Xref: utzoo comp.sys.next:17892 comp.arch:22849
Newsgroups: comp.sys.next,comp.arch
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: parity is for farmers?
Message-ID: <1991May22.155818.20148@zoo.toronto.edu>
Date: Wed, 22 May 1991 15:58:18 GMT
References: <1991May21.232331.24888@cs.umn.edu>
Organization: U of Toronto Zoology

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? 

Apple does the same thing.

>Is memory so reliable today that
>parity doesn't give enough benefit to bother with?

It's marginal, and depends on circumstances.  Modern memory *is* pretty
reliable, and it's attractive to the bean-counters to pinch pennies by
removing parity circuitry.  Doing parity requires 12.5% more memory, adds
circuitry for checking and for testing the checker (not being able to test
it, e.g. the IBM PC, is a mistake), can introduce problematic delays in
fast machines, and can be a considerable headache when writing partial
words to memory in some situations.  On well-broken-in hardware, parity
errors are quite rare.  (Utzoo gets maybe one or two a year on 24MB of
relatively old memory.)

Of course, when a bit does go bad, you'd kind of like to know about it...

>Does only ECC give a
>strong enough guarantee - and that is too expensive...

ECC is more painful in all the above ways, and tends to be used only for
server-class machines where availability sells.  In practice, with current
error rates, unless the application is one where crashes are utterly
unacceptable, parity is amply sufficient.  It's nice to be able to repair
the error, but relatively unimportant:  most parity errors will not bring
the system down if intelligently managed, and most systems crash more
often than that for other reasons anyway.
-- 
And the bean-counter replied,           | Henry Spencer @ U of Toronto Zoology
"beans are more important".             |  henry@zoo.toronto.edu  utzoo!henry