Great, thanks. I hope you'll keep it up to date so that by the time the discussion converges it can serve as a specification and rationale. We can put it in CVS too... Simon will think of where!
Until then, I'll play the role of a "human CVS server".
Ultimately it'd be worth integrating with http://www.cse.unsw.edu.au/~chak/haskell/ghc/comm/rts-libs/multi- thread. html
Of course. Some parts should be part of the user documentation, while others should probably be considered implentation details.
| A foreign exported | callback that is called from C code executing in that OS thread is | executed in the native haskell thread.
This is the bit I don't understand. Is the only scenario you have in mind here
native Haskell thread calls C which calls Haskell
and you want all that in the same native thread?
Yes, exactly.
What about this?
native Haskell thread calls C which installs a pointer to a foreign-exported Haskell function in some C data structure
Later... some other Haskell thread calls C which waits for an event which calls the callback So the callback was installed by a native thread, but won't be executed by it. Is that ok?
Definitely. It's the same way it works in C. What thread some code executes in depends on what thread the code is called from.
Anyway I think it would be worth explaining what is guaranteed a bit more clearly.
I'm not sure how... to me it looks like I already specified this exactly ;-). Anyway, I've added some examples to the proposal to clarify what I mean.
| 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.
No, I don't think so. The reason that 'safe' is cheaper than 'threadsafe' is that the current worker OS thread does not need to release the Big Lock it holds on the Haskell heap, thereby allowing other green threads to run. Instead, it holds the lock, executes the call, and returns. At least I think this is the idea, but it's all jolly slippery.
I thought that was "unsafe"? The "safe" version still does quite a lot (after all, a callbacks are allowed, so is GC). In addition, "threadsafe" may start a new OS thread in order to keep executing green threads. On the other hand, we might simply leave it unspecified: If people want to know what happens to other threads, they should use "threadsafe" or "unsafe". The exact behaviour of "safe" seems to be an implementation detail.
| Other things I'm not sure about:
Presumably if a native thread spawns a thread using forkIO, it gets just a green thread? If it used forkNativeThread it gets a distinct native thread. Better say this.
"The main program and all haskell threads forked using forkIO are green threads. Threads forked using forkNativeThread :: IO () -> IO () are native threads." I thought that was clear enough... I've added a note. Cheers, Wolfgang ***************** Native Threads Proposal, version 2 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. (Note: The type of the current haskell thread does _not_ matter when forking new 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. It is implementation dependent whether 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?) Some (not very concrete) examples: 1.) Let's assume a piece of C code --- lets call it foo() --- was called by a green haskell thread. If foo() now invokes a haskell function, the haskell function might be executed in a different OS thread than foo(). This means that if the haskell code calls another C function, bar(), then bar() doesn't have access to the same thread-local state as foo(). For example, if foo() sets up an OpenGL context, then bar() can't use it. 2.) If foo() was invoked by a native haskell thread, it is guaranteed that all haskell functions invoked by foo() run in the same native haskell thread and therefore in the same OS thread. Now if the haskell code again calls bar(), then bar() is executed in the same OS thread as foo() and the native haskell thread. This means that bar() has access to the same thread-local state as foo() (---> OpenGL works). 3.) A piece of C code creates a new OS thread and calls a haskell function in that new OS thread. I don't think it makes sense to tun the haskell function in an existing haskelll thread, so we'll create a new one. What kind of haskell thread (native or green) should the haskell function run in? I'm slightly in favour of a new native thread (after all, the C code might have it's reasons for spawning a new OS thread).