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 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.
The worst situation I've encountered is after a large conference when everyone is leaving the hotel at once. An elevator quickly fills up on the way down, but then wastes time stopping at every subsequent floor with a call, even though no one else can fit inside. At each floor the elevator stops, the doors open, the waiting people see it's already packed, the doors close, and this repeats on the next 15 floors before finally reaching the bottom where everyone gets off. If the algorithm could know an elevator is completely full and only stop at its destination floors rather than every floor along the way with a call, it would be far more efficient.
theodorejb
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.
hermanschaaf
I am currently staying in an AirBnB of a 60 floor tower, and the three elevator shafts definitely cannot handle this (I don't think the building was designed with renting to tourists in mind). The elevators are frequently completely saturated during weekends, meaning that the elevator stops at each floor, but noone can get in because the elevator is already full. This means the backlog of people wanting to use the elevators is increasing until after rush hour, and I have at one time experienced a waiting time of over half an hour.
What I take from this is, elevators need a proper check to see if they are full, and if so skip floors.
woeh
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?..
olex
Aside for the algorithms and how well they work, there is a psychological aspect to how people perceive "wait" times. It turns out just waiting is very annoying. But having people do anything during that same wait period → perceived as progress → not upset.
An example iirc was waiting for luggage at the airport. It takes min of x minutes for the first bags to show. In one version, passengers went directly from the gate to the luggage carousel and waited x mins for the first bag → unhappy. Then the airport decided to add a long, round-about "walk" path from the gate to the luggage area that burned some of those minutes → same total "wait" but happier passengers.
psadri
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.
orliesaurus
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!
comments (10)
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
peterldowns
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.
omoikane
brandonpelfrey
theodorejb
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.
hermanschaaf
What I take from this is, elevators need a proper check to see if they are full, and if so skip floors.
woeh
olex
An example iirc was waiting for luggage at the airport. It takes min of x minutes for the first bags to show. In one version, passengers went directly from the gate to the luggage carousel and waited x mins for the first bag → unhappy. Then the airport decided to add a long, round-about "walk" path from the gate to the luggage area that burned some of those minutes → same total "wait" but happier passengers.
psadri
orliesaurus
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!
cruffle_duffle