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Memory and Process Management, Major resource types, life cycle of a…

- - - -  In computing, preemption (more correctly ”pre-emption”) is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time.
      -  Preemptive scheduling incurs a cost associated with access to share data
    - -  When scheduling takes place only under circumstances 1 and 2, we say that the scheduling scheme is non-preemptive
      - once the CPU has been allocated to a process, the process keep the cpu until it release the cpu either by terminating or by switching to waiting state
  - - -  The mid-term scheduler may decide to swap out a process which :
      -  Has not been active for some time, or a process which has a low priority,
      -  A process which is taking up a large amount of memory in order to free up main memory for other processes, swapping the process back in later when more memory is available
    - -  Also known as the dispatcher
      -  Decides which of the ready, in-memory processes are to be executed (allocated a CPU) next following a clock interrupt, an IO interrupt, an operating system call or another form of signal
      -  Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
    - -  Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
      -  Long-term scheduler is invoked very infrequently
        (seconds, minutes) (may be slow)
      -  The long-term scheduler controls the degree of multi programming
- - - - common problem in multiprocessing where many processes share a specific type of mutually excluive resource known as a software lock or soft lock
        
        A set of blocked processes each holding a resource
        
        waiting to acquire a
        resource held by another process in the set.
    - - there is no general solution to
        avoid (soft) deadlocks.
  - - - Process 1 requests resource B from process 2
        
        Resource B is locked while process 2 is running.
        
        Process 2 requires resource A from process 1 to finish running.
        
        Resource A is locked while process 1 is running.
  - - - Each section of a bridge can be viewed as a resource.
        
        If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback).
        
        Several cars may have to be backed up if a deadlock occurs.
        
        Starvation is possible.
  - - - DEADLOCK OREVENTATION
        
        Mutual Exclusion
        
        Hold and Wait
        
        No Preemption
        
        Circular Wait
- - - - used to provide information (or data) to the computer system
        
        Keyboard (typing text), mouse (pointing), scanning (scanning of paper documents), microphone
    - - used to bring out information from the computer system
        
        monitor, printer, speaker
- - - - program counter
      - stack
      - data section