Priority Inversion

Priority Inversion

• Can occur in multi-tasking systems
• It is a condition where a lower priority task effectively blocks a higher priority task from accomplishing its job

Priority Inversion example

// Data acquisition thread
ImportantTask()
{
    SSetPriority(HIGHEST)
    While(TRUE)
    {
        //...
        Sleep(1 second)
        AcquireSemaphore(pDatabaseSem)
        // ... work on database
        ReleaseSemaphore(pDatabaseSem)
    }
}

// Database statistics thread
MenialTask()
{
    SetPriority(LOWEST)
    While(TRUE)
    {
        //...
        Sleep(5 minutes)
        AcquireSemaphore(pDatabaseSem)
        //... check database statistics
        ReleaseSemaphore(pDatabaseSem)
    }
}

• Can be difficult to detect
• Solutions - Don’t share semaphores in this fashion by design
• Priority Inheritance - supplied by some OSs
  • OS bumps low-priority tasks priority to that of high priority task for duration of inversion. (Sometimes only one-level deep, can be costly and non-deterministic)