Re: Tasks in Gnat3.05 for Dos?

[John Harbaugh <johnh@bunsen.ds.boeing.com>]

Robert A Duff wrote:
> 
> In article <31F3E4E7.13E4@bunsen.ds.boeing.com>,
> John Harbaugh  <johnh@bunsen.ds.boeing.com> wrote:
> >In their excellent book "Concurrency In Ada" Burns and Welling make a useful
> >distinction between parallelism and concurrency:
> >
> >"Two processes are said to be executing in parallel if at any instant they
> >are both executing...  By comparison, two processes are said to be concurrent
> >if they have the potential for executing in parallel."
> 
> Can you write a test program that can tell the difference?
> 
> - Bob

The distinction is useful but not rigorous.  I do not claim to speak for the 
authors, but my understanding is that they want the reader to think of 
program structure as independent of hardware architecture.  A correct 
concurrent design will work on one or many processors; a design which assumes 
a multi-processor architecture may not execute correctly on a 
single-processor machine (at least not without some kludge like time 
slicing).  In a recent post, Dr. Feldman related his experience with 
re-hosting a transportations simulation.  The portability was, I suspect, the 
result of understanding the inherent concurrency of the problem.

In other words, concurrency is an attribute of the problem space.  
Parallelism is an attribute of the solution space.

	- John Harbaugh
	The Boeing Company