Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!wuarchive!uunet!crdgw1!crdos1!davidsen From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr) Newsgroups: comp.arch Subject: Re: Workstation Data Integrity Message-ID: <2483@crdos1.crd.ge.COM> Date: 5 Sep 90 14:22:55 GMT References: <6797.26d6edce@vax1.tcd.ie> <56qmo1w162w@zl2tnm.gp.govt.nz> <19875@crg5.UUCP> <19208@dime.cs.umass.edu> <2201@lectroid.sw.stratus.com> <68362@sgi.sgi.com> <1990Sep4.163619.24726@zoo.toronto.edu> <68505@sgi.sgi.com> Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen) Organization: GE Corp R&D Center, Schenectady NY Lines: 36 In article <68505@sgi.sgi.com> karsh@trifolium.sgi.com (Bruce Karsh) writes: | Given that a memory system is otherwise properly designed and tested | and uses modern 4Mbit DRAM memory chips, is there any evidence that | memory parity makes a measurable difference in the silent wrong answer | rate? If the error rate for 1 bit error is 1 in N, then the rate for a 2 bit error is 1 in N^2. With N in the order of some millions (or billions), you make the chance of silent error millions of time less likely. EDAC makes it possible to *correct* errors, but the rate of 2 bit errors which would be caught is small to the point of really being insignificant. Below the rate of errors caused by noise on the bus, I believe. I would expect EDAC on a 64 bit machine, however, since it is probably cheaper. Note that for byte parity it takes 8 bits of parity memory, but for EDAC you can use 1+log2(N) or 7 bits, and get 2 bit error detection, 1 bit error correction, and use less memory. This assumes that you can (a) build fast EDAC as cheaply as parity, and (b) that you use ALL 64 bit data fetches. You can run a 71 bit data bus and put the EDAC in the bus masters (CPU and I/O controllers) which access the bus. You can still use parity on I/O devices without controllers, such as serial ports, if you have any such devices. This is a bus design issue and doesn't effect the theory at all, just the cost/simplicity ratio. Glossary: EDAC - error detection and correction BMD - bus master devices TLA - three letter acronym -- bill davidsen (davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen) VMS is a text-only adventure game. If you win you can use unix.