Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!udel!nigel.ee.udel.edu!mccalpin From: mccalpin@perelandra.cms.udel.edu (John D. McCalpin) Newsgroups: comp.unix.aix Subject: Re: Can a large RS/6000 easily replace a Convex? Message-ID: Date: 3 Jun 91 17:45:01 GMT References: Sender: usenet@ee.udel.edu Organization: College of Marine Studies, U. Del. Lines: 45 Nntp-Posting-Host: perelandra.cms.udel.edu In-reply-to: johan@dutnak2.tudelft.nl's message of 3 Jun 91 14:31:34 GMT >>>>> On 3 Jun 91 14:31:34 GMT, johan@dutnak2.tudelft.nl (Johan Haas) said: Johan> We are considering to replace our current Convex C1 with a mix Johan> of RS/6000 systems instead of upgrading to a Convex C2. This is a good idea. Johan> I would like to hear if e.g. the model 550 is any good to serve Johan> as a machine for, say, 8 PhD. students (like myself). We do a Johan> lot of program development, but also do finite difference Johan> simulations that run easily 10 hours on our C1. You should not have any trouble with such a small number of users if you have adequate memory and swap space. Johan> I know that the MIPS/MFLOPS performance of the 550 is superior Johan> to that of the Convex, but there are rumours that the multiuser Johan> performance of RISC machines is very bad. Is this true? There is no reason to call the performance "very bad". The reviews that I can recall were disappointed with the multi-user performance because it was no better than other machines with (for example) half the MIPS rating of the IBM. Unless your 8 users each insist on running *lots* of active processes all the time and all at the same time, you should not have trouble. Johan> Another issue is the single precision Floating Point Johan> performance that --according to a lettr to Unix World-- can be Johan> worse than the double precision performance. We do most of our Johan> work in single precision. It is regularly observed that single-precision (32-bit) arithmetic runs between 10% slower and 10% faster than double precision (64-bit). The moral is that (unless you are out of memory) you can switch to 64-bit for "free", with resulting increases in accuracy and reliability. But even if you run in 32-bit precision, you should not expect to see a penalty of more than 10-15%, and you might see a slight speed improvement --- it depends on how well the compiler can overlap the precision conversions for your application with other work that has to be done anyway.... Try one of your own applications and see what happens.... -- John D. McCalpin mccalpin@perelandra.cms.udel.edu Assistant Professor mccalpin@brahms.udel.edu College of Marine Studies, U. Del. J.MCCALPIN/OMNET