Elevators

A cool connection is that a spinning-disk hard drive (HDD) is actually kind of like one really long elevator, just wrapped around a spindle instead of perfectly vertical. The SCAN algorithm is actually a disk-scheduling algorithm!

https://en.wikipedia.org/wiki/Elevator_algorithm

I wonder if this is an artifact of how the author used random destinations. I worked in a building that used Destination Dispatch, and the common travel pattern seemed to be:

- Everyone who is not on the ground floor generally want to go to the ground floor.

- People who are on the ground floor generally travel in large groups to the same destination.

This happens because people who worked on the same floor often leave for lunch at the same time, and return at the same time to the same floor. Destination Dispatch helps in this case because it's batching large groups of people with the same destination.

In the game, some of the elevators are automated, and I wanted to choose an algorithm that best aligned with what players would expect an elevator to do. I ended up going with something close to LOOK, which (as the article states) is mostly what people expect. Except in case of ambiguity, I had it prioritise floors that have been waiting longer, to improve the p90, which is important in the game.

But now, add in these challenges:

- double-deck cabs (where attached cabs cover two floors at once)

- transfer floors between shafts

- express shafts

and the best (or at least, most expected) algorithm becomes much less obvious! I'll leave it as an exercise for the reader to figure out the best one.

Luckily I was in charge of a game, not a real elevator system, so I just had to find a heuristic that was good-enough, and could tweak the rules to let players manually override the elevator's plan if they don't like where it's going. This seems to have satisfied most players.

I swear no matter if it was busy or not, it seemed like everyone was assigned to the same one or two elevators. The wait was always so long too, I used the stairs most of the time if just going <6 flights.

Someone besides RCCL needs to study that algorithm.

Efficient elevator algorithms will often suggest moving an elevator into a preempted positioning strategy, based on time of day or peer elevator positions. The scheduler might move an elevator up to offset one coming down, for example. The demand-signal algorithms at least minimize movement in this way.

I think minimizing maintenance at the cost of increasing wait times is probably an important balance to get right. No doubt, the cost owners for maintenance are not going to perceive the wait time of passengers to be as important.

I can see how that could be challenging when people get off on intermediate floors, making a full car available again, changing status. And, if 3 people are waiting and there is room for 1, should it stop or not? Etc.

My favorite takeaway from this is how simplicity often beats complexity.

1. https://en.wikipedia.org/wiki/The_Intuitionist

There’s something so satisfying about watching a machine just dutifully work through queued tasks like this.

Using such a primitive elevator gave me a newfound appreciation for the complexity of modern designs. The queueing algorithms are subtle but clever, and the difference in efficiency is instantly obvious once you've dealt with the alternative.

Of course, user error is also a factor, so this isn't accounting for people not understanding how to use it and making things worse that way.

Beyond the content, the font, style, etc made it a pleasant experience for me.

[1]: https://clarkmoody.com/Moody_AgentBasedElevatorControl.pdf

I want something similar for advanced sensor stop lights...

I've long toyed with creating an elevator simulator game, but every time I build a prototype I realize it's not that fun to manually dispatch elevators, and playing with algos only goes so far! Elevator Saga is great for the programming side: https://play.elevatorsaga.com/

That will need planning and assignment I guess? But at what load figures does it become more efficient?

https://jslandy.com/elevators/

0: https://news.ycombinator.com/item?id=35152341

1: https://computer.rip/2023-03-13-the-door-close-button.html

For example, some elevators allow adjusting the door closing/opening speeds etc., I'd be curious what other things can be adjusted.

I watch it like once a year because it always tickles some part of me. Like all the different modes you can get an elevator into. The most fancy one people might encounter is when moving into or out of a building. The front office can give you a key to give exclusive control over an elevator so your movers aren’t waiting around on elevators. Put it in that mode and it will stop responding to calls from other floors. Only the person with the key can control the elevator. You get on, select the floor, door closes elevator goes, and then just chills there with the door open waiting for you. Annoying for the rest if the building (the building is down an elevator when in that mode) but is amazing for the person using it! But there are way, way more depending on the installation and function.

Fun fact: most elevator shafts are sealed at the top as tight as possible to prevent them from becoming a giant chimney in a fire. It never even occurred to me until I was in a mechanical room wondering “where is the hatch to look down the shaft?” The answer is “there is none, and it’s a feature not a bug.” You want to block all airflow so fire doesn’t chase up the shaft into neighboring floors. Who knew!

My takeaway is that the benefit of using a much more complex algorithm is marginal.

At a previous employer we had heteregenous elevators. In one bank, some went to the lower half of floors only while others went to all and the company installed a "smart" elevator system. I kind of became known because I was constantly yelling about this system in the feedback group because I hate "smart" elevators and and (IMHO) they just don't work. What actually works is express elevators and sky lobbies.

This article covers one of the deficiencies this system had: full elevators. For example, you'd want to go down and an elevator would skip your floor because another had already been assigned. That one would show up full and you couldn't go down. Down wasn't so bad because the stairs weren't a bad option but up was terrible. Going up a few floors was fine. Going up 20 was... a bigger issue.

Back in the day we had elevator operators and people in the lobby during the morning rush who would shepherd people into particular elevators. I actually think this system works way better than anything technology has come up with. Even if you nail the implementation (and I've never experienced elevators that have), people don't read and will just get into elevators anyway.

Anyway the article says that generally speaking on smaller banks simple up and down buttons work best. I absolutely agree.

There's a deeper issue here though and that is solutions looking for a problem. Nobody is making money from up and down buttons. They are fromn selling smart elevator solutions. And you see this everywhere in life. It basically devolves into rent-seeking behavior. Salespeople wine and dine a couple of people responsible for making decisions and then make bank on selling something nobody wants or needs as well as the constant maintenance and updates.

Really? Maybe I'm lucky I never had to deal a lot with elevators.