages aperiodic requests for service.
• Scheduler– Chooses the next process to be run.
• Resource manager– Allocates memory and processor resources.
• Dispatcher– Starts process execution.
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Interrupt servicing
• Control is transferred automatically to a pre-determined memory location.
• This location contains an instruction to jump to an interrupt service routine.
• Further interrupts are disabled, the interrupt serviced and control returned to the interrupted process.
• Interrupt service routines MUST be short, simple and fast.
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Metrics for real-time systems differ from that for time-sharing systems.
– schedulability is the ability of tasks to meet all hard deadlines– latency is the worst-case system response time to events– stability in overload means the system meets critical deadlines even if all
deadlines cannot be met
What’s Important in Real-Time
Time-Sharing Systems
Real-Time Systems
Capacity High throughput Schedulability
Responsiveness Fast average response Ensured worst-case response
Overload Fairness Stability
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Limited ResourcesLimited Resources
Common Component based software engineering (CBSE) technologies (JavaBeans, CORBA and COM) are seldom used as they:
Require excessive processing requirements
Require excessive memory requirements
Provide unpredictable timing characteristics
Page 37
System Level AnalysisSystem Level Analysis
At system level we analyze to determine if the system composed fulfils the timing requirements.
Several different mature analysis methods exist, for example, analysis for priority-based systems and pre-run-time scheduling techniques
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Real-time Component ModelsReal-time Component Models
Using a standard operating system in a real-time application, such as windows NT must be done carefully, as it was designed to be used so.
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Application-specific Component Models Application-specific Component Models
Maintain a component library which the application engineer can use when developing an application.
In addition to infrastructure components, domain specific component models, which in fact have been used for many years for certain domains must be considered.
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IEC 61131-3 Application Structure IEC 61131-3 Application Structure
Global and direct variables
Access path
Executioncontrol path
Variableaccess path
FBTask
Program Program
FB FB
Task
Program
Task
Program
FB FB
Task
Resource Resource
Configuration
Communication Function
FunctionBlock
Variable
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A Configuration in IEC 61131-3A Configuration in IEC 61131-3
Encapsulates all software for an application and consists of one or several resources which provide the computational mechanisms.
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A Program in IEC 61131-3A Program in IEC 61131-3
A program is written in any of the languages proposed in the standard, for example:
Instruction lists
Assembly languages
Structured text
A high level language similar to Pascal
Ladder diagrams
Function block diagrams (FBD)
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A Port-based Object ApproachA Port-based Object Approach
The model is based upon the development of domain-specific components which maximize usability, flexibility and predictable temporal behavior.
Independent tasks are the bases for the PBO model.
Whenever a PBO needs data for its computation, it reads the most recent information from its in-ports, irrespective of its producer.
The PBOs are in their nature periodic and the system can be analyzed using traditional schedulability analysis.
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A Port-based Object A Port-based Object
Port-based objectVariable input ports
Variable output ports
Resource ports for communication with sensors and actuators
Configuration parameters
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Designing Component-based RTSDesigning Component-based RTS
System specification
Top-level design
Detailed design
Scheduling / interface check
Obtain components timing behavior on
target platform
System verification Final product
Component library
Create specifications for the new components
Implement and verify new components using classical development
methods
Add new components
to library
Architecture analysis
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Top-level Design Top-level Design
The first stage of the development process involves de-composition of the system into manageable components
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Detailed Design Detailed Design
At this stage a detailed component design is performed, by selecting components to be used from the candidate set.
Page 48
Architecture Analysis Architecture Analysis
At this stage it is time to check that the system under development satisfies extra-functional requirements such as:
Maintainability
Reusability
Modifiability
Testability
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Scheduling Scheduling
At this point we must check that the temporal requirements of the system can be satisfied, assuming time budgets assigned in the detailed design stage.
In other words, we need to make a schedulability analysis of the system based on the temporal requirements of each component
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WCET Verification WCET Verification
Performing a worst-case analysis can either be based on measurements or on a static analysis of the source code.
What is more interesting in the test cases is the execution time behavior shown as a function of input parameters as shown in the following slide.
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An Execution Time GraphAn Execution Time Graph
Execution time
Input domain 1 domain 2 domain 3
The execution time shows different values for the different
input sub-domains.
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Maximum execution time per sub-domainMaximum execution time per sub-domain
Execution time
Input domain 1 domain 2 domain 3
Page 53
Implementation of New Components Implementation of New Components
New components; Those not already in the library must be implemented. The designer of the component has two requirements:
The functional requirements
The assigned time budget
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System Build and Test System Build and Test
Finally, we build the system using old and new components.
We must now verify the functional and temporal properties of the system obtained.
If the verification test fails, we must return to the relevant stage of the development process and correct the error.
Page 55
Component Library Component Library
Is the most central part of any CBSE system as it contains binaries of components and their descriptions.
A component library containing real-time components should provide the following:
Memory requirements
Worst cast execution time (WCET) test cases
Dependencies
Environment assumptions
Page 56
Composition of ComponentsComposition of Components
Component 1
(C1)
Component 2
(C3)
Component n
(C2)
in1_Cnew
in2_Cnew
in3_Cnew
in4_Cnew
in_C1
in2_Cn
in1_C2
in2_C2
out_C1
out_C2
out1_Cn
out2_Cn
out1_Cnew
out2_Cnew
out3_Cnew
New Component (Cnew)
Page 57
End-To-End DeadlinesEnd-To-End Deadlines
End-to-end deadlines
Are set such that the system requirements are fulfilled in the same way as the time budgets are set
Should be specified for the input to and output from the component since the WCET cannot be computed since its parts may be executing with different periods.
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Specification Of Timing Attributes Specification Of Timing Attributes
We specify virtual timing attributes of the composed component, which are used to compute the timing attributes of sub-components, ie:
IF virtual period is set to P,
THEN the period of a sub-component A should be fA * P
AND the period of B is fB * P,
WHERE fA and fB are constants for the composed component, which are stored in the component library
Page 59
RT Components in Rubus OSRT Components in Rubus OS
Rubus:
Is one of a few real-time operating systems currently available which have some concept of components.
Is a hybrid operating system, in the sense that it supports both pre-emptive static scheduling and fixed priority scheduling.
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A Task and Its InterfacesA Task and Its Interfaces
The timing requirements are specified by release-time, deadline, WCET and period
Task: BrakeLeftRight Period: 50 ms Release time: 10 ms Deadline: 30 ms Precedes: outputBrakeValues WCET: 2 ms
oil pressure
speed
….
brake left wheel
brake right wheel
Task state information
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A Composed System in the Red Model of RubusA Composed System in the Red Model of Rubus
The task depicted below is required to execute before the outputBrakeValues task, (i.E. Task BrakeLeftRight precedes task outputBrakeValues).
Component: BrakeLeftRight
oil pressure
speed
brake left wheel
brake right wheel
State information
input 1
input 2
Component:
OutputBrakeValues
State information
Page 62
Composition of Components in Rubus Composition of Components in Rubus
Task:BrakeLeftRight
oilpressure
speed
brake left
brake right
Task state information
Task:OutputBrakeValues
Task state information
Component: BrakeSystem
pressure
speed
An embedded realtime application using Rubus An embedded realtime application using Rubus
(CBSE) model in the design development(CBSE) model in the design development
Page 63
Conclusion
• The Rubus component model (CBSE) provides:– Methods to express the infrastructure of software
functions, i.e., the interaction between software functions in terms of data-flow and control-flow
– The resulting architecture is formal enough for analysis of timing and memory properties
– The components and the infrastructure allow for a resource efficient mapping onto a run-time structure
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