I've now written up a slightly more formal proposal for native threads. (OK, it's only a tiny bit more formal...) I doubt I have explained everything clearly, please tell me which points are unclear. And of course please tell me what you like/don't like about it. I have some rough ideas on how to implement the proposal. I would be ready to invest some time, but I don't have enough free time to make any promises here. The discussion has to be finished first, anyway. Cheers, Wolfgang ******************* Native Threads Proposal, version 1 Some "foreign" libraries (for example OpenGL) rely on a mechanism called thread-local storage. The meaning of an OpenGL call therefore usually depends on which OS thread it is called from. Therefore, some kind of direct mapping from Haskell threads to OS threads is necessary in order to use the affected foreign libraries. Executing every haskell thread in its own OS thread is not feasible for performance reasons. However, perfomance of native OS threads is not too bad as long as there aren't too many, so I propose that some threads get their own OS threads, and some don't: Every Haskell Thread can be either a "green" thread or a "native" thread. For each "native" thread, there is exactly one OS thread created by the RTS. For a green thread, it is unspecified which OS thread it is executed in. The main program and all haskell threads forked using forkIO are green threads. Threads forked using forkNativeThread :: IO () -> IO () are native threads. Execution of a green thread might move from one OS thread to another at any time. A "green" thread is never executed in an OS thread that is reserved for a "native" thread. A "native" haskell thread and all foreign imported functions that it calls are executed in its associated OS thread. A foreign exported callback that is called from C code executing in that OS thread is executed in the native haskell thread. A foreign exported callback that is called from C code executing in an OS thread that is not associated with a "native" haskell thread is executed in a new green haskell thread. Only one OS thread can execute Haskell code at any given time. If a "native" haskell thread enters a foreign imported function that is marked as "safe" or "threadsafe", all other Haskell threads keep running. If the imported function is marked as "unsafe", no other threads are executed until the call finishes. If a "green" haskell thread enters a foreign imported function marked as "threadsafe", a new OS thread is spawned that keeps executing other green haskell threads while the foreign function executes. Native haskell threads continue to run in their own OS threads. If a "green" haskell thread enters a foreign imported function marked as "safe", all other green threads are blocked. Native haskell threads continue to run in their own OS threads. If the imported function is marked as "unsafe", no other threads are executed until the call finishes. Finalizers are always run in green threads. Issues deliberately not addressed in this proposal: Some people may want to run several Haskell threads in a dedicated OS thread (this is what has been called "thread groups" before). Some people may want to run finalizers in specific OS threads (are finalizers predictable enough for this to be useful?). Everyone would want SMP if it came for free (but SMP seems to be too hard to do at the moment...) Other things I'm not sure about: What should we do get if a foreign function spawns a new OS thread and executes a haskell callback in that OS thread? Should a new native haskell thread that executes in the OS thread be created? Should the new OS thread be blocked and the callback executed in a green thread? What does the current threaded RTS do? (I assume the non-threaded RTS will just crash?)