Resources

• Classification of resources-I, Two general categories of resources can be distinguished:
  • Reusable: something that can be safely used by one process at a time and is not depleted by that use.
    • Processes obtain resources that they later release for reuse by others.
    • Examples are processors, I/O channels, main and secondary memory, files, specific I/O devices, databases, and semaphores.
    • In case of two processes and two resources, deadlock occurs if each process holds one resource and requests the other.
  • Consumable: these can be created and destroyed.
    • When a resource is acquired by a process, the resource ceases to exist.
    • Examples are interrupts, signals, messages, and information in I/O buffers
    • Deadlock may occur if a Receive message is blocking
    • May take a rare combination of events to cause deadlock
• Classification of resources-II, One other taxonomy again identifies two types of resources:
  • Preemptable: these can be taken away from the process owning it with no ill effects (needs save/restore). E.g., memory or CPU.
  • Non-preemptable: cannot be taken away from its current owner without causing the computation to fail. E.g., printer or floppy disk.
• Deadlocks occur when sharing reusable and non-preemptable resources