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From: danw@hob8.prime.com (Dan Westerberg)
Newsgroups: comp.sys.next,comp.arch
Subject: Re: parity is for farmers?
Message-ID: <1991May22.093355@hob8.prime.com>
Date: 22 May 91 13:33:55 GMT
References: <1991May21.232331.24888@cs.umn.edu>
Reply-To: danw@hob8.prime.com (Dan Westerberg)
Organization: Prime Computer
Lines: 34
Nntp-Posting-Host: 129.122.50.108

Parity is by no means a dead issue with memory.  We recently installed a 
number of SPARCstation2s here and ran into a memory problem.  One of our 
machines was max-ed out on the motherboard with 64MB of memory and we began
to take 'transient' parity errors with alarming frequency.  Sun maintained that
this wasn't a known problem of any type and we ended up tracing the problem
down to the 3rd party 4MB simms we were using.  The Sparc2 *requires* 80ns
ram parts, the 3rd party simms were spec'd to 80ns but it turned out that the
particular DRAMs used could not maintain 80ns access times with a fully loaded
memory bus.  Our vendor replaced all of these 4MB simms and we haven't had a 
problem since.

Without parity checking, I have no idea how this problem could have been tracked
down.  I do believe, however, that parity is no longer useful for intra-chip
busses.  Parity on busses was useful when those busses spanned long lengths of
trace and/or crossed a backplane going through several connectors.  But, on 
today's motherboards where we have similar busses traveling a few inches
at most, I think the usefullness of parity wanes.

ECC is the only way in which to provide reliable error protection/correction
in today's large memory architectures.  Also, in order to maintain extremely
high *availability* of a machine (i.e. # of hours down vs. # of hours running),
ECC is the only way to go.

Dan

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~  "These walls that still surround me      ~                                 ~
~   Still contain the same old me           ~     Dan Westerberg              ~
~   Just one more who's searching for       ~        danw@toucan.prime.com    ~
~   The world that ought to be"             ~                                 ~
~                             - Neil Peart  ~  Prime Computer, Framingham, MA ~
~                                           ~                                 ~
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