Path: utzoo!attcan!uunet!van-bc!
From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Parity Checking / ECC RAM on the A3000
Message-ID: <1710@lpami.wimsey.bc.ca>
Date: 7 Jun 90 23:53:10 GMT
Lines: 73
Return-Path: <lpami!lphillips>
To: van-bc!rnews

In <3649@tymix.UUCP>, pnelson@hobbes.uucp (Phil Nelson) writes:
>
> You may want to consider that what you have reading is the opinion of some
>people that memory chips are so reliable that "parity is useless". The facts
>(if we had any) may be otherwise. If the Amiga had parity, it would easy to
>get good data on the reliability of the memory IN THE BOX (not in some chip
>test lab) and IN THE FIELD (not some clean, quiet final test area).

Since I am the one that used the words "parity is useless", I think I will say
that you should refrain from placing words in my mouth that were never there. I
did _not_ say make that statement because I think that memory is too reliable.
I said it because I see no real use for adding extra memory, at extra cost,
thereby statistically reducing reliability, for the sole purpose of either (a)
informing the user that a partity error has occurred, or (b) crashing the
program or system.


>These are good points. I think it very likely that the memory system is not
>the greatest cause of unreliability on the Amiga. Certainly not if you
>include software bugs. This does not prove that parity checking is useless,
>but that other measures are needed too. The order in which to take measures
>to improve reliability is not determined exclusively by which is the worst
>problem, it may be reasonable to start with a problem that is not the worst,
>if a solution is easily implimented (memory parity checking, for example).

In what way do you see parity checking as 'measures to improve reliability'?
I think you are confusing reliability with some other parameter. Parity
checking, if it only informs you of a parity error, does not change the
reliability of a system at all. If it is used to halt a task or a system, it
does, in fact, reduce reliability.

You might want to ask yourself what the benefits of parity checking are, vs.
the cost of it.

Benefits:

	Information. You know you had a memory error, and have the option of
rerunning anything that might possibly have been affected by it.

  Information. You know that after running any particular program, if you were
not informed of a parity error, that any errors you may have, were caused by
something else. Note that the lack of a parity error says nothing about the
accuracy of your results, and that the presence of a parity error likewise says
nothing about the accuracy of your results.

Costs:
  Parts.

  Wasted time/resources. If a parity error occurred in a non-important part of
memory (including the parity bit memory itself), you have no way of knowing
that you didn't need to rerun a program. The mere presence of a parity error
indication tells you nothing but that there was a parity error, but encurages
users to rerun things, and lulls them when the little light doesn't come on.

> I think the cost of ECC cannot be justified on the Amiga, unless for special
>applications. The added cost of simple parity checking (not very great) might
>easily by justified because it would help by allowing the early detection
>and repair of machines with memory problems. It would be especially useful
>for machines with flaky, intermittent memory.

The most useful thing for machines with flaky, intermittent memory is a trip to
the repair shop. Flaky, intermittent memory will show up in other ways, without
having to add more flaky, intermittent memory.

-larry

--
The raytracer of justice recurses slowly, but it renders exceedingly fine.
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|   //   Larry Phillips                                                 |
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