Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!ncar!midway!quads.uchicago.edu!rtp1 From: rtp1@quads.uchicago.edu (raymond thomas pierrehumbert) Newsgroups: comp.arch Subject: Re: RISC vs. CISC -- SPECmarks Message-ID: <1991Apr30.163153.18568@midway.uchicago.edu> Date: 30 Apr 91 16:31:53 GMT References: <3423@charon.cwi.nl> <11602@mentor.cc.purdue.edu> Sender: news@midway.uchicago.edu (NewsMistress) Organization: University of Chicago Lines: 39 > Comments on Cyber 205, page faults, etc. I suffered with a Cyber 205 for 6 or 7 years, and I know the machine from a scientific user's viewpoint. For just about all fluids codes, we found virtual memory to be essentially worthless. For out of core codes, we had to manage I/O by hand, and about all virtual memory did for us was to enable us to create blocks of virtual storage that we could move things in and out of "by hand" As for it's architecture, the Cyber is a good case of why architectural classifications aren't a good way to classify computers as "super" vs. otherwise. What made the Cyber a pain to program, and killed it vs. the Cray, was that (1) it had a long pipeline, hence a big hit for short vectors, which meant that codes had to be rewritten to use very long vectors, (2) it couldn't execute vector instructions unless data was contiguous in memory, so for non-unit strides, you had to do scatter-gather. The bottom line is that lots of codes never ran very efficiently on our machine, because it was too hard to recode. The IMSL general eigenvalue finder ran about half the speed on the Cyber 205 as it does on my IBM RS/6000 model 730 (using tuned code). The cache machines, like IBM, also have problems with non-unit strides, but I'm finding that using strip-mining, it is relatively easy to get around them, provided your algorithm has re-use of data. While we're on the subject of vector architectures, anybody remember the Texas Instruments ASC? It had its hardware problems, but it had the nice feature that it had vector instructions for multiply-dimensioned arrays, and the compiler (when it worked at all, which wasn't often) used them intelligently. This machine was ahead of its time, and went beyond what the real technology could support, but I think its designers deserve to get some credit. Our old TI ASC at Princeton was junked and went to Texas to be recycled, sharing a truck with some crates of N.J. lettuce. It's too bad the architecture was junked as well, as it had a lot going for it. .