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From: pratt@cs.stanford.edu
Newsgroups: comp.parallel
Subject: Re: parallel programming model
Message-ID: <12316@hubcap.clemson.edu>
Date: 17 Dec 90 17:53:38 GMT
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In-Reply-To: Your message of Mon, 17 Dec 90 10:37:04 -0500.
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	From: caliban!ig@uunet.UU.NET (Iain Bason)
	I don't see the point of this argument.  The original poster was...
	...
	In fact, Von Neumann machines and parallel processors
	are the same at some level (e.g., they are both digital computers).

I don't see the point of this argument in this context.  The effect of
levels on complexity is conjunctive: if all levels save one are
distributed and that one is shared memory, and the shared memory turns
out to be a bottleneck, then it kills your performance no matter how
fast the other levels are.

At the level at which Von Neumann machines and parallel computers are
the same, both are distributed.  The discussion was whether a
particular level was distributed or centralized (i.e. von Neumann).
Assuming this is the only level that hasn't yet committed to
distributed, that's the only level that pertains to the discussion.

Protocol implementations that mistake protocol levels for an
implementation guideline instead of merely a way of structuring the
semantics take a big performance hit due to this conjunctiveness of
per-level computational complexity: every level slows you down a little
bit more.  If *any* level of Internet were implemented by looking
things up over at NIC per packet it would slow you down a *lot* more.

However it would not impact the main purpose of networking, which is
not performance but rather simply to spare people the inconvenience,
cost, and unreliability of moving data around on magnetic tapes and
other portable media.

Great performance is great, but not as great as convenience, economy,
reliability, or sex.

	Vaughan Pratt