"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.
