What real-life analogies do you use when explaining threading related problems to co-workers?

Question

It seems that often it is better to use real-life analogies to explain some things. But I can't find a single all-encompassing analogy to talk about all things multithreaded. Sometimes I use the room and the locking door analogy, but it doesn't explain deadlocking very easily. The flashcards and TV-watching paradigms are even more limited in number of scenarios they cover...

Have you found a good analogy for this area? If so, what is it?

Edit: I'd like to be able to explain deadlocking AND race conditions AND high contention scenarios AND thread affinity using the same analogy, if possible

Yes, I agree that it's better when it's real. In other words I agree that it's better to avoid analogies for important problems like this. Unfortunately, life doesn't work this way and sometimes developers with little or no multithreading experience are contributing to parts of the system where the above mentioned aspects become critical and quick path to mutual understanding is required. Hence is the question.

Thank you very much for your input!

Answer 1

Try patting your head and rubbing your stomach at the same time.

Hard isn't it.

Threading's harder.

Answer 2

There are a lot of "real world examples" on the text books. If I recall correctly the Tanenbaum has a lot of them. From what I can recall the Dining Philosophers explained deadlocking pretty well.

Edit: It isn't the Tanenbaum (I've checked)... maybe it was the Stallings, I can't remember)

From the same all mighty wikipedia:

Explaining race conditions:

In many work environments, it is often difficult for software developers or engineers to explain a race condition to managers, who may not be familiar with technical details. In this case, a horse race metaphor example can be provided:

Example: A horse race represents the computer application. Each horse represents a specific element in the application, that are all running in parallel, such as user interface graphics versus network communications running concurrently. The software may require that a specific horse must win the race in order for the application to run properly. The application may work flawlessly if horse #5 finishes first, while the application crashes if another horse finishes the race before horse #5. One possible solution to this problem, is to add synchronization to guarantee that horse #5 finishes before #2. Metaphorically, this is similar to jockey #2 and #5 teaming up to let #5 run ahead of #2.

Sleeping barber problem

Answer 3

I wouldn't really go with analogies. They tend to confuse people, without explaining the real problem; or even worse, give people the impression they know about the domain, when they actually dont :)

If you really want to go with analogies, the good old deposit/withdrawal story is fine for race-conditions. For deadlocks, assume 2 persons who can only do 1 thing at a time. Now, at one point person A waits for person B to finish, while person B waits for person A to finish => stalemate :)

Answer 4

Imagine a chicken, a goat, and a bobcat are all tied up in a burlap sack and then tossed into a river...

Answer 5

There is an intersection near my house that I've seen get into a deadlock state. A line of cars in one direction gets backed up trying to make a left turn but they can't make progress because they are blocked by another line of cars in the opposite direction also trying to make left turns. Every time I get stuck in that I think about deadlock and I'm the only person laughing about it while everyone else is quickly getting frustrated.

Answer 6

Byzantine Generals are very in vogue and can be easily extended to rendition protocols for more complex issues.

Dining philosophers are old favourites but sadly do not cover such a broad scope now they have been dumbed down with TV dinners.

Cigarette Smokers are no longer appropriate, in fact there are moves to make the whole analogy illegal within 50 meters of a food service establishment which does make discussing it over lunch problematic.

Atomic isn't any good anymore, people just start talking about quantums, cat's in boxes alive or deadlocked until you open the box and before you know it it's two entangled phases and you've no idea which one you commit but the docs say either way your server's wave functions going to collapse.

Answer 7

My favorite real-world analogy for Threading is Traffic. Everyone understands it and threading (and especially threading problems) map onto it very easily and simply:

• Simultaneity is obvious: multiple vehicles driving at the same time
• Blocking is also obvious: single-lane road with a vehicle in your way (can also be done with filled parking spots)
• Deadlocks -> gridlocks
• External/imposed locks: traffic lights, stop signs (4-way stops signs are great)
• Priority: emergency vehicles
• Queuing: duh.
• Stacks: parking lots w/ attendants (watch how they pack the cars in)
• Programming with threads: A fleet of delivery trucks in a local municipal area, with new packages to deliver all the time.