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From: brooks@lll-crg.ARpA (Eugene D. Brooks III)
Newsgroups: net.arch
Subject: Re: Help on Thinking Mach. Inc.
Message-ID: <1533@lll-crg.ARpA>
Date: Wed, 21-May-86 00:22:56 EDT
Article-I.D.: lll-crg.1533
Posted: Wed May 21 00:22:56 1986
Date-Received: Sat, 24-May-86 01:50:40 EDT
References: <655@ur-helheim.UUCP>
Reply-To: brooks@lll-crg.UUCP (Eugene D. Brooks III)
Organization: Lawrence Livermore Labs, CRG Group
Lines: 64

In article <655@ur-helheim.UUCP> dave@ur-helheim.UUCP (Raver Dave) writes:
>Can anyone present a version of what exactly Thinking Machines
>computer is?  Articles from WSJ etc have been vague at best.

Lets try prodding them into discussion:

I will try to provide a meager description, collected from a paper about the
Connection Machine I recently read.  There may be inaccuracies, due to
lack of information in the paper, but I think that my description in general
is reasonably close.

The machine is a SIMD computer in which each cpu is bit serial and communicates
with others by passing messages.  Every cpu executes the same "nanoinstruction"
which is farmed out by a single instruction decoder than takes
"macroinstructions" from a host.  Each cpu in the CM executes the same
instruction, with at least the one proviso (there may be more) that it may sit
the instruction out based on the value of an internal flag.  I think that it is
reasonable to consider the DAP a close ancestor of the Connection Machine.  The
Thinking Machines people may not agree, and may strongly disagree, and I do not
speak for them.  In the sprit of stirring up trouble I will compare the two
machines.

Like the DAP, the CM is a SIMD message passing machine where each cpu is bit
serial, making sqrt as fast as floating add, and the collective memory of
the CM "cpus" is visible to a "host" as memory.  Each bit serial cpu in the CM,
like the DAP, does not share memory with the other cpus.  They communicate by
passing messages.  The basic ammount of memory available on each cpu is a few
K bits or so.  Roughly the same ammount of memory, per cpu, was available on
the DAP.  The number of cpus in the CM, 2^16, is quite a bit larger than that
of the DAP.  The C.M. is implemented with much more recent technology and one
expects N to increase due to VLSI advances.  Due to the VLSI implementation
each cpu has quite a bit more functionality than that of the DAP.

There are two reasons why the C.M. is more interesting than the DAP.  The
software for the C.M. is of course much more advanced.  The significant
hardware difference is the communication topology used in the C.M.  Instead of
the 2 D mesh without message routing hardware, which was used in the DAP,
the CM uses the direct 2-ary n-cube topology used in the Caltech Hypercube
machine.  The C.M. also has hardware support for message routing from any cpu
to any other which is critical to applications which do not "fit" on the
network.  Because of the Hypercube network topology, which has many interesting
networks imbedded within it, the C.M. will be able to run many diverse
algorithms efficiently, using nearest "hypercube" neighbor communication.
Because of its routing hardware, algorithms that do not map well onto the
Hypercube, and might otherwise suffer in performance, will also do well on
the machine.

The bottom line for performance on the "hard" parallel algorithms is high
peformance communcation between processors, which I perfer to have in the
form of SHARED MEMORY.


The fundamental shortcoming of the CM is that it is a SIMD machine.  Each
cpu executes the same instruction, or sits out the instruction cycle,
which is a very restrictive programming model.  The DAP certainly did some
things well, and given the fact that the CM is much more flexible and has
message routing hardware for long distance message passing, it should do well
on a much larger class of parallel applications.  It remains to be seen whether
the SIMD message passing system is sufficiently general to become a general
purpose parallel computer.  It also remains to be seen if the CM will be a
success in the commercial market, the DAP was not.


							Eugene