I am a new learner of Haskell and I am interested in Haskell's
concurrent model. Can somebody give me a brief intro about Haskell's
thread model, like how the use-level threads are mapped to kernel thread
and what scheduling mechanism that Haskell uses, or point me to some
links or documents that I can learn from.
In the interpreter Hugs, all Haskell threads are run in one kernel thread. They are scheduled cooperatively; thread switches only take place when a function from the "Concurrent" module is called.
In the currently released version of GHC, all Haskell threads are run in one kernel thread, too; however, thread switches can take place whenever memory is allocated --- and in Haskell, that means "almost always". The optimizer manages to compile a fibonacci function that doesn't allocate any memory, but in the real world, it's as good as real preemption.
If you compile the bleeding-edge GHC from the CVS HEAD, you'll get something else; while "most" threads (those created using "forkIO") are still light-weight threads that are scheduled in just one kernel thread, you can also create threads that get their own operating system thread. This is solves all the problems that lightweight threads can have with foreign (i.e. non-Haskell) libraries.
You should also note that no Haskell implementation currently supports SMP; even when multiple kernel threads are used, there is a mutual exclusion lock on the Haskell heap, so a multithreaded Haskell program will use only one CPU on an SMP system.
I hope my answer was useful...
Cheers,
Wolfgang
P.S.: If you want to do me a favour, you could tell your mail program not to send multipart or HTML messages to the list; they look terrible to people like me who get a daily digest from the mailing list.
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