Real Time - Operating System - Lecture Slides, Slides of Computer Science

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Real-Time OS’s &

Applications

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Outline

• Examples of real-time systems
• Real-time OS implementation
• Real-time CPU scheduling
  • Rate-monotonic scheduling
  • Admission control
  • Earliest Deadline First (EDF) Scheduling
  • Schedulability
• POSIX
• VxWorks
• Wind Microkernel
• RT Mach
  • CPU & disk scheduling

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Other Examples

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Other Examples

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Other Examples

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Types of Real-time

• Hard real-time
  • Correct operation depends not only upon the logical correctness of the computation but also upon the time at which it is performed. Computations performed after a deadline are, by definition, incorrect, and usually have no value.
• Safety-critical systems
  • Incorrect operation, such as a missed deadline, results in a catastrophe
  • E.g., weapon systems, antilock braking systems, flight control systems
• Soft real-time
  • Value of an operation declines steadily after the deadline expires

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Soft Real-Time

Value of completion of operation

Time

deadline

(Shape of curve after deadline is defined by the application & users)

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Real-Time Systems

• Computer system often completely dedicated

to the control application

• Utilization of computer system is less important than meeting deadlines
• Single purpose systems
• May be relatively simple
• Thousands of lines of program code, not millions
• Physical space constraints
• E.g., a computer system for a hand-held game needs to be relatively small
• Cost constraints
• E.g., 8 or 16 bit processors & 1 MB ram

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Real-Time O/S

Implementation - 2

• Delays in system must be minimized or

bounded (for hard real-time)

• I.e., bounded by constant time values
• Delays include
• interrupt latencies
• Time from when interrupt occurs (I.e., when hardware device generates interrupt) until interrupt service handler is invoked
• dispatch latencies
• Amount of time required for dispatcher to stop one process and start another
• system calls

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Interrupt Latencies

• Important contributing factor: amount of time

interrupts turned off while updating kernel

data structures

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p : period

t : processing time

d : deadline Docsity.com

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Assumption of Periodic

Processes

• See Section 1.2.1 of Li & Yao (2003)
• It seems clear that assumption of periodic

processes accounts for only some of the

events in a system

• Aperiodic: Occurrence of incoming missiles

for weapons defense system

• Periodic: control signals to weapon firing

system

• High frequency, but periodic, firing coordinates