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From: rej@cornell.UUCP (Ralph Johnson)
Newsgroups: net.arch
Subject: Re: Re: Caltech's Cosmic Cube
Message-ID: <198@cornell.UUCP>
Date: Wed, 20-Feb-85 09:28:43 EST
Article-I.D.: cornell.198
Posted: Wed Feb 20 09:28:43 1985
Date-Received: Mon, 4-Mar-85 04:46:02 EST
References: <333@oakhill.UUCP> <21294@lanl.ARPA> <7268@watrose.UUCP> <166@cmu-cs-wb1.ARPA>
Reply-To: rej@gvax.UUCP (Ralph Johnson)
Organization: Cornell Univ. CS Dept.
Lines: 33
Summary: 

In article <166@cmu-cs-wb1.ARPA> avie@cmu-cs-wb1.ARPA (Avadis Tevanian) writes:
>
>What puzzles me is why use point to point channels between processors (and
>do routing if a connection does not exist)?  Wouldn't it be much simpler to
>use a dedicated ethernet?  A 10mb ethernet should easily provide the
>necessary bandwidth for 64 or more processors.  Since the ethernet would be
>dedicated, minimal protocols could be used, thus keeping the costs of
>managing the ethernet down.  If 10mb is not enough bandwidth (which I highly
>doubt), it shouldn't be too tough to increase the bandwidth considering that
>the wire will be dedicated and can be very short (it won't need to run all
>around a building).
>

I assume that most of the communication between processors consists of very
short packets, i.e., a single floating point number.  Ethernet is very
inefficient when it is handling short packets, since it has a lot of overhead
per packet.  In actual practice, the 10mb bandwith is approximated only when
packets are very long (perhaps 10KB, I forget).

Also, I bet most of the algorithms for the Cosmic Cube are fairly
synchronous, so all the processors would want to be broadcasting at the
same time.  Ethernet assumes that the net is not very loaded.  A 10%
loaded Ethernet is very rare.

Also, Ethernet is not that cheap.  Each connection runs a few hundred
dollars.  A straightforward serial connection would only be a few dollars,
and a parallel port is even faster and almost as cheap (wiring costs, you
know).  As long as the interconnection pattern is regular and there are
not too many processors (too many is more than the number that fit in one
or two cabinets) the Cosmic Cube interconnection scheme should be cheap
and simple.

Ralph Johnson