An Experimental Time-Sharing System

What kind of paper is this?

• Describes a system
• Motivates need for a new type of system
• Presents a new scheduling algorithm

Paper places itself in context.

• Two interpretations of time-sharing
  1. multiprogramming -- goal is to use hardware more effectively.
  2. "real" time-sharing -- run different jobs for multiple people
• Focus on man-computer interaction (is this a systems paper or an HCI paper?)
• A lovely quote, as true today it was then (if not more so): "In part, this effect has been due to the fact that as elementary problems become mastered on the computer, more complex problems immediately become of interest. As a result, larger and more complicated programs are written to take advantage of larger and faster computers."

Problems in "today's" systems

• System complexity is increasing
• Systems are becoming larger.
• Larger systems implies more bugs.
• Debugging is time-consuming.
• Turn-around on debugging is too long.

This idea of time-sharing can be thought of as one element in a line of programmer-productivity tools!

Problems in hardware:

• Memory protection.
• Dynamic relocation.
• I/O device protection.
• More permanent storage.
• Timer interrupts.
• Larger main memories.

System software requirements:

• Accounting.
• Swap out processes waiting on I/O.
• Programming tools.
• Programmer flexibility.

Usage problems:

• Handle excessively large programs.
• Accurate accounting reporting.
• Identify running process; handle errors gracefully.

709 system:

• Not only developed, but ported once!
• Hardware modifications
  • attach typewriters (3)
  • add accounting/interrupt clock
  • supervisor mode (i.e. protection, relocation, trap on I/O)
• 3 interactive users, 1 batch user
• Timesharing supervisor takes up low 5000 words of memory
• Each user has two tapes (private, dump)

System subtleties (or why this is an important paper):

• Commands are queued
• Overlap CPU and I/O (read-ahead) .
• Buffer flow control

What does the time-sharing supervisor subsume in today's systems?

• shell
• text editor

Research areas identified

• Editors
• User interfaces
• Symbolic debuggers (tracing, dump, post-mortem)
• Shared programs (i.e. groupware)
• File systems
• Integration of background / foreground users

Derivation of multi-level scheduling

• programs enter system at level based on ratio of program size to the number of bytes that can be moved to/from secondary storage in a quantum (let L = log ₂ ratio)
• supervisor runs programs with lowest ratio.
• run program for 2 L quanta.
• if program runs to completion but is not done (not waiting user input), elevate the program to the L + 1^st queue.
• what scheduling discipline is this.

So What?

• bounded response time.
• bounded system efficiency.
• bounded efficiency relative to run-to-completion.
• describe how efficient I/O can be utilized to improve algorithm.

Opinion?

• Great paper.
• Important ideas.
• Analytical evidence.
• Well -integrated paper (i.e. hardware + software + tools)