knack

A really really bad implementation of the Ackermann function.

Playing with distributed programming in Erlang.

How to Run

Edit knack.erl. Change the cookie value if you want, and make sure you change the introducer_node ID to a node that actually exists (or will exist) on your cluster/network.
Compile: erlc knack.erl
Copy knack.beam into some directory on each node of your cluster.
On each node, run:

$ erl -sname SOME_NAME
> knack:join_pool().

(The names can all be the same, as long as the machines have different hostnames.)

On one of the nodes, run: > knack:ack(2, 2) It should return 7 fairly quickly.

"Cool, lemme try `ack(123, 456)`!"

Watch out! The Ackermann function is profoundly evil, and returns ludicrously huge values for even small inputs. Execution times get very long even for relatively small output values. ack(3, 5) takes several minutes on my setup, and only returns 61.

"Will this melt my clusters' CPUs?"

Nope. The Ackermann function doesn't take advantage of concurrency at all: it still can only calculate one level of recursion at a time. Your clusters' CPUs will spend most of their time in blocking states, waiting for the results of deeper recursions.

"Okay, but will it fill up all the nodes' memory?"

No again. Erlang processes are surprisingly light. I suppose if you ran the function with large enough inputs you could run out of memory, but it would take an absurdly long time.

"sigh Will it at least flood my network with traffic?"

Also no. At any given moment, across the whole cluster, only one process is running, either spawning a new process or sending results back up to the process that spawned it. So there is at most one communication going on at once. Your network will have a lot of packets flying around, and the lights on your router will blink pleasantly, but in terms of bandwidth usage it's not much.

"That's boring!"

Yes, it's a very slow and convoluted way of doing not much. But it's also a cool demo of distributed Erlang, a parable about when concurrency is beneficial and when it isn't, and maybe an effective method of making CS professors cry.

It was fun for me to make and test, and I leaned a bunch.

cassvs/knack