.. _scheduler:


#######################
The Process Scheduler
#######################

We define :term:`process scheduling` as:

    The act of determining which process in the ready state should be moved
    to the running state. That is, decide which :term:`process` should 
    run on the CPU next.

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The **goal** of the scheduler:

    Implement the :term:`virtual machine` in such a way the user perceives that
    each process is running on it's own computer.

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Modern operating systems are generally able to achieve this because of the
statistical properties of processes.  Over a short period, processes can be
categorized according to what is limiting the completion of the process. 

.. index:: I/O Bound Process

.. describe:: I/O Bound Process

   * Processes that are mostly waiting for the completion of input or output
     (I/O) are :dfn:`I/O Bound`.  
   
   * Interactive processes, such as office applications are mostly I/O bound
     the entire life of the process.  Some processes may be I/O bound for only
     a few short periods of time.

   * The expected short run time of I/O bound processes means that they will
     not stay the running the process for very long.  
   
   * They should be given high priority by the scheduler.

.. index:: CPU Bound Process

.. describe:: CPU Bound Process

   * :dfn:`CPU Bound` processes are ones that are implementing algorithms with
     a large number of calculations.  
   
   * They can be expected to hold the CPU for as long as the scheduler will
     allow.  
   
   * Programs such as simulations may be CPU bound for most of the life of the
     process.  
   
   * Users do not typically expect an immediate response from the computer when
     running CPU bound programs.

   * They should be given a lower priority by the scheduler.

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Schedulers fall into one of two general categories:

1. .. describe:: Non-preemptive scheduling 

      when the currently executing process gives up the CPU voluntarily.

2. .. describe:: Preemptive scheduling

      when the operating system decides to favor another process, preempting
      the currently executing process.

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**Contents**

.. toctree::
   :maxdepth: 2

   sched_parts
   metrics
   fcfs
   sjn
   rr
   priority_queues