Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site cornell.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!mhuxn!mhuxj!mhuxr!ulysses!allegra!bellcore!decvax!tektronix!uw-beaver!cornell!rej 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