Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!ut-sally!husc6!bloom-beacon!think!ames!lll-tis!ohlone!nelson From: nelson@ohlone.UUCP (Bron Nelson) Newsgroups: comp.arch Subject: Re: What with these Vector's anyways? Message-ID: <343@ohlone.UUCP> Date: Fri, 24-Jul-87 02:47:52 EDT Article-I.D.: ohlone.343 Posted: Fri Jul 24 02:47:52 1987 Date-Received: Sat, 25-Jul-87 13:55:35 EDT References: <218@astra.necisa.oz> <142700010@tiger.UUCP>, <363@astroatc.UUCP> <8344@utzoo.UUCP> Organization: Cray Research Inc., Livermore, CA Lines: 33 Summary: Vector speed up on a Cray XMP In article <8344@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes: > > BTW: The Cray-1 & XMP are also very-good Scalar machines! > > It should be noted that this is an important reason why they sell so well > (the Cray production runs are an order of magnitude larger than those of a > lot of other supercomputer projects in the last two decades). If a program > is 90% vectorizable, than magically making the vector part of it *infinitely* > fast will only speed it up by a factor of ten. A good rule-of-thumb is that running a problem vectorized is about 10 times faster that doing the same problem scalar on an XMP (single processor). Thus, if some old problem you have is 90% vectorizable, it will spend half of its execution time doing the part that is (still) scalar. So scalar performance is very important. Now, it is very true that only a small fraction of the *number* of jobs run in a typical day have any significant vectorizeable parts even if you look at the sites that buy our machines: compilers and editors (and your news reading program :-)) do not vectorize very well. However the kind of jobs that take the majority of the *cpu* time DO tend to have significant vectorizable parts. The big cpu/memory hogs tend to cycle through huge volumes of data, frequently doing pretty much the same thing to each datum (this is of course a vast over-simplification, but you get the idea). Some codes (for example, oil company's analysis of seismic data at a potential drilling site) can be almost embarrassingly vectorizable - 99+%. And of course people designing large new codes tend to consider how to make the solution vectorizable/parallelizable. But it IS a big selling point to be able to say "slap that crufty old monster on this baby and she'll run like lightning from the word go." ----------------------- Bron Nelson {ihnp4, lll-lcc}!ohlone!nelson Not the opinions of Cray Research