1. Context-related issues in real-time systems

2. Context awareness Context Context-awareness
The state of the environment in which an application operates
Potential attributes: location, identity, activity, bandwidth, power, etc.
Context-awareness
Ability to extract, interpret, and use context information obtained from its environment

3. Context-Awareness
Sensors/Actuators
Context-Awareness

4. In the Core … Various different content formats are emerging … WML
Real
Flash
PNG
GIF
DHTML
HTML
ASP
MPEG1
Quicktime
Windows Media
JPEG
PHP
MPEG4

5. … At the Edge …
Various different user agents and platforms are emerging …
Embedded Devices
WAP Phone
Desktop
Integrated Chip
PDA
Laptop
Palmtop

6. Real-time requirements in context-aware applications
Context-aware applications interact with physical environment
Inherently has timeliness requirements.
Correctness of context information
Correctness of actions based on context information
Types of timeliness requirements
Hard real-time
Soft real-time
Time-free

7. Challenges and requirements for context-aware real-time services
Application level
Capture of context data
Gathering data from unreliable multi-modal sensors
Uncertainty of context data
Representation of context data
Interpretation of context data
Synchrony
Synchronous operation cannot be adopted to embedded systems.
Context data should be fed asynchronously.
Privacy
Scalability

8. Challenges and requirements for context-aware real-time services
Middleware level
Adaptability
Provides various levels of QoS
Cope with resource constraints
Resource preservation
Resource allocation and scheduling for satisfying various QoS
Collaboration
Context-aware applications can collaborate between entities.
Group communication
Synchrony
Dynamic reconfiguration of connections and entities
Scalability
Fault-tolerance & consistency

9. Challenges and requirements for context-aware real-time services
Networking level
Adaptability
Provides various levels of QoS
Cope with resource constraints
Limited bandwidth availability
Unpredictable latency
Heterogeneity
End-to-end QoS

10. What makes up Context-Aware Framework?
We’ve got information
(context)!
We’ve got agents!
We’ve got sensors!

11. Agents Agents are a software abstraction.
An agent is a program that assists people and acts on their behalf.
Agents are: reactive, autonomous, and goal-driven.
For OO programmers…
Objects are described in terms of methods
Agents are described in terms of behaviors.

12. Information Context in our framework is Semantic data.
Context is described in a semantic ontology. (OWL and RDF).
An ontology is typically a hierarchical data structure containing all the relevant entities and their relationships and rules within a domain of knowledge.
Quoting wikipedia
Image Creative Commons by Claudecf

13. Making Agents Smarter…
Using formal ontologies allows agents to share beliefs in a precise manner.
Ontologies create inferred knowledge…
Example
I know Abernethy 210 is a space in a structure called Abernethy, and that structure is different from a structure called Manning.

14. Making Agents Smarter…
Using a Rules Engine allows agents to make deductions.
Example
If this is a classroom, and there is a class scheduled, and you are the teacher, and you have a syllabus =>
I should fire up your presentation and start recording.
I should take attendance.

15. What is a Rules Engine?
Rules engines are tools for building intelligent software, or expert systems.
Rules collect facts about the world, and continuously apply rules to those facts.
Rules engines allow agents to make deductions and plans by looking at the context information.

16. When should I use a Rule Engine?
Complicated logic (not 1+1 = 2)‏
Changes often (whatever that means)‏
Traditional approaches are unmaintainable

17. Rules Rules are declarative Follow the pattern
IF <condition> THEN <consequence>
A rule firing (execution) can change the state of the Working Memory therefore possibly triggering other rules to fire

18. Problems with rule engines
How can it be implemented?
Dynamically loadable
Heterogeneous environment
Changes often
Solutions
Machine-independent language
Interpreter/script language
Open standard
 Use of XML
 JAVA is (virtually) the only solution (until now).

19. Problems of JAVA applications in real-time systems
Thread management
Priority-based scheduling is not supported.
Class loading
Class loading is delayed until it is referenced.
How long it takes? – We don’t know.
Garbage collection
System is suspended during GC
How long it takes? How often it occurs?
Compile
Java byte codes are interpreted at run time.
How long it takes to interpret?
How long it takes to run?

20. Real-Time Specification for Java
RTSJ for short
First JSR request
Still not completely finished
Implementations
TimeSys RI
Purdue OVM
Real Time Java

21. RTSJ Guiding Principles
Backward compatibility to standard Java
Write Once, Run Anywhere
Current real-time practice
Predictable execution
No Syntactic extension
Allow implementation flexibility
Real Time Java

22. RTSJ Overview Clear definition of scheduler
Priority inheritance protocol
NoHeapRealtimeThread
Scoped memory to avoid GC
Low-level access through raw memory
High resolution time and timer
Real Time Java

23. RTSJ: Scheduling Standard Java offers no guarantee Fixed priority
Even non preemptive JVM possible
Fixed priority
FIFO within priorities
Minimum of 28 unique priority levels
GC priority level not defined
Real Time Java

24. RTSJ: Memory Areas GC collected Heap Immortal memory Scoped memory
LTMemory
VTMemory
Physical memory
Different time properties
Access to HW devices!
Real Time Java

25. RTSJ: Thread Types Extensions to java.lang.Thread
RealTimeThread
NoHeapRealTimeThread
AsyncEventHandler
Scoped and immortal memory for NHRTT
Strict assignment rules
Not easy to use
Real Time Java

26. RTSJ: Synchronization
Use synchronized
Priority inversion possible in standard Java
Priority inheritance protocol
Priority ceiling emulation protocol
Real Time Java

27. RTSJ: Scoped Memory Cumbersome programming style
New class for each code part
class UseMem implements Runnable {
public void run() {
// inside scoped memory
Integer[] = new Integer[100];
... }
// outside of scoped memory
// in immortal? at initialization?
LTMemory mem = new LTMemory(1024, 1024);
UseMem um = new UseMem();
// usage
computation() {
mem.enter(um);
Real Time Java

28. Asynchronous Event Handler
Difference between bound an unbound
Implementation hint at application level
No functional difference for the application
Better: only one type
Specify a minimum latency at creation
Runtime system decides about implementation
Real Time Java

29. Garbage Collection? An essential part of Java
Without GC it is a different computing model
RTSJ does not believe in real-time GC
Real-time collectors evolve
Active research area
For You?
Real Time Java

30. RTSJ Issues J2SE library: On small systems: Heap usage not documented
OS functions can cause blocking
On small systems:
Large and complex specification
Expensive longs (64 bit) for time values
Real Time Java