1. Real-Time Embedded Systems
Krithi Ramamritham

2. Outline Special Characteristics of Real-Time Systems
Scheduling Paradigms for Real-time systems
Operating System Paradigms
Real-Time Linux
Real-Time NT

3. What is “real” about real-time?
computer world real world
e.g., PC industrial system, airplane
average response events occur in environment at
for user, interactive own speed
occasionally longer reaction too slow: deadline miss
reaction: reaction:
user annoyed damage, pot. loss of human life
computer controls speed computer has to follow real speed
of user of environment
“computer time” “real-time”

4. Why real-time, why not simply fast?
“Fast enough”: dependent on system and its environment and
turtle: fast enough to eat salad
mouse: fast to enough steal cheese
fly: fast enough to escape

5. what if environment is changed? “systems” not fast enough
mouse trap
fly-swatter
time scale depends on - or dictated by - environment
cannot slow down environment
…is the real world

6. Average vs. Worst Case
“There was a man who drowned crossing a river with an average depth of 20cm.”
Standard computing is concerned with average behavior of system e.g., if it responds fast enough on average, it is acceptable word processing, etc.
Real-time computing requires timely behavior in all given situations e.g., a single value delivered too late can cause the system to fail, even when the average timing is kept air planes, power plants, etc.

7. Real-Time Systems
event
action
I/O - data
time
Real-time
computing system
A real-time system is a system that reacts to events in the environment by performing predefined actions
within specified time intervals.

8. Real-Time Systems: Properties of Interest
Safety: Nothing bad will happen.
Liveness: Something good will happen.
Timeliness: Things will happen on time -- by their deadlines, periodically, ....

9. Correctness of results depends on value and its time of delivery
In a Real-Time System….
Correctness of results depends on value and its time of delivery
correct value delivered too late is incorrect
e.g., traffic light: light must be green when crossing, not enough before
Real-time:
(Timely) reactions to events as they occur, at their pace: (real-time) system (internal) time same time scale as environment (external) time

10. Types of RT Systems
Dimensions along which real-time activities can be categorized:
how tight are the deadlines? --deadlines are tight when the laxity (deadline -- computation time) is small.
how strict are the deadlines? what is the value of executing an activity after its deadline?
what are the characteristics of the environment? how static or dynamic must the system be?
Designers want their real-time system to be fast, predictable, reliable, flexible.

11. Hard, soft, firm Hard result useless or dangerous if deadline exceeded
value
time
deadline (dl) +
soft
Soft result of some - lower - value if deadline exceeded
Firm
If value drops to zero at deadline
hard
Deadline intervals: result required not later and not before -

12. Examples Hard real time systems Aircraft Airport landing services
Nuclear Power Stations
Chemical Plants
Life support systems
Soft real time systems
Mutlimedia
Interactive video games

13. Example of Real time systems
Temperature sensor
Input port
CPU
Memory
Output port
Heater

14. Code for example While true do { read temperature sensor
if temperature too high
then turn off heater
else if temperature too low
then turn on heater
else nothing }

15. Comment on code Code is by Polling device (temperature sensor)
Code is in form of infinite loop
No other tasks can be executed
Suitable for dedicated system or sub-system only

16. Extended polling example
Temperature Sensor 1
Conceptual link
Heater 1
Task 1
Temperature Sensor 2
Heater 2
Task 2
Computer
Temperature Sensor 3
Heater 3
Task 3
Temperature Sensor 4
Heater 4
Task 4

17. Polling Problems Arranging task priorities Round robin is usual
Urgent tasks are delayed

18. Other methods Use interrupt driven system
Use general purpose operating system plus tasks
Use task handling language (e.g ADA)
Use special purpose operating system

19. Interrupt driven systems
Advantages
Fast
Little delay for high priority tasks
Disadvantages
Programming
Code difficult to debug
Code difficult to maintain

20. How can we monitor a sensor every 100 ms
Initiate a task T1 to handle the sensor
T1:
Loop
{Do sensor task T2
Schedule T2 for +100 ms }
Note that the time could be relative (as here) or could be an actual time - there would be slight differences between the methods, due to the additional time to execute the code.

21. An alternative… Initiate a task to handle the sensor T1 T1:
Do sensor task T2
Repeat
{Schedule T2 for n * 100 ms
n:=n+1}
There are some subtleties here...

22. Clock, interrupts, tasks
Processor
Examines
Job/Task queue
Task 1
Task 2
Task 3
Task 4
Tasks schedule events using the clock...

23. Real-Time: Items and Terms
Task
program, perform service, functionality
requires resources, e.g., execution time
Deadline
specified time for completion of, e.g., task
time interval or absolute point in time
value of result may depend on completion time

24. activity occurs repeatedly number of instances
Periodic
activity occurs repeatedly
number of instances
e.g., to monitor environment values, temperature, etc.
period
time
periodic

25. no arrival pattern given
Aperiodic
can occur any time
no arrival pattern given
time
aperiodic
aperiodic

26. minimum time between arrivals
Sporadic
can occur any time, but
minimum time between arrivals
mint
time
sporadic

27. Who initiates (triggers) actions?
Example: Chemical process
controlled so that temperature stays below danger level
warning system: distance level to danger point, so that cooling can still occur
Two possibilities:
action whenever temp raises above warn; event triggered
look every int time intervals; action when temp if measures above warn time triggered

28. t
time TT ET

29. t
time TT ET

30. ET vs TT Time triggered Stable number of invocations Event triggered
Only invoked when needed
High number of invocation and computation demands if value changes frequently

31. Apollo 11, 1969, first lunar landing
Michael Collins (astronaut Apollo 11)
“At five minutes into the burn (6000 feet above the surface)
... "Program alarm", barks Neil, "Its a 1202", what the hell is that?I don't have the alarm numbers memorized for my own computer, much less for the LM's I jerk out my checklist and start thumbing through it, but before I can find 1202 Houston say, "Roger, we're GO on that alarm" no problem in other words. My checklist says 1202 is an "executive overflow" meaning simply that the computer has been called upon to do too many things at once and is forced to postpone some of them. A little farther, at just 3000 feet above the surface the computer flashes 1201, another overflow condition, and again the ground is superquick to respond with assurances.
Excerpt from Apollo, Expeditions to the Moon,
Scientific and Technical Information Office, NASA, Washington D.C., 1975

32. interrupt handler
radar

33. interrupt handler
radar

34. interrupt handler
radar
error 1201

35. Slow down the environment?
Importance
which parts of the system are important?
importance can change over time e.g., fuel efficiency during emergency landing
Flow control who has control over speed of processing, who can slow partner down?
environment
computer system
RT: environment cannot be slowed down

36. Performance Metrics in Real-Time Systems
Beyond minimizing response times and increasing the throughput:
achieve timeliness.
More precisely, how well can we predict that deadlines will be met?

37. What is Predictability
A simplified definition:
The ability to reliably determine whether an activity will meet its deadline.
A complete definition can be quite complex and -- subject to assumptions about failure, etc.

38. Achieving Predictability
Layer by layer: Predictability requirement of an activity percolates down/up the layers.
Top-layer: Do your best to meet the deadline of an activity. If deadline cannot be met, handle the exception predictably.

39. Layer-by-layer Predictability
Applications: Activities having deadlines.
Processes: Processes with deadlines.
one or more processes make up an activity.
Tasks: Tasks with deadlines
multiple (precedence-related) tasks make up a process.
Operating System: Bounded code execution time.
Bounded Operating System primitives.
Bounded synchronization costs.
Bounded scheduling costs.
Architecture: Bounded memory access times.
Bounded instruction execution times.
Bounded inter-node communication costs.

40. Top-layer Predictability
An activity has
a component executed in the usual case with deadline (d -- r) executed using best-effort approaches
a component executed in the exception case -- typically simple execution (if needed) is guaranteed.

41. Issues to worry about
Meet requirements -- some activities may run only:
after others have completed - precedence constraints
while others are not running - mutual exclusion
within certain times - temporal constraints
Scheduling
planning of activities, such that required timing is kept
models
methods

42. Issue: Running time of programs
Depends on hardware, memory wait cycles, clock etc.
Depends on virtual memory and caching
Depends on data
Depends on program control
Depends on pipelining
Depends on memory management

43. More Issues… Operating Systems Off-the shelf (COTS)
Application-specific
Dataflow
data are read from environment - sensors
data are processed by tasks
results are processed by other tasks…
data are transferred to environment
– actuators

44. Further Issues Programming languages Maximum execution times
Difficult with recursions, unbounded loops, etc
Networks
Bounded transmission times
e.g., ethernet, CSMA/CD large number of collisions
Synchronization of clocks
Clock drift apart
Faulty clocks can cause wrong synchronization, e.g., for average

45. Even more issues… Fault tolerance
Deadlines cannot be kept if computer or network has hardware errors
Tolerate certain faults within timing
Real-Time Databases
maintain data consistent and fresh
Design
derive and specify timing