Transcript CSC 506: Software Engineering and Knowledge Engineering

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
Dr. Syed Noman Hasany
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Review of known methodologies
Analysis of software requirements
Real-time software
Software cost, quality, testing and measurements
Object programming
Knowledge engineering issues: knowledge representation
using rules, frames & logic, basics of logical inference, and
basics of search.
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 To
explain the concept of a real-time system
and why these systems are usually
implemented as concurrent processes
 To describe a design process for real-time
systems
 To explain the role of a real-time operating
system
 To introduce generic process architectures
for monitoring and control and data
acquisition systems
Designing
embedded software
systems whose behaviour is
subject to timing constraints
Real-time embedded systems are
defined as those systems in which the
correctness of the system depends not
only on the logical result of computation,
but also on the time at which the results
are produced.
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• 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. Internet
telephony, Skype.
• Real real-time — systems which are hard real-time and which the
response times are very short. E.g. Missile guidance system.
• Firm real-time — systems which are soft real-time but in which there is
no benefit from late delivery of service. E.g. Banking
A single system may have a combination of these.
In reality many systems will have a cost function associated with missing each
deadline
Systems which monitor and control their
environment.
 Sometimes associated with hardware devices
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◦ Sensors: Collect data from the system environment;
◦ Actuators: Change (in some way) the system's
environment;
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Time is critical. Real-time systems MUST
respond within specified times.
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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.
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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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Question: Is a payroll processing system a real-time system?
o It has a time constraint: Print the pay checks every two weeks.
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Perhaps it is a real-time system in a definitional sense, but
it doesn’t pay us to view it as such.
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We are interested in systems for which it is not a priori
obvious how to meet timing constraints.
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Real Time Embedded:
 Nuclear reactor control
 Flight control
 Basically any safety critical system
 GPS
 MP3 player
 Mobile phone
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Real Time, but not Embedded:
 Stock trading system
 Skype
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Embedded, but not Real Time:
 Sprinkler system
 Washing machine, refrigerator, etc.
 Blood pressure meter
 What
is it?
o Myspace?
o Email
o Forums
o Wikis
 Instant
Messaging
o Receive Real Time Interaction When All Parties are Able
To
o Can Participate When You Want
o Able to Connect Many to Many
o Doesn’t Support Storing the Same Way Forums and
Wikis Do
Metrics for real-time systems differ from that for time-sharing systems.
Time-Sharing
Systems
Real-Time
Systems
Capacity
High throughput
Schedulability
Responsiveness
Fast average response
Ensured worst-case
response
Overload
Fairness
Stability
– 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
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A real-time system is a software system where the correct
functioning of the system depends on the results produced
by the system and the time at which these results are
produced
A ‘soft’ real-time system is a system whose operation is
degraded if results are not produced according to the
specified timing requirements
A ‘hard’ real-time system is a system whose operation is
incorrect if results are not produced according to the timing
specification
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Given a stimulus, the system must produce a response
within a specified time
Periodic stimuli. Stimuli which occur at predictable time
intervals
o For example, a temperature sensor may be polled 10 times per
second
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Aperiodic stimuli. Stimuli which occur at unpredictable
times
o For example, a system power failure may trigger an interrupt which
must be processed by the system
 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
 Timing demands of different stimuli are different so
a simple sequential loop is not usually adequate
 Real-time systems are usually designed as
cooperating processes with a real-time executive
controlling of these processes
S en so r
S en so r
S en so r
S en so r
S en so r
S en so r
R eal-tim e
con tro l sys tem
Act uat or
Act uat or
Act uat or
Act uat or
 Sensors
control processes
o Collect information from sensors. May buffer
information collected in response to a sensor
stimulus
 Data
processor
o Carries out processing of collected information
and computes the system response
 Actuator
control
o Generates control signals for the actuator
S en so r
Act uat or
S t im u lu s
S en so r
con tro l
Resp on se
Dat a
p ro ces so r
Act uat or
con tro l
 Design
both the hardware and the software
associated with system. Partition functions to
either hardware or software
 Design decisions should be made on the basis on
non-functional system requirements
 Hardware delivers better performance but
potentially longer development and less scope for
change
Es t ab l is h s ys tem
requ irement s
Parti ti on
requ irement s
S o ftware
requ ir em ent s
Hardw are
requ irem ent s
S o ftware
d es ig n
Hardw are
d es ig n
 Identify
the stimuli to be processed and the
required responses to these stimuli
 For each stimulus and response, identify the timing
constraints
 Aggregate the stimulus and response processing
into concurrent processes. A process may be
associated with each class of stimulus and
response
 Design
algorithms to process each class of
stimulus and response. These must meet the given
timing requirements
 Design a scheduling system which will ensure that
processes are started in time to meet their
deadlines
 Integrate using a real-time executive or operating
system
 May
require extensive simulation and experiment
to ensure that these are met by the system
 May mean that certain design strategies such as
object-oriented design cannot be used because of
the additional overhead involved
 May mean that low-level programming language
features have to be used for performance reasons
 Hard-real
time systems may have to be
programmed in assembly language to ensure that
deadlines are met
 Languages such as C allow efficient programs to be
written but do not have constructs to support
concurrency or shared resource management
 Ada is a language designed to support real-time
systems design so includes a general purpose
concurrency mechanism
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Java supports lightweight concurrency (threads and
synchonized methods) and can be used for some soft real-time
systems
Java 2.0 is not suitable for hard RT programming or
programming where precise control of timing is required
o Not possible to specify thread execution time
o Uncontrollable garbage collection
o Not possible to discover queue sizes for shared resources
o Variable virtual machine implementation
o Not possible to do space or timing analysis
Sun Java Real-Time System
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Requires a Real Time OS underneath (e.g., no Windows
support)
–
 Real-time
executives are specialised operating
systems which manage the processes in the RTS
 Responsible for process management and
resource (processor and memory) allocation
 May be based on a standard RTE kernel which is
used as it is i.e. unchanged or modified for a
particular application
 Does not include facilities such as file
management.
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 Real-time
clock
o Provides information for process scheduling.
 Interrupt
handler
o Manages aperiodic requests for service.
 Scheduler
o Chooses the next process to be run.
 Resource
manager
o Allocates memory and processor resources.
 Despatcher
o Starts process execution.
 Configuration
manager
o Responsible for the dynamic reconfiguration of the
system
software and hardware. Hardware modules may be
replaced and software upgraded without stopping the
systems
 Fault
manager
o Responsible for detecting software and hardware faults
and
taking appropriate actions (e.g. switching to backup
disks) to ensure that the system continues in operation
S ch edul in g
i nfo rmat io n
R eal-t im e
clo ck
Int errup t
h an dl er
S ch edul er
P ro ces s reso urce
requ irem ent s
P ro ces ses
await in g
reso urces
R eady
p ro ces ses
R eady
l is t
Avail able
reso urce
l is t
Reso ur ce
m anag er
Releas ed
reso urces
P ro ces so r
l is t
Des pat ch er
Ex ecut in g
p ro ces s
 The
processing of some types of stimuli must
sometimes take priority
 Interrupt level priority. Highest priority which is
allocated to processes requiring a very fast
response
 Clock level priority. Allocated to periodic processes
 Within these, further levels of priority may be
assigned
 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
 In
most real-time systems, there will be several
classes of periodic process, each with different
periods (the time between executions),
execution times and deadlines (the time by
which processing must be completed)
 The real-time clock ticks periodically and each
tick causes an interrupt which schedules the
process manager for periodic processes
 The process manager selects a process which
is ready for execution
 Concerned
with managing the set of concurrent
processes
 Periodic processes are executed at pre-specified
time intervals
 The executive uses the real-time clock to determine
when to execute a process
 Process period - time between executions
 Process deadline - the time by which processing
must be complete
S ch ed u l er
C h oo se p ro ces s
fo r execut io n
Reso u rce m an a ger
Al l ocat e m em ory
and p ro ces so r
Des p a tch er
S t art execut io n on an
av ail ab l e pro ces so r
 The
scheduler chooses the next process to be
executed by the processor. This depends on a
scheduling strategy which may take the process
priority into account
 The resource manager allocates memory and a
processor for the process to be executed
 The despatcher takes the process from ready list,
loads it onto a processor and starts execution
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Non pre-emptive scheduling
o 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)
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Pre-emptive scheduling
o The execution of an executing processes may be stopped if a higher
priority process requires service
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Scheduling algorithms
o Round-robin
o Shortest deadline first
 Important
class of real-time systems
 Continuously check sensors and take actions
depending on sensor values
 Monitoring systems examine sensors and
report their results
 Control systems take sensor values and control
hardware actuators
A
system is required to monitor sensors on doors
and windows to detect the presence of intruders in
a building
 When a sensor indicates a break-in, the system
switches on lights around the area and calls police
automatically
 The system should include provision for operation
without a mains power supply
 Sensors
o Movement detectors, window sensors, door sensors.
o 50 window sensors, 30 door sensors and 200 movement
detectors
o Voltage drop sensor
 Actions
o When an intruder is detected, police are called automatically.
o Lights are switched on in rooms with active sensors.
o An audible alarm is switched on.
o The system switches automatically to backup power when a
voltage drop is detected.
 Identify
stimuli and associated responses
 Define the timing constraints associated with each
stimulus and response
 Allocate system functions to concurrent processes
 Design algorithms for stimulus processing and
response generation
 Design a scheduling system which ensures that
processes will always be scheduled to meet their
deadlines
 Power
failure
o Generated aperiodically by a circuit monitor.
When received, the system must switch to
backup power within 50 ms
 Intruder
alarm
o Stimulus generated by system sensors.
Response is to call the police, switch on building
lights and the audible alarm
S tim ulus/R e sp on se
P ow er fa il in ter r u p t
D oor a la r m
W in d ow a la r m
M ovem en t d etector
A u d ible a la r m
L ig h ts sw itch
C om m u n ica tion s
V oice syn th esiser
T im in g r e q uir e m e n ts
T h e sw itch to ba ck u p p ow er m u st be com p leted
w ith in a d ea d lin e of 5 0 m s.
E a ch d oor a la r m sh ou ld be p olled tw ice p er
secon d .
E a ch w in d ow a la r m sh ou ld be p olled tw ice p er
secon d .
E a ch m ovem en t d etector sh ou ld be p olled tw ice
p er secon d .
T h e a u d ible a la r m sh ou ld be sw itch ed on w ith in
1 /2 secon d of a n a la r m bein g r a ised by a sen sor .
T h e lig h ts sh ou ld be sw itch ed on w ith in 1 /2
secon d of a n a la r m bein g r a ised by a sen sor .
T h e ca ll to th e p olice sh ou ld be sta r ted w ith in 2
secon d s of a n a la r m bein g r a ised by a sen sor .
A syn th esised m essa g e sh ou ld be a va ila ble
w ith in 4 secon d s of a n a la r m bein g r a ised by a
sen sor .
4 00 Hz
6 0Hz
M ovem ent
d et ecto r p ro ces s
1 00 Hz
Door sen so r
p ro ces s
Det ecto r s tat us
W i nd ow sen so r
p ro ces s
S en so r st at us
S en so r st at us
5 60 Hz
Al ar m s ys tem
C o m m u ni cat io n
p ro ces s
B u il di ng m on it or
p ro ces s
P ower fai lu re
i nt erru pt
B u il di ng m on it or
Pow er swi t ch
p ro ces s
Roo m n um b er
Al arm s ys tem
p ro ces s
Al ert m ess ag e
Ro om nu mber
Al arm
s ys tem
Al arm
s ys tem
Au di bl e alarm
p ro ces s
Al arm s ys tem
Ro om nu m ber
Li ghti ng co nt ro l
p ro ces s
Vo ice s yn th esi zer
p ro ces s
// S ee http://w w w .softw are-engin.com / for links to the com plete
// Jav a code for this ex am ple
class B uildingM onitor ex tends T hread {
B uildingS ensor w in, door, m ov e ;
S iren
siren = new S iren () ;
Lights lights = new Lights () ;
S ynthesizer synthesizer = new S ynthesizer () ;
D oorS ensors doors = new D oorS ensors (30 ) ;
W indow S ensors
w indow s = new W indow S ensors (50 ) ;
M ov em entS ensors m ov em ents = new M ov em entS ensors (200 ) ;
P ow erM onitor pm = new P ow erM onitor () ;
B uildingM onitor()
{
// initialise all the sensors and start the processes
siren.start () ; lights.start () ;
synthesizer.start () ; w indow s.start () ;
doors.start () ; m ov em ents.start () ; pm .start () ;
}
public void run () {
int room = 0 ;
while (true) {
// poll the movement sensors at least twice per second (400 Hz)
move = movements.getVal () ;
// poll the window sensors at least twice/second (100 Hz)
win = windows.getVal () ;
// poll the door sensors at least twice per second (60 Hz)
door = doors.getVal () ;
if (move.sensorVal == 1 | door.sensorVal == 1 | win.sensorVal == 1)
{
// a sensor has indicated an intruder
if (move.sensorVal == 1)
room = move.room ;
if (door.sensorVal == 1)
room = door.room ;
if (win.sensorVal == 1 )
room = win.room ;
lights.on (room) ; siren.on () ; synthesizer.on (room) ;
break ;
}
}
lights.shutdown () ; siren.shutdown () ; synthesizer.shutdown () ;
windows.shutdown () ; doors.shutdown () ; movements.shutdown () ;
} // run
} //BuildingMonitor
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A burglar alarm system is primarily a monitoring system. It
collects data from sensors but no real-time actuator control
Control systems are similar but, in response to sensor
values, the system sends control signals to actuators
An example of a monitoring and control system is a system
which monitors temperature and switches heaters on and
off
5 00 Hz
S en so r
p ro ces s
S en so r
valu es
5 00 Hz
Th erm o st at
p ro ces s
5 00 Hz
S w it ch co m m and
Ro om n u m ber
Heat er con tro l
p ro ces s
Th erm o st at pro ces s
F u rnace
con tro l p ro ces s
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Real-time system correctness depends not just on what the
system does but also on how fast it reacts
A general RT system model involves associating processes
with sensors and actuators
Real-time systems architectures are usually designed as a
number of concurrent processes
Real-time executives are responsible for process and
resource management.
Monitoring and control systems poll sensors and send
control signal to actuators