The Structure of the THE Multiprogramming System

What kind of paper is this?

• Describes a system.
• Describes use of semaphores in an OS (not the introduction of semaphore; this happened in an earlier Dutch paper circa 1963).
• First attempt at proving a system correct.
• Context: contemporary of OS/360

Some of my favorite quotes:

• an interrupt system to fall in love with is certainly an inspiring feature
• experience does by no means automatically lead to wisdom
• the intellectual level needed for system design is in general grossly underestimated
• Qualitatively speak- ing, I am afraid, we became more and more immodest as the work progressed.
• the resulting system is guaranteed to be flawless

Designed for the WL XS machine

• 32 K words (27 bits) of 2.5 microsecond memory.
• 512 K word drum (1024 words per track; 25 RPM) [this is the persistent memory -- this disk!].
• Indirect addressing.
• Large number of low capacity channels.

System Goal

• Decrease turn-around time for short jobs.
• Use Peripherals economically.
• Automatic control of backing store.
• OK to use the machine for jobs that aren't compute intensive.

Testing

• Provably correct.
• Exhaustively tested.
• "Guaranteed to be flawless."

Storage Allocation

• Write anywhere paging.
• Allocation becomes trivial.

Software Structure

• Implemented as a set of sequential processes (we take this for granted in many ways today).
  • Process abstracts out timing information.
• Example
  • One process per user program.
  • One process per peripheral device.
• Use semaphores to synchronize across processes, thereby guaranteeing that there are no misunderstood interactions. What is the problem with this strategy?
• Strict hierarchical structure
  • Level 0: Process scheduling.
    • real time clock to generate interrupt.
    • priorities to provide quick turn around.
    • abstraction: each process has its own machine.
  • Level 1: Segment controller.
    • essentially the pager.
    • does bookkeeping.
    • abstraction: programs refer to segments, independent of their location.
  • Level 2: Message interpreter.
    • manages keyboard.
    • keyboard and printing two separate devices.
    • abstraction: each process has its own console.
  • Level 3: I/O streams.
    • multiplexes peripherals among processes buffering input.
    • unbuffering output.
    • abstraction: peripherals are logical communication units.
  • Level 4: User programs.
  • Level 5: The operator.

Can any system be made hierarchical?

• Examples: memory management versus file system.
  • MM manages memory but must use the FS to move pages to/from disk.
  • FS uses MM for buffering.
• Eliminating such circularities will make system easier to build and maintain.

Tips for doing research

1. Never underestimate the time to debug!
2. Select advanced project (keep mundane work to a minimum).
3. Select a machine with a sane architecture.
4. Beware that experience does not equate to wisdom.