Good demo for showing benefits of parallelism

[Mikola Lysenko]

There seems to be a great deal of confusion between concurrency and 
parallelism.  Parallelism is a natural part of many problems, and it is 
relatively easy to exploit in order to enhance performance.  Parallel 
algorithms naturally scale to arbitrary numbers of processors, and are 
not particularly difficult to develop.

Concurrency on the other hand is very difficult.  When multiple 
processes must communicate, the programming complexity quickly spirals 
out of control resulting in unmanageable chaotic programs.  Locks, 
channels and STM are all useful concurrency primitives, but no single 
one can be considered a complete solution.  The difficulty of concurrent 
programming was recognized early on by programmers like Dijkstra who 
worked on the first operating systems.  To this day, it is still an 
unsolved problem and must be approached very carefully on a per-case basis.

In this light, GPU programming should not be considered concurrent 
programming, since it is impossible for threads on the GPU to 
communicate since all shader memory is read-only.  GPU programs are 
parallel however, and they are typically not very difficult to write 
(beyond some annoyances in the API/shader language).  Similarly futures 
do not help with concurrent programs, since they only improve the 
parallelism inherent within a program.

Shaders, futures and array operations are all helpful, since they 
provide convenient mechanisms for utilizing parallelism.  However they 
utterly fail to address the most difficult aspects of multi-threading, 
which means they are not a complete solution.