Multiprocessors I
=================
Lecture 24, 4/22/2004

1) Went through slides:
    - mostly examples of multiprocessor machines of different sizes
      and organizations.
      . multi-chip multiprocessors
      . single-chip multiprocessors
        *** We're going to see lots of these in the future, now that
	    we can fit many of them on a die and ILP parallelism is
	    reaching its limits.

2) What is multiprocessing?
   - Diagram of SIMD machine (shared PC and instruction decode)
   - Diagram of MIMD machine (PC and decode for each processor)
   - Contrast with ILP parallelism (one PC, but each instruction only
      executed on one piece of data, unlike SIMD machine)

3) Examples of uses:
   - ATM transaction processing system
   - Web server
   - Graphics processors

4) Writing parallel programs can be hard...
   - static-page web serving -- easy
   - parallelizing the final project assignment (cache sim) -- hard

5) Granularity of parallelism:

   Granularity		Example
   -----------		-------
   bit			32-bit adder
   instruction (ILP)	superscalar processor or out-of-order processor
   procedure		...
   task			MSWord + Web browser running on two-processor machine

6) Communication models
   Asked question: how do two program communicate?
   Message passing:
     - e.g. network sockets
     - e.g. special API like MPI
   Shared memory:
     - Showed how page tables could implement this on operating system,
       for allowing two processes to share memory
     - Briefly mentioned issues with cache consistency