alt Hacker NewsElevators
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Back in highschool, simulating different elevator algorithms was one of the projects I implemented during my CS class. It wasn't for the class — AP CS did not require anything like actual programming — but it was a fun project.
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!
I was on a Royal Caribbean ship a couple weeks ago (Utopia, 18 decks, 5,668 passengers). It has 24 main elevators split between front and back, left and right. So a bank of 6 in front of you each time you needed a lift. You press your floor on a touchpad and it assigns you an elevator A through F.
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.
For folks that have never seen elevator scheduling the game: https://play.elevatorsaga.com/ Enjoy this rabbit hole :D
> Destination Dispatch [...] are in general worse
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.
Hah, I thought a lot about this problem while developing Sky Lobby, a mobile game (iOS / Android) about controlling and automating elevators.
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.
In these various elevator algorithms, it's not commonly discussed the overall wear and tear on the elevator. More movements would typically translate to faster repair times. Hydraulic fluids needing to be replaced, parts breaking, etc.
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.
Oh my god, every time I'm waiting for the elevator, I think of how annoying it must be to be someone who's building the algorithm to make sure that you minimize the amount of wait times between picking up people and taking them to their destination. Then I also wonder if the people who apply the logic and program the logic into the elevators are actually evil sadists that are doing it on purpose to make us wait longer enough and suffer a little extra.
The biggest problem I always have with elevators is not the algorithm, it's the people seemingly being unable to grasp the concept of pressing either the up or the down request button, depending on where they want to go. Almost always I find someone will press both, because "then the elevator comes faster". Completely ignoring the fact that they end up going the wrong way first half the time, and adding an unnecessary halt for everyone already in there. How hard can it be to understand?..
No mention of an algorithm/sensor for checking capacity of the car. It's a pain when your elevator car shows, it's full, and your request is canceled, meaning you have to press again. Better if the car was full and just moved on.
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.
This is commonly referred to as the "dial-a-ride" problem, which is an optimization problem with a multi-variate cost. Back in college for my senior design project we built a multi-axis closed loop servo system which would emulate a slide guitar. The dial-a-ride problem is identical to optimizing travel for a string of notes/chords.
Destination dispatch should be no worse than standard up/down buttons, right (at least in theory)? It provides additional information about the destination, but it should be possible for that information to be interpreted as if it were just an up/down button press, so RSR could still be used. I have a feeling a better algorithm could in fact make destination dispatch slightly better than RSR... or am I missing something?
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.
I've never really liked Destination Dispatch elevators. Besides struggling to find the car I've been assigned to, sometimes you really have to hustle across the lobby to catch it. Then you also run into visitors that assume it's a normal car and can hit a floor button when the get in, only to realize they've walked into a car without buttons.
Slightly OT, but this reminded me of Colson Whitehead's _The Intuitionist_[1], an amazing speculative mystery novel about two dueling schools of elevator inspection philosophies: the Empiricists and the Intuitionists.
If you optimize for initial wait times, someone who is going to a floor not usually visited might effectively get trapped on an elevator which shuttles between more trafficked floors to reduce wait for elevators overall. Maybe this is very unlikely in practice.
It also depends on the person understanding what the elevator button does: https://blog.jgc.org/2010/06/elevator-button-problem.html
Makes me want to play Sim Tower again.
There’s something so satisfying about watching a machine just dutifully work through queued tasks like this.
My friend lives in a building with one of those super old elevators with the manual double doors and no queuing. It's the sort of elevator that someone probably got paid to operate once upon a time.
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.
I feel like most cases of wait times are due to elevators "waiting" empty in the wrong floor. They either all wait in the bottom floor, or all wait where they were last left at. Would be better if their waiting floors were distributed throughout the floors so that first response is fast.
This is super interesting. I've always wondered how the complicated things work!
Enjoyed the read, thanks!
Beyond the content, the font, style, etc made it a pleasant experience for me.
Super interesting. I've always wondered how exactly they work.
What this doesn't include: not all people are equal. Do you want the CEO of your new office building to have to wait behind a bunch of plebeians? At least one car is going to have to be reserved for the better people.
Lol, love this post. Every single day when I take the elevators at the office I try and figure out what the algorithm is behind it. Been too lazy to just research it. Now I know!
Parking is a big factor too. Where you put the elevator when it is not being used can mean the difference between the doors opening instantly and worst case scenario.
One of the most frustrating assignments[1] (2011) in my computer science education was trying to build a better mousetrap for elevator control.
[1]: https://clarkmoody.com/Moody_AgentBasedElevatorControl.pdf
This is beautiful. By the end of it I started to feel like I was calculating complicated chess lines in a middle game.
This is absolute catnip to me. Fantastic job to the author! It really scratches the itch of wanting to understand elevator dispatch algos!
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/
I wonder how a TWIN system with 2 cars per shaft factors in here: https://www.tkelevator.com/us-en/products/elevators/all-elev...
That will need planning and assignment I guess? But at what load figures does it become more efficient?
You have no idea how excited I was to see this article today. I've long been fascinated with elevators and the algorithms that control them. Thank you!
I recently wrote an article about this problem, and detail one reasonable algo for minimizing wait time. (Doesn’t consider concept of cars being full though, interesting addition)
This reminds me of elevator saga (https://play.elevatorsaga.com/) - little programming game where you have to program elevator schedules.
Is there a comprehensive overview of a) multi-bank elevator algorithms that are out there, b) the config parameters that the elevator tech/company can set on an elevator system?
For example, some elevators allow adjusting the door closing/opening speeds etc., I'd be curious what other things can be adjusted.
In a similar vein, there was an interesting discussion [0] a while back about elevator buttons [1] and how timing for door close/open works, along with a bit of the history and regulations that go into them.
0: https://news.ycombinator.com/item?id=35152341
1: https://computer.rip/2023-03-13-the-door-close-button.html
Anybody knows what algorithm HITACHI is using on their elevators? Their elevators feels most “responsive” in high rises.
I think this comparison is also assuming the distribution of request & requested floor is uniform? When actually near pairs are less likely (I'll just take the stairs) than bottom to top, so even analysing it is more complex than at first glance.
My biggest pet peeve is elevator banks where they don’t make some number of idle elevators automatically come back to the lobby. Half your traffic is over there! Everyone coming in has to wait an extra minute or two because all your elevators were chilling on the 35th floor and above.
Reminds of something I haven't thought about in a very long time... SimTower
I absolutely adore this, I am always trying to guess whichever elevator will come in first in large crowded buildings. Thanks for this article.
I think a small improvement that could be made is to have the dot representing the person be colored the same as a dot on each floor, which would obviate the destination floor for each individual.
I've always wondered about elevator algorithms. This is awesome.
I didn't realize that neal.fun had some competition.
Don't forget the executive elevator algorithm (EEA), which uses a priority interrupt to take passengers directly to the top floor.
This article gives me war flashbacks about a particularly irritating OO design interview from 5 years ago.
If you like elevator hacking don’t forget the seminal DEF CON talk by Deviant Ollam and Howard Payne: https://youtu.be/oHf1vD5_b5I
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!
Maybe I missed it, but the algorithm should also redistribute empty elevators that stop too closely to each other.
As someone who lives in a penthouse, elevator’s frustrations are real struggle especially in rush hours, because the elevator has to travel dedicated to you rather than you piggy backing on one that is already going up or down.
did he factor in the lifting speed?
I love elevator simulations.
>All this technology just to underperform the S&P
My takeaway is that the benefit of using a much more complex algorithm is marginal.
I don't think it was mentioned, but the algorithm would be different based on the behavior of the majority. In some buildings most traffic is either up from lobby or down to lobby, e.g. it's very rare for people to go from floor 5 to floor 6. In some it's the other way around, where majority of moves don't involve lobby.