Date: Tue, 28 Dec 1999 21:57:56 -0700 From: "Kenneth D. Merry" <ken@kdm.org> To: Michael VanLoon <MichaelV@EDIFECS.COM> Cc: Mike Smith <msmith@FreeBSD.ORG>, Lance Costanzo <lance@costanzo.net>, freebsd-hardware@FreeBSD.ORG Subject: Re: ECC RAM useless with FreeBSD? Message-ID: <19991228215756.A94267@panzer.kdm.org> In-Reply-To: <8070C3A4E99ED211A63200105A19B99B317471@mail.edifecs.com>; from MichaelV@EDIFECS.COM on Tue, Dec 28, 1999 at 08:54:46PM -0800 References: <8070C3A4E99ED211A63200105A19B99B317471@mail.edifecs.com>
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On Tue, Dec 28, 1999 at 20:54:46 -0800, Michael VanLoon wrote: > From: Kenneth D. Merry [mailto:ken@kdm.org] > Sent: Tuesday, December 28, 1999 8:36 PM > > >FWIW, I generally run with parity detection turned on, but not ECC, since > >I've heard (i.e. I haven't looked in any Intel datasheets to verify this) > >that there may be a performance penalty for running with ECC turned on. > > > >You could probably verify the performance penalty by doing a dd test for > >memory bandwidth with ECC enabled and simple parity checking enabled. > >(e.g. "dd if=/dev/zero of=/dev/null bs=1m count=1024") > > There is a performance penalty, but it's very slight, especially if you have > decent cache (any modern processor (PII, PIII, Athlon) or a decent Super-7 > motherboard with a K6). > > Parity only detects one-bit errors. ECC can detect two-bit errors. You're > doing yourself a disservice by buying that more expensive memory, then not > really using it, especially on a server (where reliability is much more > important than a slight performance increase). I seriously doubt you could > determine the performance difference between having it on or off, except > with some sort of very specific benchmark. I've never had a memory problem (and I've had a number of memory problems) that wasn't detected with simple parity. Maybe I'm just lucky. If you have a slightly less modern processor (like a Pentium Pro), I think the performance loss can be around 10% or so. Unfortunately I'm not in a position at the moment to do the dd test above, so I can't say for sure, only what I remember. > And, does that hardly discernable performance loss make up for the time you > lose when your machine crashes, or you have to track down some malfunction > that is simply a flipped bit? I suppose I've never had a 2-bit error. Another way to look at it is that I'd rather be notified of memory problems, so that I can then turn on ECC to work around them, than have ECC silently work around the problem. If I had faster machines, and if we had some method of notifying the user when there are bad bits that get ECC corrected, I probably would run with ECC turned on. As it stands, though, you won't know about a 1-bit memory problem if you turn ECC on. Ken -- Kenneth Merry ken@kdm.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
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