.. _priority_queues:

Multi-level Priority Queue Scheduler
=====================================

.. index:: Multi-level Priority Queue Scheduler

* A more complex scheme is often used to shift the behavior of the :ref:`rr`
  slightly in favor of short, interactive processes like the :ref:`sjn`.  

* New processes begin in the highest priority queue.

* A process is rewarded by being moved to a higher priority queue if it
  voluntarily blocks for I/O before its time quantum expires.  Similarly
  processes that are preemptively removed are penalized by being put in a lower
  priority queue.  
  
* Queues with a lower priority use a longer time quantum.  

* Higher priority queues must be empty before processes from lower priority
  queues are allowed to run.  Thus, it is possible for :term:`starvation` to
  occur.


.. figure:: priority_queues.png
   :align: center