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From: russell@minster.york.ac.uk
Newsgroups: comp.ai,comp.ai.neural-nets
Subject: Re: Parallelism, Real vs. Simulated: A Query
Message-ID: <624298345.14843@minster.york.ac.uk>
Date: 13 Oct 89 16:12:25 GMT
References: <17820@bellcore.bellcore.com>
Reply-To: russell@uk.ac.york.minster (russell)
Organization: Department of Computer Science, University of York, England
Lines: 43

In article <17820@bellcore.bellcore.com> srh@wind.UUCP (Stevan Harnad) writes:
>
>I have a simple question: What capabilities of PDP systems do and
>do not depend on the net's actually being implemented in parallel,
>rather than just being serially simulated? Is it only speed and
>capacity parameters, or something more?
>
>Stevan Harnad
>harnad@confidence.princeton.edu

I think this is an interesting point, but the simple answer is that there
is nothing more to be gained from a parallel implementation over a serial
simulation (except speed and possibly capacity - but even that depends on
the parallel architecture vs. the serial machine).

The reason:

The only thing that can happen in a parallel system that can't be mimiced
on a serial one is the occurance of simultaneous events - but is this a 
problem?  If we consider time at a sufficiently fine granularity, then we
can guarantee that any node/unit in the net will only have had one operation
performed on it - there may be many nodes that actually encounter a simultaneous
update or operation, but there will still have been only one per node.  If our
simulation on a sequential machine operates at this level of granularity, and
completes all the required updates to each affected node before advancing the
`clock' by one time increment, then the sequential machine will be functionally
identical to the parallel implementation.

The only problems occur when the simulation is run at insufficiently fine granularity,
or when limiting assumptions about the number of simultaneous events are broken.

Russell.

____________________________________________________________
 Russell Beale, Advanced Computer Architecture Group,
 Dept. of Computer Science, University of York, Heslington,
 YORK. YO1 5DD.  UK.               Tel: [044] (0904) 432762

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