Path: utzoo!attcan!sobmips!uunet!samsung!gem.mps.ohio-state.edu!tut.cis.ohio-state.edu!pt.cs.cmu.edu!MATHOM.GANDALF.CS.CMU.EDU!lindsay From: lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) Newsgroups: comp.arch Subject: Re: Surges Message-ID: <7000@pt.cs.cmu.edu> Date: 16 Nov 89 16:44:47 GMT References: <503@ctycal.UUCP> <15126@haddock.ima.isc.com> <28942@shemp.CS.UCLA.EDU> <31097@winchester.mips.COM> <28985@shemp.CS.UCLA.EDU> <29018@shemp.CS.UCLA.EDU> Organization: Carnegie-Mellon University, CS/RI Lines: 35 In article aglew@urbana.mcd.mot.com (Andy-Krazy-Glew) writes: >I'm a wide datapath proponent myself, but some of my contacts have >responded that so many signals simulataneously changing state at the >same time => huge instantaneous power demand. > >How much of a problem is this *really*? This is well-known as one of Seymour Cray's concerns. In the Cray-1, he was worried that standing waves could develop in the copper ground plane of a circuit board. His answer was to make a machine with power demand that was (I believe) completely independant of the data flowing through it. In the Cray-3, there is some reason to believe that the circuits will have power demand that is also independent of the clock, that is, absolutely constant rather than cyclic. 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. Another fun program is the one which puts many identical settings into a content-addressable memory such as the TLB. Sixty four registers all putting their hands up at once is enough to fratz a naively designed TLB. 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. -- Don D.C.Lindsay Carnegie Mellon Computer Science