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Lab 2: Scheduler
====================

**Handed out Wednesday October 23, 2019**

**Due Friday Wednesday November 6, 2019**

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1. Getting Started
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Download a new directory of starter code of XV6 for Lab2 from this
`link <https://github.com/mit-pdos/xv6-public>`_.

To get started, look at the file ``proc.c``.

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2. Assignment (95%)
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In this part of the assignment, you will change the scheduler from
a simple round-robin to a priority scheduler.  Add a priority value to
each process (lets say taking a range between 0 to 31).  The range
does not matter, it is just a proof of concept.  When scheduling from
the ready list you will always schedule the highest priority
thread/process first.  All the processes should have a default initial
priority of 10.

Add a system call to change the priority of a process.  A process can
change its priority at any time.  If the priority becomes lower than
any process on the ready list, you must switch to that process.

**Goals**: Understand how the scheduler works.  Understand how
to implement a scheduling policy and characterize its impact on
performance.  Understand priority inversion and a possible solution
for it.

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3. Bonus Section (upto 10%)
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Implement up to two of the next three items.  Each is a 5% bonus.
To get credit for a bonus part, you must also develop a user test that
will illustrate it.

* To avoid starvation, implement aging of priority.  If a process
  waits increase its priority.  When it runs, decrease it.
  (Possible Bonus 1)

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* Implement priority donation/priority inheritance. (Possible Bonus 2)

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* Add fields to track the scheduling performance of each process.
  These values should allow you to compute the turnaround time and
  wait time for each process.  Add a system call to extract these
  values or alternatively print them out when the process exits.
  (Possible Bonus 3)

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4. Questions to Explore
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* Explain how the current scheduler works, including interaction with
  the timer.