phillips@convex.com (Steve Phillips) (11/19/89)
In article <150@csinc.UUCP>, rpeglar@csinc.UUCP (Rob Peglar x615) writes: > In article <7000@pt.cs.cmu.edu>, lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes: [deleted comments about Cray] > > CMOS isn't like that. It's asymmetric, and cares about transitions: > > 0=>1 takes more power than 1=>1. So, one can write worst-case > > programs, which generate on-chip noise (mass transitions on the wide > > datapath), or which generate board noise and heat (mass transitions > > on the address and data pins). I'm not sure what cache activity > > generally produces the most heat: it may depend on implementation. > > > (more deletions) > > > CMOS is wonderful, but the ECL/GaAs/BiCMOS folks talk an awful good > > fight about how it's a different world on the other side of 50 MHz. > > Sure is. The ETA-10 had this problem in spades. This is why the > ETA-10 air-cooled was limited to about a 12-13 ns clock (~80 MHz) > before fratzing. There was a 15ns model (the model R) under development > when CDC killed it. > > The liquid-nitrogen cooled behemoths could have gone just about as > fast as one dared. The successor machine to the ETA-10G (the I > model) was to be around 5 ns (200 MHz) in a breadbox-sized container > of liquid nitrogen. Folks were talking about a 500 MHz board, but > that was just hall talk. > > Note, the parts were ASIC CMOS. > > [ deleted Neil quote ] > >-- >Rob Peglar Control Systems, Inc. 2675 Patton Rd., St. Paul MN 55113 >...uunet!csinc!rpeglar 612-631-7800 The ETA10-E and ETA10-G machines were not LN2 cooled to prevent the gate arrays from melting. While it is true that the LN2 removed the heat generated, that was not the reason that LN2 cooling was chosen. They were LN2 cooled because the gate delays were reduced by a factor of 0.6 at those temperatures. This is due to the simple fact that transistors switch faster at -190 degrees celsius than they do at room temperatures. In addition, the minimum clock cycle was not limited by the increasing amounts of heat generated as the clock cycle is reduced. It was limited by the fact that the machine was designed to run at 7 ns in LN2 and, if the clock was pushed beyond this, then long paths became a problem. If we apply our 0.6 speed up factor in reverse ( 7ns / 0.6 ), we get a minimum "warm" clock cycle of ~11.7 ns. It should be noted that the 7 ns clock cycle was chosen based on the estimates of the speed of the process from our vendor. These estimates proved to be wildly optimistic. Near the end of the design phase, a new "improved" spec arrived and we suddenly found that our 7 ns machine would run no faster than 14 ns. Over a two year period the silicon process was improved and we eventually began getting arrays that performed up to the original spec. This was one of several reasons for the delayed introduction of the 7 ns ETA10. Actually, getting rid of the heat in the air cooled machines was the least of our problems. Lousy CMOS gate array output drive, poorly terminated transmission lines, impedance mismatches, process variations, and clock tuning were all more of a headache. These are the type of problems that the 'ECL/GaAs/BiCMOS folks' are talking about. Steve Phillips | "The New Force in Supercomputers" Convex Computer Corporation | "We Will Win the Supercomputer Race" phillips@convex.com | -former slogans of ETA
desnoyer@apple.com (Peter Desnoyers) (11/21/89)
In article <3217@convex.UUCP> phillips@convex.com (Steve Phillips) writes: > The ETA10-E and ETA10-G machines were not LN2 cooled to prevent the > gate arrays from melting. While it is true that the LN2 removed the > heat generated, that was not the reason that LN2 cooling was chosen. > They were LN2 cooled because the gate delays were reduced by a factor > of 0.6 at those temperatures. Actually, isn't LN2 supposed to be a pretty lousy thermal conductor? Just because it's cold doesn't mean it's a good coolant :-) Peter Desnoyers Apple ATG (408) 974-4469
lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) (11/21/89)
In article <5290@internal.Apple.COM> desnoyer@apple.com (Peter Desnoyers) writes: >Actually, isn't LN2 supposed to be a pretty lousy thermal conductor? Just >because it's cold doesn't mean it's a good coolant :-) You're probably thinking of liquid helium. The superconductivity researchers were thrilled to switch from helium to nitrogen. Also, note that liquids are three orders of magnitude denser than air. Air is even worse than helium (but cheap). -- Don D.C.Lindsay Carnegie Mellon Computer Science
schumach@convex.com (Richard A. Schumacher) (11/21/89)
The LN2 cooled the chips by boiling, not by conducting away the heat. The resulting vapor was then recondensed and re-used (at all user sites) or just thrown away (at the manufacturing facility). It turned out that at about $0.25/liter it was cheaper just to throw it away, rather than pay electricity costs, amortized capital cost, etc., for the closed system! (Most customers realized this and wanted an open system, but ETA dithered over it for two years or so...)
rpeglar@csinc.UUCP (Rob Peglar x615) (11/23/89)
In article <3266@convex.UUCP>, schumach@convex.com (Richard A. Schumacher) writes: > > The LN2 cooled the chips by boiling, not by conducting away the heat. > The resulting vapor was then recondensed and re-used (at all user sites) or > just thrown away (at the manufacturing facility). It turned out that at > about $0.25/liter it was cheaper just to throw it away, rather than pay > electricity costs, amortized capital cost, etc., for the closed system! > (Most customers realized this and wanted an open system, but ETA dithered > over it for two years or so...) True enough. There were sites in the US that were considering open-loop operation. In the US, LN2 is quite cheap. However, in Europe and Japan, not the case, so closed-loop was a necessity. As for dithering, ETA dithered over a lot of things, the least of which was open-loop cryogenics. Rob -- Rob Peglar Control Systems, Inc. 2675 Patton Rd., St. Paul MN 55113 ...uunet!csinc!rpeglar 612-631-7800 The posting above does not necessarily represent the policies of my employer.