Real-Time Scheduling

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Real-Time Scheduling

• COSC 513
• Operating Systems
• Yue Dou

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Real-Time System

• Robotics
• Air traffic control
• Military command and control systems
• Space station
• Undersea exploration

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Correctness of Real-Time System

• Correctness of the real-time system depends not
  only on the logical result of the computation,
  but also on the time at which the results are
  produced

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Used Terms

• Hard real-time task must meet its deadline (
  deadline specifies either a start time or a
  completion time)
• Soft real-time task has an associated deadline
  that is desirable but not mandatory, it still
  makes sense to schedule and complete the task
  even if it has passed its deadline

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Five Characteristics of Real-Time Operating System

• Determinism concerned with how long an
  operating system delays before acknowledging an
  interrupt
• Responsiveness concerned with how long after
  acknowledgment, it takes an operating system to
  finish the interrupt service routine
• Determinism and responsiveness together make up
  the response time to external events which are
  critical for real-time systems

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Five Characteristics of Real-Time Operating
System (cont.)

• User control allow the user fine-grained control
  over task priority
• Reliability a transient failure may cause
  financial loss or major equipment damage or even
  loss of life.
• Fail-soft operation when data corruption
  happen, system should attempt corrective action
  and same time continues operation perhaps at a
  reduced level of service

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Two Popular Classes of Scheduling Algorithms (1)

• Deadline Scheduling the task which has the
  earliest deadline, will be scheduled first

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Example of Deadline Scheduling

• A system that collects and processes data from
  two sensors, A and B. The deadline for collecting
  data from sensor A must be met every 20 ms, and
  that for B every 50 ms. It takes 10 ms to
  process each sample of data from A and 25 ms to
  process each sample of data from B.

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Example of Deadline Scheduling (cont.)
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Two Popular Classes of Scheduling Algorithms ( 2 )

• Rate Monotonic Scheduling ( RMS ) used for
  resolving multitask scheduling conflicts for
  periodic tasks, it assigns priorities to tasks on
  the basis of their periods ( rate 1 / periods )

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High Rate, High Priority ( rate monotonic )
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Bound Equation

• C1/T1 C2/T2 .Cn/Tn lt n(21/n 1)
• Cn processing time for each task
• Tn each tasks period
• Cn/Tn utilization of the processor

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Example of Rate Monotonic Scheduling

• P1 C1 20 T1 100 C1/T1 0.2
• P2 C2 40 T2150 C1/T1 0.267
• P3 C3 100 T3350 C1/T1 0.286
• Total utilization 0.753
• Since 0.753 lt 3 (21/3 1) 0.779
• So, if RMS is used, all tasks will be
  successfully scheduled