Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!bellcore!texbell!wuarchive!cs.utexas.edu!uunet!mcsun!cernvax!hjm From: hjm@cernvax.UUCP (Hubert Matthews) Newsgroups: comp.arch Subject: Re: SRAM vs. DRAM, 33MHz 386 UNIX-PC Message-ID: <1082@cernvax.UUCP> Date: 8 Sep 89 09:42:20 GMT References: <21936@cup.portal.com> Reply-To: hjm@cernvax.UUCP (Hubert Matthews) Organization: CERN European Laboratory for Particle Physics, CH-1211 Geneva, Switzerland Lines: 55 In article <21936@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes a lot of stuff about PCs and UNIX and RAMs. To save on bandwidth, I shall not not quote great tracts, but I hope the context is obvious. Why do you want zero-wait state RAM for all of your memory? A data cache is there to provide zero-wait states for most accesses for most sorts of program. If the data cache hit rate is 90%, then changing all of the memory to zero-wait state will give you 0.9*1+0.1*3=1.2 cycles per access (assuming 3 cycle main memory), on average, which is 0.2 wait-states assuming a one cycle cache. Are you prepared to pay 5 times as much for your machine for a 20% increase in performance on that type of program? You may be running software that has a very low cache hit rate if you are doing CAD work or scientific calculations. Take this little loop for example: SUM = 0.0 DO 10 I = 1, 1000000 SUM = SUM + VEC(I) 10 CONTINUE A data cache is *no use at all* for this problem. You will get a cache miss on every data access. Similarly, copying data from one bit of memory to another will be limited by the raw memory speed. For these sort of programs, a fast main memory is essential. The factor of 5 in cost starts to look not too bad if you get about a factor of 5 increase in performance. Why do Crays have fast memories? Because this is exactly the type of problems they are designed for. I/O is another area where raw memory speed is important as a data cache cannot help at all. The AT bus is indeed slow, but so is the memory attached to it. If you speed up the bus, you need to speed up the memory to go with it or your fast disk will wait for memory. In essence, the PC architecture was a reasonably well balanced system when the 8088 and the 8086 were used as CPUs, but most top-end machines are now hopelessly imbalanced with a 386 inside them. If you really want the performance from a 386, you are going to have to pay about 5 times as much to get the memory and the I/O to feed it at a decent rate. There ain't no such thing as a free lunch. >Please POST your comments. OK, you asked for war, you got it... -- Hubert Matthews ...helping make the world a quote-free zone... hjm@cernvax.cern.ch hjm@vxomeg.decnet.cern.ch ...!mcvax!cernvax!hjm