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From: chuck@irene.mit.edu (CHUCK PARSONS 617-253-4157)
Newsgroups: comp.sys.next,comp.arch
Subject: Re: parity is for farmers?
Message-ID: <22MAY91115737@irene.mit.edu>
Date: 22 May 91 15:57:37 GMT
References: <1991May21.232331.24888@cs.umn.edu>
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In article <1991May21.232331.24888@cs.umn.edu>, scott@poincare.geom.umn.edu (Scott S. Bertilson) writes...
> 
>  Does anyone else get nervous about the fact that NeXT ships their machines
>with 8 megabytes of non-parity memory?  Is memory so reliable today that
>parity doesn't give enough benefit to bother with?  Does only ECC give a
>strong enough guarantee - and that is too expensive, so we should just
>go without?
>  I might be paranoid, but I don't ever remember buying memory for a
>"real" computer before that didn't have some sort of error checking
>included.  Please help me to find a good answer to this question -
>I've got to add memory to a number of NeXTStations and I'd sure prefer
>to use parity memory, but not if it is a waste of time.

  The parity memory costs you and extra bus wait state, on the NeXT
or so I'm told.

  If you are a bank, a hospital or SDI get ECC. If you are a university
research or programming enviorment then IMHO 

 Probability(serious program bug) > 10^4* Probabilty(non detected memory fault)

 Most memory problems will make themselves apparent, of course some won't.
but in many areas the majority of programs running have serious bugs.
The chance of an ocassional cosmic ray bit flip going undetected is real.
Lets say something like 1/per 5 years. At least on the parity systems
I've used thats about how often you see it. But how does that affect
your productivity vs buying an extra 1gigabyte disk drive with the
money you save?


  I find that properly designed memory systems are pretty reliable. The
biggest problem is cutting the timing specs too close. The NeXT calls
for 100ns ram. I'm sure that spec is well thought out. You should
be fine.

chuck@mitlns.mit.edu