QUESTIONS TO ASK ABOUT OPERATINGS SYSTEMS

Or, why are studying this stuff?
Why are operating systems important?
• They consume more resources than any other program.
They may only use up a small percentage of the CPU time, but consider how many machines use the same program, all the time.
• They are the most complex programs.
They perform more functions for more users than any other program.
• They are necessary for any use of the computer.
When "the (operating) system" is down, the computer is down. Reliability and recovery from errors becomes critical.
• They are used by many users.
More hours of user time is spent dealing with the operating system. Visible changes in the operating system cause many changes to the users.
Why are operating systems difficult to create, use, and maintain?
• Size - too big for one person
Current systems have many millions lines of code. Involve 10-100 person years to build.
• Lifetime - the systems remain around longer than the programmers who wrote them.
The code is written and rewritten. Original intent is forgotten (UNIX was designed to be cute, little system - now 2 volumes this thick). Bug curve should be decreasing; but actually periodic - draw.
• Complexity - the system must do difficult things.
Deal with ugly I/O devices, multiplexing-juggling act, handle errors (hard!).
• Asynchronous - must do several things at once.
Handles interrupts, and must change what it is doing thousands of times a second - and still get work done.
• General purpose - must do many different things.
Run Doom, Java, Fortran, Lisp, Trek, Databases, Web Servers, etc. Everybody wants their stuff to run well.
________________________________________
Operating systems are an unsolved problem.
• Most do not work very well.
Crash too often, too slow, awkward to use, etc.
• Usually they do not do everything they were designed to do.
• Do not adapt to changes very well.
New devices, processors, applications.
• There are no perfect models to emulate.
________________________________________
(No, UNIX is not it! Nor is Windows!) Unlike fields like electronics where there are such models (zero distortion, flat response), any real system has (a large number of) flaws.
________________________________________

Copyright © 2001, 2002, 2008 Barton P. Miller
Non-University of Wisconsin students and teachers are welcome to print these notes their personal use. Further reproduction requires permission of the author.