Difference between revisions of "Priority Inversion"

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* There is a class called javax.realtime.MonitorControl which specifies the monitor priority control Policy.
 
* There is a class called javax.realtime.MonitorControl which specifies the monitor priority control Policy.
 
* The PriorityInheritance class extends the MonitorControl class.
 
* The PriorityInheritance class extends the MonitorControl class.
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== Blocking ==
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* If a SO has m critical sections, the max no of times it can be blocked is m, ( with Priority Inheritance.)
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== Deadlock ==
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* Deadlock can still occur with priority inheritance.
  
 
[[Category:RealtimeJava]]
 
[[Category:RealtimeJava]]

Revision as of 17:24, 6 January 2012

Intro

  • All synchronization mechanisms that are based on mutual exclusion suffer from priority inversion.
  • This is where a low-priority SO enters into a mutual-exclusion zone which it shares with a high-priority SO.
  • Now the low priority SO is in a mutex zone. Now, consider a medium priority thread which doesnt use the mutex zone, it preempts the low priority thread and starts performing a computationally intensive task.
  • So the low priority SO cant run and get out of the mutex zone. Which means, the high priority thread is blocked.
  • The medium-priority thread thereby indirectly blocks the progression of the high-priority thread for a potentially unbounded period of time.
  • Solutions ?
    • Priority Inheritance
    • Non Blocking Communication

Priority Inheritance

  • If thread p is blocking thread q, then p runs with q's priority.

RTSJ Model

  • RTSJ requires all system Q's to be priority ordered.
    • Q's of SO's waiting for an object lock.
    • SO's blocked on wait().
  • If more than one SO in the Q has same priority, then FIFO is followed.
  • Has facilities to specify priority inversion control algorithms.
    • By default, RTSJ requires simple priority inheritance to occur whenever a SO is blocked waiting for a resource.
  • There is a class called javax.realtime.MonitorControl which specifies the monitor priority control Policy.
  • The PriorityInheritance class extends the MonitorControl class.

Blocking

  • If a SO has m critical sections, the max no of times it can be blocked is m, ( with Priority Inheritance.)

Deadlock

  • Deadlock can still occur with priority inheritance.