Exercise 1 SRS

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• Exercise 1 SRS
• Due date 26.11.04
• The specification should contain the following
• System overview - submitted today and will be
  given back next week.
• Use case diagram
• External interfaces (according to SRS document)
• Sequence diagram

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UML Example
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Class Diagram
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Component diagram
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Deployment diagram
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Activity diagram(for Lend Item)
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Lecture 4Schedulability and Tasks
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• Time-Based Systems
• Time based systems are systems whose behavior is
  controlled by time.
• This can be due to arrival of an absolute time or
  the elapse of a time interval.
• Absolute time is an independent real world time.
• System/Mission time is the time elapsed from
  system startup. It starts when the systems starts
  and ends when the system shuts down, it may
  hold or stop at different points in the
  mission.
• (Time) interval is a particular start and end
  points, and not a length of the time. Two
  identical intervals must have both started and
  ended at the same time.
• (Time) duration is a relative time measure, it is
  a scalar value independent of a start time.
• It is possible that two non-identical intervals
  have the same duration if they started at
  different time.
• A periodic event is one that repeats with a
  constant duration once it begun.

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• Reactive Systems
• Reactive systems are systems whose behavior
  controlled by internal or external events.
  Execution of tasks is initiated in response to
  these events.
• During requirement analysis phase the events are
  identified and characterized.
• Example
• Respond to a user turning a knob within 20 msec
  (aperiodic).
• Respond to a heart beat to invoke a ventricular
  pacemaker pulse within 10 µsec (aperiodic).
• Adjust the ailerons for a banked turn in response
  to flight control computer commands sent every 50
  µsec (periodic).
• Periodically clamp and release brake s while the
  wheels of a car are in a skid.

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Timing diagrams
A common way to represent change in state over
time is via a timing diagram. A simplified
version shows binary task states
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Time Concepts
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Timing diagrams more elaborate from
Slanted line transition from one state to
another takes time. Leading Jitter, Trailing
Jitter time to start state transition. Leading
and Trailing Jitters are usually non significant
time period relatively to the whole view.
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Timing diagrams more elaborate from
Inter-arrival Time duration between task
invocations Minimum Inter-arrival Time lower
bound Maximum Burst maximum number of events
that can occur within a period of time.
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• Priority
• Two independent properties of actions are
  important in real time system
• Urgency is the timeliness constraint of action.
• Importance is the value of computational action
  to the system.
• Scheduling systems do not commonly contain these
  abstractions. They provide a low level
  abstraction called priority. The priority of a
  task is used to resolve disputes over which tasks
  execute when more than one task is waiting and
  ready to execute.
• Do not get confused
• Importance of a task is the value of the
  completion of the action relative to the overall
  system goals.
• The tasks priority has to do with which task
  wins when more than one is available to run.
• High priority is represented by low or high
  numerical OS dependent.

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• Example
• Monitoring device.
• There is a high bandwidth waveform data.
• An alarm must be reported within 10 sec.
• Waveform displays with more than 100ms delay
  would be unacceptable to user. It is not
  critical, but has tight timeliness with a hard
  deadline.
• Alarm handling is crucial for the system, but the
  user would not even notice half second delay in
  alarms. It is critical, but has broad timeliness
  with a soft deadline.
• Therefore, alarm handling will have lower
  priority to ensure that high bandwidth waveform
  data usually wins and its display is smooth.
• Task with hard deadline has higher priority than
  a task with a soft deadline.

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Schedulability Timeliness of an action has to do
with the action meeting its time constraints. The
basic concept of timeliness in real-time systems
is that action must begin in response to event
arrival or due time arrival, and it must complete
within a certain time after it begun. Schedulable
task task that can be determined to always meet
its timeliness constraints. Deterministically
schedulable task as a special case, always
guaranteed to meet all its deadlines. Events
worse-case response time is less than tasks
deadline.
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• Schedulability
• Scheduling analysis of hard real-time system is
  more straightforward, deterministic and
  predictable.
• Scheduling analysis of soft real-time system is
  more complicated, not always deterministic and
  not always predictable.
• Considering all deadlines to be hard is an
  approximation making the analysis easier but too
  strict. Assuming worst-case execution time,
  analysis may derive a conclusion that the system
  is not schedulable. But a soft-real system may
  still meet all its mission requirements.

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Latency and Interrupt latency
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Latency and Interrupt latency
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Latency and Interrupt latency
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Latency and Interrupt latency
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Latency and Interrupt latency
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Thread Quantum
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Thread Quantum
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Timer Granularity
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Utility Function u(t) Measures utilityusefullness
value of action as a function of time. In Hard
real-time systems 1 valuable 0 not valuable
(completed after hard deadline) -1
counterproductive (completed after hard deadline)
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Utility Function u(t) hard real-time systems
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Utility Function u(t)
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Utility Function u(t)
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Utility Function u(t)
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Utility Function u(t)
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Utility Function u(t)
Early completion
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Utility Function u(t)
Early completion
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• Progressive Utility Function u(t,w)
• Measures not the action completion, but the
  action progress in time.
• t time
• w percentage completion of the action
• Many actions increase in value as work
  progresses, these actions may still have a value
  even if not performed to the completion.
• For example it is very important that an action
  performs on time 75 percent of the way, but the
  actual completion may be deferred until later.

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Progressive Utility Function u(t,w)
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Progressive Utility Function u(t,w)
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Progressive Utility Function u(t,w)
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Scheduling Determination of the policies that
decide which tasks execute when multiple tasks
are available. Is always a design concern!
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Scheduling terms Preemptive operating system
may swap out tasks, Non-preemptive the task
must voluntarily to give up control. A task is
blocked when it is ready to run but cannot
because a lower-priority task owns a required
resource. Blocking is unavoidable in preemptive
scheduling in which resources are shared among
tasks. However, the blocking must be bounded so
that worst case blocking may be
computed. Unbounded priority inversion high
priority tasks may be blocked from execution by
an indefinite set of other tasks.
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Unbounded priority inversion example
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• Unbounded priority inversion example
• Task A has the highest priority
• Every task by itself can be scheduled
• Even if task C locks the resource, task A will
  still meet its deadlines.
• So will task A always meet its deadline?

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Unbounded priority inversion example
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Scheduling algorithms
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than fixed-priority case.
(?????)
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Because LL algorithm (and ED )assume that all the
deadlines are hard.
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• Minimizing Maximum Lateness Scheduling
• Sometimes one task cannot start until another
  completes.
• In addition, the completion of each action j is
  associated with a cost function hj.
• An optimal scheduling minimizing cost can be
  constructed using the following off-line static
  algorithm

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Minimizing Maximum Lateness Scheduling J ordered
set of tasks already scheduled Jc set of task
still to be scheduled(complement of J) J set of
task that can be scheduled immediately before J(J
cant start before J completes) 1. J empty, Jc
all the tasks, Jall tasks with no
successors 2. Select j from Jc with minimum hj
that has no predecessor in Jc (so j must be in
J) 3. Add j to J and remove from Jc 4. Adjust
J 5. If Jc is empty stop, otherwise go to step 2