Stop Using “Real Time” Wrong

It is pretty much the definition for real-time for anyone who has worked on embedded, safety, and critical systems. Preemption is about priority, which is a property of parts of the real-time system. A real-time system with only one kind of thing it's doing doesn't have to deal with priority (too much), but in communication systems (like between aircraft and air traffic control) there may be messages which will always be processed within some unit of time, and others which can be deferred or even discarded if they are preempted by a higher priority message. The whole system is still real-time. Usually with a fixed processing cycle. For a radio, maybe every 1/100th of a second it will receive, process, and transmit, repeat. Priorities determine what gets transmitted and processed in any given cycle based on various inputs from the aircraft, the aircraft operator, or other radios in the network.

That 1/100th of a second cycle time and the guarantees associated with it are what make it a real-time system (a hard real-time system, in this example).

Software engineers tend to think in deterministic terms. My code tells the machine exactly what to do and that's it. In real world systems there is uncertainty which requires different thinking. A real-time process needs both, a deadline and the concept of errors. A process must be completed within a deadline or there is an error. As the author said, the absolute time (or latency) is irrelevant. Think of a tennis match. you either hit the ball or you lose. There is a deadline and an error. It's not that complicated yet so many people are confused by the concept.

No, I will not stop. I like using realtime for low latency messaging.

Indeed. I stopped reading after she said:

> Because, brace yourselves, “real time” refers to process prioritization not response time.

She latter back pedals and does define it correctly.

Originally, the term has really been just opposed to batch processing, in connection with systems like Whirlwind, SAGE, the DEC PDP-1 (as the first commercial real-time system) and is tightly connected to the idea of interactive computing. (Another early real-time system outside of the MIT tech path was MIDSAC, 1953.)

Compare Digital founder Ken Olsen's use of the term in his oral history, "The original computing was based on the way people had done computations before. You'd collect all the data, bring it together, process it and send the answers back. The idea of processing it, real time, took a long time to develop. In the world of commercial processing, it's just in the last few years that batch processing has started to disappear. The replacement for it is now called transactional processing, where if you make a transaction with a bank, it is instantly taken care of." (https://www.computerhistory.org/collections/catalog/10263035...)

There are many applications to this, each coming with their own set of implications. E.g., if you want a smoothly running interactive program on an early PDP computer, you want to have program paths of about equal runtime duration. Which may mean in turn that you would want opt out of no-operation paths as late as possible, by this stabilizing run-time, rather than as early as possible. (E.g., we perform calculations anyway, but apply the result only under a certain condition.) Or, if it is about complex processes, where it may mean that we will fulfil a contract in a guaranteed span of time to facilitate cooperations of any kind (e.g, what Olsen calls transactional processing or matching sampling rates with digital computers as a replacement for analog lab computers). Or it may be bound to a particular domain, where stale data isn't of any use (e.g., compare Whirlwind's origin in a digital flight simulator.) Or it may be just about a system being able to respond to input at all.