Eda222 - Real Time Systems

Front 1

What is Real-Time Systems?

Back 1

Real-Time Systems is one which the correctness of the system depends not only on the logical result of computation, but also on the time at which results are generated

"Misconceptions of Real-Time Computing"

Front 2

Properties of Real-Time Systems?

Back 2

• Strict timing constraints
• Application-specific design
• High-reliability

Front 3

Design phase of Real-Time Systems?

Back 3

1. Specification
2. Implementation
3. Verification

Front 4

Examples of application constraints?

Back 4

• Timing constraints
• Exclusion constraints
• Precedence constraints

Front 5

What is timing constraints?

Back 5

A task must complete its execution within given time frames

e.g. task periodicity or deadline

Front 6

Where timing constraints come from?

Back 6

• Laws of Nature
• Mathematical theory
• Component limitation
• Artificial derivation

Front 7

Criticality of constraints?

Back 7

Hard

• non-critical
• critical
• safety-critical

Soft

Front 8

What are critical choices during design time?

Back 8

• Application software
• Hardware architecture
• Run-time systems

Front 9

How we verify the system?

Back 9

Ad-hoc testing

Let the system runs for a while then let the absence of failures prove the correctness

Exhaustive testing

Verify all the combinations of input data, time and faults

Formal analysis

Using proof machine and schedulability analysis

Front 10

Run-time system shared resources are?

Back 10

CPU Time

Memory Pool

I/O Device Access

Data Structure (Table, Buffers, ...)

Front 11

Application-program shared resources are?

Back 11

Displays

Data Entities

Data Structure

Front 12

Resource classification & operations?

Back 12

Classification

1. Exclusive
2. Shared

Operation

1. Acquire
2. Release

Front 13

How to make acquire non-blocking?

Back 13

Let acquire returns a status code that indicates whether the resource is available or not

Front 14

Problems with resource management?

Back 14

• Deadlock
• Starvation

Front 15

How to avoid starvation?

Back 15

Granting an access in FIFO manner

Dynamic priority depends on waiting time

Front 16

Conditions for deadlock to occur?

Back 16

1. Mutual exclusion
2. Hold and wait
3. Non preemptive
4. Circular Wait

Front 17

Hardware level mutual-exclusion method?

Back 17

Single Processor

Disable-Interrupt

Multi Processor

Atomic Instructions (test-and-set lock)

Front 18

Implement semaphore in C!

Back 18

• object should receive an initial value
• object should have wait and signal methods
• methods should support call-back functionality

WAIT(s)

if s > 0 then:

...s := s-1

else

...block()

SIGNAL(s)

if there is blocked tasks

...allowBlockedTask

else

...s := s+1

Front 19

What causes component faults?

Back 19

Specification faults

• incomplete
• lack of techniques

Component faults

• manufacturing defects
• wear and tear

Environmental effects

• high stress
• electromagnetic/elementary-particle rad

Front 20

Types of faults?

Back 20

Permanent - total failure of component

Transient - temporary malfunction

Intermittent - repeated occurrences of transient

Front 21

How errors are handled at run-time?

Back 21

Error detection

erroneous data/program behaviour is detected

Error correction

originally data intended is recovered

Fault masking

hide erroneous data

Front 22

Consideration for fault-tolerant real-time system?

Back 22

1. What is fault model used?
2. How should the model implemented?
3. What scheduling policy should be used?

Front 23

Cause of message delay in network?

Back 23

OVERHEADS

• formatting
• queueing
• medium
• notifying (at receiver)
• decoding

Front 24

Cause of queueing delay?

Back 24

corresponding time slot (FlexRay)

transmission token (Token Ring)

contention-free transmission (Ethernet)

network priority negotiation (CAN)

removal from priority queue (Switch Ethernet)

Front 25

How can you draw synchronous network comm?

Back 25

Front 26

And for asynchronous network?

Back 26

Front 27

How is the message transmitted onto medium?

Back 27

Contention-free communication

dedicated time slot, one sender for one line

e.g. FlexRay, TTP/C, TTCAN

Token-based communication

utilize a token for the arbitration of the turn

e.g. TokenRing

Collision-based communication

used for retransmission and identifier

e.g. Ethernet, CAN

Front 28

How CAN protocol works?

Back 28

1. each node is waiting until bus is idle
2. node with highest priority send first
3. identifier first, then MSB to LSB in order
4. check the recessive bits, '1' to '0', if different then it's not transmitting highest-priority msg
5. node that transmit identifier bit and pass number 4, then continue transmit the body

Front 29

How do we verify the RTS?

Back 29

SCHEDULABILITY Analysis

Front 30

Two steps of Schedulability Analysis?

Back 30

Introduce an abstract-model

e.g. task model, processor, run-time model

Predict whether tasks meet time-constraint

e.g. WCET, timing-correct abstract model

Front 31

How do you facilitate schedulability analysis?

Back 31

Concurrent and reactive programming

+ thread & object enabled

+ WCET should be derived from entities (objects)

Deterministic task execution

+ time tables with task priorities

+ preemptives task execution

Front 32

3 basic states of run-time model are?

Back 32

1. Waiting

2. Ready

3. Running

Front 33

Different type of tasks?

Back 33

Periodic task

- arrive at time interval Ti

Sporadic task

- arrive at time interval ≥Ti

Aperiodic task ~ not applicable on hard RTS

- intermittent periodicity