Path: utzoo!utgpu!water!watmath!clyde!ima!think!ortiz
From: ortiz@think.COM (Luis F. Ortiz)
Newsgroups: comp.arch
Subject: Re: Connection Machine
Message-ID: <13470@think.UUCP>
Date: 11 Dec 87 14:55:44 GMT
References: <3516@husc6.harvard.edu>
Sender: usenet@think.UUCP
Reply-To: ortiz@thyestes.think.com.UUCP (Luis F. Ortiz)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 80

In article <3516@husc6.harvard.edu> reiter@harvard.UUCP (Ehud Reiter) writes:
>The Connection Machine is usually presented as 64,000 1-bit SIMD processors
>hooked up with a hypercube router.  However, from talking to a few people who
>have experience with it (the CM2, not the CM1), I get the impression that many
>applications basically ignore the 1-bit processors and the hypercube router
>(which is very slow - it takes 1 ms to send a message), and instead do all
>their computing with the 2000 Weitek FPU's, and do all their communicating
>with the "NEWS" system, which basically is a fast 2-D mesh interconnect.

First, I would like to correct something.  The CM-2 has no 2-D
mesh interconnect between processors like the CM-1 does.  It turns out
that the hypercube connections between processors are suffcient to
embed a N-D lattice in the M dimensional hypercube (where N < M).  


>That is, the alternative model of the Connection Machine which these
>applications use is that it is a 2-D mesh of 2000 fast FPU's, backed by
>512MB of memory and a very high bandwidth disk I/O system.  Like a
>modern-day version of the Illiac IV, I guess (but one that actually works,
>unlike the Illiac).  To me, anyways, this "Illiac" model seems much more
>useful (for getting real work done) than the "hypercube" model.



>My question is, how do people who are running real applications on the CM2
>use it?  Does anyone who actually has a CM care to comment?  I realize that
>there is one serious application, the news-wire retrieval program, which does
>use the 1-bit CPU's to do its communicating (although it doesn't use the
>hypercube router very much) - are there other (commercial) applications
>which use either the 1-bit CPU's or the hypercube router?
>

Just to mention a few of the projects that we have worked on:

	1) FFT's:  It turns out that you can directly use the
	hypercube communications structure of the CM to great
	advantage while still looking at the machine as a one
	dimensional sequence of processors.

	2) 3-D Air flow:  Here we map a 3-d lattice onto the hypercube
	and use each processor to simulate a vlume of air.

	3) QED:  Quantum electrodynamics can map nicely into a 4 torus
	with each processor keeping track of the field strength in
	eight directions.

	4) Cellular Automata:  This kind of a research application
	uses almost no (if any) floating point, and spends its time doing
	nothing but 1-bit operations interspersed with aribtrary
	dimensional lattice communications.
	


I guess my real point is that the machine has a special features (like
fast 1-bit arithmetic, fast floating point, and hypercube
connectivity, that are used differently by different applications.
The field of parallel processing is still young and, quite frankly,
there is still a lot left to learn.  When people try to use the
machine, oftentimes they one idea as to how the problem is to be
solved, but end up implementing it in a totally differnt manner. Often
you are pleasantly surprised by something you though was difficult,
turning out to be elegantly simple (like diffusion equation
solutions).

I think that viewing the CM-2 as being a 2-D mesh of floatinbg point
processors is too limited.


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Luis F. Ortiz
Thinking Machines Corporation

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