Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!cwjcc!tut.cis.ohio-state.edu!rutgers!elbereth.rutgers.edu!hardees.rutgers.edu!patterso From: patterso@hardees.rutgers.edu (Ross Patterson) Newsgroups: comp.sys.next Subject: PaRITY (was NeXT Memory - No Error Checking or Parity !) Summary: No-parity is for data corruption. Keywords: Memory,errors,parity Message-ID: Date: 2 Nov 88 15:35:53 GMT References: <549@gt-eedsp.UUCP> <7493@well.UUCP> Organization: Rutgers Univ., CCIS Lines: 53 In article <7493@well.UUCP>, ejf@well.UUCP (Erik James Freed) writes: > I think it was Seymour Cray who was quoted as saying > "Parity is for farmers" A strange attitude, given that his machines spew out data at a rate far beyond the abilities of mere mortals to validate. Ever watch the output of a dynamic molecular modeling run? Dumping the contents of a multi-megabyte array every thousand or so iterations, over a 100,000+ iteration run produces more than a few numbers. Does Cray want to be the one to go through them one by one and check that they didn't get corrupted by a passing subatomic particle (or just a weak DRAM chip)? > I would tend to support NeXT's decision. Parity is supposed to allow > you to pinpoint where errors reside, but the software is rarely written > so that information is easily available. Parity also allows your hardware to correct the error, and inform you of it later. You *CAN* have your cake and eat it too. Systems I've worked with before have had SC/DD (Single-bit Correct, Double-bit Detect) hardware that informs the error logger of exactly which chip on which memory board got nailed. The SC hardware lets you get your data out, and in theory points out the problem before the chip deteriorates into causing a DD (which doesn't get the data out). I understand that IBM's 3090 series has Double-bit Correct/Triple-bit Detect logic. Neato. > In general if your system memory > is flakey, you will soon realize that something is up and then you can > run memory tests to isolate the particular simm module. (I assume that a > good memory checking diagnostic will be available at a standalone level > for the NeXT) A useable (thorough) memory test takes a lot of time. It is > not something that you want to run every boot up. That's why continual parity checking is so nice. You don't have to run a thorough memory test at every startup, only when you've logged an unacceptable number of errors. I agree that a real good beating on the NeXT's standard 8MB would take much longer than the average user would want to wait. But do you think that IBM's 3090 customers run one over all 1024MB of real memory at every startup (yes, the largest 3090 can have over a gigabyte of RAM)? > And parity memory in > my experience just is not really that useful. (at least to justify the PC > real estate) Perhaps not. But that's a decision for the person using the machine, not the manufacturer. Joe Owner, running the accounts receivable for his Subchapter S corporation, Small Business Inc., no doubt feels that he's entitled to accurate data. If Itty Bitty Machines feels that the machine they designed for game use doesn't need parity, that's probably all the use it will get. The need for reliable data has absolutely no relation to the size of the machine. And just as a sideline, I doubt JoBS would appreciate your calling the NeXT a PC anymore than Bill Joy would like you to call a Sun 3/60 a PC. Ross Patterson Rutgers University, CCIS