FWIW, I dipped into the higher levels of real-time-systems development
several times in my career. The earliest being a control system (circa
1981) for the LANL Proton Storage Ring where one naturally can't afford
anything *but* failsafe implementations, etc. The stakes are just too
'ffing high and the coupling to electrooptomechanical systems quite
intimate.
The "digital" components of such systems might have had the opportunity
to ignore timing issues and simply "execute the same steps" regardless
of timing. But in fact many software-driven (sub)systems represented
time-critical processes and sometimes were up agains the timing limits
of the analog components which had no leeway in their "execution".
There are all kinds of analogies in federated (distributed) simulation
environments which Glen (and others here) probably know much better than
I, where different "clocks" matter, and different levels of
synchronization and reproducibility are in play. The Postscript
interpreters, printers, and film recorders were also pseudo real-time
systems since some of the timing components were in fact software
controlled (for example, the film recorders were "stroke" devices with
software driving D-A converters to "sweep" out vectors and "clip" the
on/off of the beam with appropriate analog component delays/biases/gains
needing to be calibrated for. Fortunately failures in this step did
not (usually) damage anyone or risk anyone's health and safety (like the
beam in the PSR did).
Regarding identity and equivalence, I prefer the phrase: "close enough
for who it's for"...
On 10/21/22 11:18 AM, glen wrote:
Ha! If we're going to argue about words, then let's stick with the
word "identity" and skip the "metaphor" nonsense. You and Frank seem
to be using the word in a weird way. Identity means "the exact same
particular thing over any differencing available" or somesuch. I mean,
it's used that way in phrases like "identity theft" as well as
mathematical identity. It's equivalence sets all the way down. I just
can't imagine any working computationalist would ever say anything
like "executed identically" unless ... well ... the exact same
process, with the exact same steps, happened.
I suppose there are deep philosophical intuitions pried at by the
words "emulation" versus "simulation". And one can argue (again with
help from Christian List) about whether there exist fully closed
ontological walls like we try to create with things like Jails,
HyperV, Docker, VM's like Java's, etc. But "execute identically" is a
phrase that would only be used by someone who worked *way* above such
levels (assuming levels even exist at all). It's a bit like talking to
the kids programming websites these days, with access to infinite disk
space, infinite memory, steeped in continuous delivery, etc. [⛧]
Layers of abstraction are fine. Use 'em when you need 'em. But we
shouldn't posture by invoking things like "instruction sets" and
"execute identically" in the same breath. (Not that you did that ...
just sayin'.)
[⛧] Rant: This is a good talk
<https://www.youtube.com/watch?v=8Ab3ArE8W3s>. But I get super
irritated when people use *toy* code in their rhetoric and leave large
scale deployment as an exercise for the reader. Yeah, fine. The REPL
is cool and all. But when my simulation takes a fvcking WEEK to
execute, it's difficult to sympathize. I've recently been playing
around with VSCodium, which is pretty cool. But whatever, man. I still
have to upload the code somewhere and execute it. Get off my lawn!
On 10/21/22 09:24, Steve Smith wrote:
As a counter-example, we ran film recorders whose "guts" were built
by Ed Fredkin's Information International company and were built to
the spec of Dec PDP-11 (I think 11?) and it was anecdotally agreed
among the user community (of a few thousand delivered units in the
world?) that these PDP-clones *never* failed to execute the code
identically to the machines they were patterned after. I don't
remember the details of implementation of these 70's era hardware
designs, but I understood that they III designed their own PCBs but
(obviously?) used the same CPU chips... I don't know about all the
other support components... A likely answer to this pondering is that
these machines did not run a general purpose OS and the III
software/system people probably made up for any differences in
Software/Timing/Error Handling?
If Owen is listening in here, I think he was there for more than a
little of this from inside Apple/Sun?
- Steve
PS. To concede/confront glen's sentiment that: " 'Metaphor' is an
evil word, used only by manipulators and gaslighters", I would
offer that the use of *conceptual metaphor* is to thinking as noise
is to simulated annealing, and his point about "tighter or looser
equivalence" might well be the best argument *for* the use of
metaphorical thinking? I can't believe I'm stirring/kicking this can
of worm-hornets down the street again...
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