| I've started looking into the docs on GHC's | implementation of Concurrent-Haskell, and I got | confused about the architecture. Different sources | seem to indicate that it either: | - Uses one OS thread, and blocking IO calls will | therefore block all the Haskell-threads, or | - Uses one OS thread, occasionaly (50 times a second?) | calling select() to simulate blocking IO calls without | blocking other Haskell-threads, or | - Uses a pool of OS threads so that when a blocking IO | call is made, another thread takes over execution.
Basically the last of these. (At least in the most recent GHC.) Simon Marlow implemented a web server using Concurrent Haskell, and has a paper about it too. http://www.haskell.org/~simonmar/bib.html
But we should really point out that you get the second method by default; the "pool of OS threads" method is currently experimental and not enabled unless you rebuild GHC's RTS with the right flags. The default single-threaded IO multiplexing works pretty well, although it doesn't scale to multiple processors of course. Cheers, Simon