CS452 - Real-Time Programming - Fall 2008

Lecture 27 - Occam 2

Questions & Comments

1. Coyotos

Hardware

Fourth Generation

1. Multi-(multi-core CPUs) called servers
   • communicate and synchronize using shared memory
     • using techniques from CS343
   • multi-core, cache, and shared memory don't play well together
   • performance limited by threading
     • explicit support for threading, e.g. register windows (Sun) increases performance hugely
     • communication still costly
2. Multi-core microprocessors
   • performance strongly hampered by lack of multi-threading
   • multi-core, cache, and shared memory don't play well together
3. Stream processors
   • good for SIMD
   • treat as co-processors

The Transputer: a Second Generation Offsoot

Use many co-operating, medium capability microCPUs to do a big job.

• An idea whose time has now come.
• Google

Problem is communication

• Big granularity
  • minimizes communication
  • maximizes replicated data
  • The Google approach: a problem nobody thought about.
• Small granularity
  • minimizes replicated data
  • maximizes communication
  • Your system, like threading solutions, relies on shared memory for communication
    • How would you handle caching?

Comminication requires either

• a common bus, star topology
  • system bus,
  • LAN
  • internet
• passing messages along
  • emphasizes the switches

The transputer was an early, now vanished, example of the latter

• CPU, memory, switch on one chip
• chips connected in an array
• presumably a run-time system decides where tasks will go in order to
  • maximize CPU throughput
  • minimize communication overhead

Occam 2

Basic idea

1. processes (tasks)
   • may be named, take arguments and return values
2. channels
   • need a protocol (for type safety)
   • must be named
3. time

Combining processes

1. sequential
2. conditional
   • if/then
   • selection by case
3. looping
   • without test/break
   • with test/break
4. parallel
   • initiated when the keyword PAR occurs.
5. alternation
   • guarded alternatives
   • if more than one guard is true then select at random
     • can be prioritized

Channels

1. input

   channel ? variable

   • variable type must match protocol
   • real-world hardware acknowledged by the anarchic protocol
   • blocks when channel is empty
2. output

   channel ! value // the value of a variable or the result of an procedure

   • variable type must match protocol
   • blocks until an input process accepts the data
3. input & output provide synchronization
   • similar to sockets except for output blocking

Time

• timer returns time as channel input

  clock ? now

• AFTER can be used to combine times

  IF now AFTER yesterday THEN

• AFTER can make timer input blocking

  clock ? AFTER tomorrow

The Result

You can write a type-safe server, BUT

• all possible clients must be known at compile time.

Program structure is more static than is allowed in your system.

• This might be a good thing.

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