Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!swrinde!ucsd!ucbvax!agate!shelby!eos!amelia!wilbur.nas.nasa.gov!eugene From: eugene@wilbur.nas.nasa.gov (Eugene N. Miya) Newsgroups: comp.arch Subject: Re: Computer time measurements (Was Re: 64 bits for times....) Message-ID: <7945@amelia.nas.nasa.gov> Date: 23 Aug 90 03:12:42 GMT References: <26012@bellcore.bellcore.com> <11187@alice.UUCP> <1990Aug22.044826.18572@portia.Stanford.EDU> Sender: news@amelia.nas.nasa.gov Reply-To: eugene@wilbur.nas.nasa.gov (Eugene N. Miya) Organization: NASA Ames Research Center, Moffett Field, CA Lines: 44 >| I am unclear as to the self measuring precision required of a computer >| system. For example, I don't see the relevence of marking file time >| stamps to the 1ns increment... I did not see the original post. News has been flakey and I've not been reading all of c.a. See a paper I wrote for the Usenix 1988 Supercomputing Workshop. To quote William Hewlett gave at an MIT graduation: "Sometimes noise is significant." I did not go to MIT, but I read this neat quote. Basically, there are in my opinion 5 classes of measurement environments: A environments: have the best facilities: cycle time clocks, non-intrusive performance measuring hardware, software to use all this and other software tools. Examples: Cray Y-MP, Cray-X-MP, some machines which never saw the light of day. Other machines I've under non-disclosure. A machines can migrate to B environments as better "A" machines come out. I can tell you half a dozen Cray HPM limitations (they know them). B environments: Cycle time clock and some software tools. Cray-2, Cray-1, Convex, some IBM hardware. AT least good compilers, etc. C environments: average. 50/60Hz clock average software Not great: IBM PC, early workstations, VAX-11/780. Lots of variance in cycle. D environments: machines without clocks, poorer software, etc. E environments: components, not complete systems, but something to be measured. VLSI was a perfection of linear measurement: micron space realm. Unless similar improvements take place in time, you don't get faster machines. Fortunately people at places like the NBS realize this and they make things like atomic clocks. If you are not interested in faster machines, just ignore this posting. Andy: it was close, real close ;^). Not quite those words, but close. --e. nobuo miya, NASA Ames Research Center, eugene@orville.nas.nasa.gov {uunet,mailrus,other gateways}!ames!eugene