Newsgroups: comp.arch
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Workstation Data Integrity
Message-ID: <1990Aug4.231129.1358@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <1990Aug3.204358.330@portia.Stanford.EDU>
Date: Sat, 4 Aug 90 23:11:29 GMT

In article <1990Aug3.204358.330@portia.Stanford.EDU> jackk@shasta.stanford.edu (Jack Kouloheris) writes:
>I'm a bit puzzled by the lack of any type of memory error detection/
>correction on many workstations and high-end PCs. These workstations
>are beginning to have memories that rival or exceed those of
>the previous generation of minicomputers, which almost always used
>some sort of ECC protection...

DRAM chips have improved a great deal in the last decade.  Thank heavens.
Speed pressures have also increased a lot, and ECC in particular tends to
incur speed penalties.

And it's a tempting thing to leave off when timing or board space gets tight.
After all, the thing still works...

>Some SUNs have parity checking on the memory system, but what does
>the OS do when a parity error occurs, since correction is not
>possible ?

Depends on the situation.  A parity error in a code page is harmless --
just bring in a fresh copy from disk.  A parity error in data in an
ordinary user program can be dealt with by killing that program.  You
get into difficulties only when the error hits the kernel or some vital
system daemon.  If errors are rare enough, parity is adequate.

(Many people -- e.g. the imbeciles who have their kernels kill processes
at random when swap space is short -- overlook the fact that some of
the daemons are every bit as vital to proper operation as the kernel.
Fortunately they're often not all that large, and are less likely to
get hit by memory errors than elephantine user programs.)

If you want something to be concerned about, consider that while most
PCs have parity, almost all PC software ignores parity errors.
-- 
The 486 is to a modern CPU as a Jules  | Henry Spencer at U of Toronto Zoology
Verne reprint is to a modern SF novel. |  henry@zoo.toronto.edu   utzoo!henry