I think you're telling fibs. :-p I double-checked srv.arc (which defines 'defop), and the code there opens a thread for every request. This is true in Anarki, in official Arc 3.1, and even way back in Arc0.
Even without parallelism, this would come in handy to prevent I/O operations from pausing the whole server.
Arc does have threads, yes, but it also has a style of mutating in-memory globals willy-nilly. As a result, all its mutator primitives run in atomic sections (http://arclanguage.github.io/ref/atomic.html#atomic) at a deep level. The net effect is as if the HN server has only one thread, since most threads will be blocked most of the time.
I can't find links at the moment, but pg has repeatedly said that HN runs on a single extremely beefy server with lots of caching for performance.
Edit: Racket's docs say that "Threads run concurrently in the sense that one thread can preempt another without its cooperation, but threads do not run in parallel in the sense of using multiple hardware processors." (http://docs.racket-lang.org/guide/concurrency.html) So arc's use of atomic wouldn't matter in this case. It does prevent HN from using multiple load-balanced servers.
Looking back, I see that I did indeed inaccurately answer zck's question about "running single-threaded". I'd like to amend my answer to "No, it runs multi-threaded, but the threads use a single core." Rocketnia is right that arc has concurrency but not parallelism.
"The net effect is as if the HN server has only one thread, since most threads will be blocked most of the time."
Well, in JavaScript, I do concurrency by manually using continuation-passing style and building my own arbiter/trampoline... and using it for all my code. If I ever do something an easier way, I have to rewrite it eventually. Whenever I want to try out a different arbiter/trampoline technique, I have to rewrite all my code.
Arc's threading semantics are at least more automatic than that. Naive Arc code is pretty much always usable as a thread, and it just so happens it's especially useful if it doesn't use expensive 'atomic blocks (or the mutators that use them).
"Automatic" doesn't necessarily mean automatic in a good way for all applications. Even if I were working in Arc, I still might resort to building my own arbiters, trampolines, and such, because concurrency experiments are part of what I'm doing.
All in all, what I mean to say is, Arc's threads are sometimes helpful. :-p
Absolutely. I was speaking only in the context of making use of multiple cores.
I see now that I overstated how bad things are when I said it's as if there's only one thread. Since I/O can be done in parallel and accessing in-memory data is super fast, atomic isn't as bad as I thought for the past 5 years.
