Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!ihnp4!inuxc!pur-ee!uiucdcs!uxc.cso.uiuc.edu!uicsrd!turner From: turner@uicsrd.UUCP Newsgroups: comp.arch Subject: Re: Josephson Junction computers (was: Message-ID: <43700014@uicsrd> Date: Sat, 14-Mar-87 00:24:00 EST Article-I.D.: uicsrd.43700014 Posted: Sat Mar 14 00:24:00 1987 Date-Received: Sun, 15-Mar-87 23:39:59 EST References: <492@cpocd2.UUCP> Lines: 58 Nf-ID: #R:cpocd2.UUCP:492:uicsrd:43700014:000:3245 Nf-From: uicsrd.CSRD.UIUC.EDU!turner Mar 13 23:24:00 1987 /* Written 3:28 pm Mar 11, 1987 by howard@cpocd2.UUCP in uicsrd:comp.arch */ > There are still better reasons. JJs give you very fast gates, but are > hard to integrate (as noted). Now, where does delay come from .... Good points - > Communication is more important than gate speed, and integration is > the only way to get improved communication speeds because we already > know how to send signals at more than half the speed of light. This > limits the gains to be gotten from superconducting wires. The wires > must get shorter; the system must get smaller. That means putting > more circuitry on a chip. Very true, and quite a problem but not necessarily unsolvable. The gain problem is only one of many that JJ technology faces in integration. But: > Then look at the reliability/maintainability issue. To do repair work > on a JJ computer, you need to warm it from 4K to room temp. What does > that do to all your delicate wires and transistors? How many temp > cycles like that will it take to utterly destroy the chip? You could > try to change the temperature slowly, but then what happens to the > mean repair time? What does this have to do with high-temp superconductors though? There is said to be a possibility of room-temp superconductivity, just read this week's Science, or any one of several other sources. These new materials are oxides, not immediately applicable to LSI, but we must walk before we can run, no? No one claims that the problems will be solved this decade (or even this millenium?) > The conclusion is that superconductivity in general, and JJs in > particular, just won't be of much use in general-purpose computers. > We're far better off looking for good architectures to make use of the > fabrication technology we have. Try reading "The Connection Machine" > by W. Danny Hillis for a taste of what computers might look like in 10 > or 20 years, when the Von Neumann architecture (and bottleneck) is > fading from the scene. Here we have a potential gain of several > orders of magnitude, not just a piddling 50%. > > Howard A. Landman > ...!intelca!mipos3!cpocd2!howard At the end there you lost me; the serial bottleneck will NEVER fade away completely. Some things just have to be done in order. Admittedly parallel processing is a good thing, but haven't you ever heard of Amdahl's Law? Parafrased as: The amount of speedup due to paralleism in a program in inheirently limited to the reciprocal of the sequential fraction. That is if a program is 99% parallelizable then on an unlimited number of processors (even given perfect communication) the maximum possible speedup is only 100. To achieve parallelism of several orders of magnitude we will have to design algorithms that compute in parallel more than 99.99% of the time, a difficult feat - but what better challenge to the modern computer scientist. --------------------------------------------------------------------------- Steve Turner (on the Si prairie - UIUC CSRD) UUCP: {ihnp4,seismo,pur-ee,convex}!uiucdcs!uicsrd!turner ARPANET: turner%uicsrd@a.cs.uiuc.edu CSNET: turner%uicsrd@uiuc.csnet *-)) Mutants for BITNET: turner@uicsrd.csrd.uiuc.edu Nuclear Power (-%