🚀 go-pugleaf

andrewm@connect.com.au (Andrew McNamara) writes:

>cdewick@orb.apana.org.au (Craig Dewick) writes:

>>I'd never really given much time to think about the benefits of parity
>>checking of RAM, but this afternoon I have come home to find that my 3/80
>>had self-rebooted at 17:19 when the kernel detected a parity error. I'll
>>let the log file speak for itself.... 

>[...]

>>Anyway, I'm a little confused as to how this error was actually detected,
>>and since it hasn't occured again would I be right to assume that it might
>>have been a random thing and not a *real* RAM problem? I was under the
>>impression that RAM integrity checking was only ever done as part of the
>>power-on self test. 

>The whole point of parity ram is that you detect errors while the machine
>is running. Dynamic ram, being essentially a capacitor, can be discharged
>by radiation - it's actually not that uncommon for single bit errors to
>occur.

Parity RAM is next to useless.  It is really a waste of valuable bits.
The only logical approach is to use no parity, or use ECC RAM.
Parity memory can't even detect but SOME memory errors, and can't do
ANYTHING about it, except warn (or as pointed out, just go ahead and
crash the system anyway).

>The NCR machines that I used to use had error correcting ram - they would
>log a correction about once every six months (these machines generally had
>uptimes in the order of years).

Our Altos 15,000 uses ECC Ram.  In 2.5 years, it has found and corrected
13 single bit errors.  Any one of those errors had the potential to bring
the system down, but ECC prevents it from happening.  The ECC system can
detect a lot more than single bit errors, and can correct single, double, and
some other memory errors (don't have the docs in front of me). The Altos 15000
has an INCREDIBLE uptime- it has never failed and seems to keep running
forever.  ECC RAM is a must for a serious computer system.

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