Resource management and priorities in RealTime Systems

1
Resource management and priorities in Real-Time
Systems

• Jan Lindström

2
Contents

• Objectives
• Resources
• Priority inversion problem
• Methods to avoid priority inversion
• Priority inheritance protocol
• Summary

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Objectives

• What are shared resources ?
• Why resources influence scheduling of the
  real-time tasks ?
• What is priority inversion problem ?
• What methods there are to avoid priority
  inversion problem ?
• How does priority inheritance work ?

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Shared resources

• Several tasks access software and hardware
  objects where mutual exclusion is necessary.
• Resource allocated to one job at time. Once
  allocated, held by the job until no longer
  needed.
• Examples semaphores, locks, servers,
• Operations lock(resource) --- ltcritical
  sectiongt--- unlock(resource)

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Priority inversion problem
lock(R)
unlock(R)
lock(R)
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Pre-emption of tasks in their critical sections

• Negative effect on schedulability and
  predictability.
• Traditional resource management algorithms fail.
  ( They decouple resource management decisions
  from scheduling decisions).

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Mars Pathfinder

• Landed 4.7.1997
• Experiences software glitches.
• http//research.microsoft.com/mbj/Mars_Pathfinder
  /Mars_Pathfinder.html

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Priority inversion in Mars Pathfinder
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Avoiding priority inversion

• Non-preemptive critical sections
• Causes unpredictable waiting time.
• Usable only to short critical sections.
• Entry protocols for critical sections
• Priority Inheritance Protocol.
• Priority Ceiling Protocol.

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Priority inheritance protocol

• Under this protocol, if a higher priority task
  TH is blocked by a lower priority task TL,
  because TL is currently executing critical
  section needed by TH, TL temporarily inherits the
  priority of TH. When blocking ceases (i.e., TL
  exits the critical section), TL resumes its
  original priority. Unfortunately, priority
  inheritance may lead to deadlock.

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Example of priority inheritance
blocked
T1
T2
T3
without priority inheritance
prio(T1) gt prio(T2) gt prio(T3)
T1
T2
T3
with priority inheritance
prio(T3) prio(T1)
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Complexity of scheduling

• When there are mutual exclusion constraints in a
  system, it is impossible to find an optimal
  on-line scheduling algorithm (unless it is
  clairvoyant). (Mok, 1983)
• The problem of deciding feasibility for set of
  periodic tasks which use semaphores to enforce
  mutual exclusion is NP-hard. (Mok, 1983).

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Summary

• Critical sections have negative effect on
  schedulabitity and predictability.
• Critical sections can cause priority inversion
  problem.
• Priority inheritance can control priority
  inversion.