howard@cpocd2.UUCP (03/12/87)
This discussion should move into comp.arch or comp.lsi or ? as it has little to do with space. In article <7718@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >> Then there are Josephson junctions... >> This wasn't possible before because the transistors would dissipate too >> much heat to cool to liquid helium temperatures. Room temperature >> Josephson junctions would blow computing wide open. > >Well, maybe not. The cooling problem wasn't the only reason why JJs haven't >swept the field. They are also fundamentally low-gain devices, which makes >it very hard to build working JJ LSI -- the precise control of characteristics >needed to make low-gain devices work well is nearly impossible in LSI. IBM >concluded that the problem wasn't fixable. There are still better reasons. JJs give you very fast gates, but are hard to integrate (as noted). Now, where does delay come from in large-scale high- performance computers (the kind JJs would be used in)? Gene Amdahl believed (~5 years ago, answering a question at a Berkeley talk) that it was: 1/3 gate delay 1/3 on-chip interconnect 1/3 system interconnect Assume this is approximately right. Now suppose you have a magical technology with *ZERO* gate delay. How much does your system speed up if all other things are held constant? (If you get an answer other than 50% faster, check your arithmetic.) This limits the gains to be gotten from JJs. And if they have less dense integration levels, the system might actually run *slower*. 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. 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? 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%. The only factor in recent discussions which might argue in the other direction is that the brittleness of the reported liquid-N2-temp superconductors is not really a problem for ICs, since the wires would not be drawn but rather etched from a sheet. This suggests that it will be *possible* to make ICs with high-temp superconducting elements, but not that it is worth doing so. -- Howard A. Landman ...!intelca!mipos3!cpocd2!howard