RE: Native Threads in the RTS
Improving. Want to put it in CVS? Simon M can suggest where. Simon | Definitions | ~~~ | A native thread (aka OS thread) is a thread as defined by the operating | system. | A Haskell thread is [*** FIXME - How shall I put this? ***] the thing | you see from Haskell land. A Haskell thread encapsulates the execution of a Haskell I/O action. A Haskell thread is created by forkIO, and dies when the I/O action completes. A Haskell thread is always executed by a native thread. The Haskell RTS creates one or more worker native threads to execute Haskell threads. | Haskell threads may be associated at thread creation time with either | zero or one native threads. Each Native thread is associated with zero | or more native threads. zero or more *Haskell* threads. Can you give an example of when a native thread is associated with more than one Haskell thread? To avoid A Haskell thread associated with a native thread I'd prefer to define the term a bound Haskell thread. I would also like to describe a bound native thread as one that has associated Haskell thread(s). | If a native thread is associated with one or more Haskell threads, | exactly one of the following must be true: | *) Exactly one Haskell thread associated with the native thread is | executing. | *) The native thread is executing foreign code. | *) The native thread and all Haskell threads associated with it are | blocked. You don't say (but you do mean) A bound Haskell thread can be executed only by its associated native thread You don't say (and I'm not sure if you mean) If a bound native thread blocks, all of its associated Haskell threads are blocked too If a bound Haskell thread blocks, its associate native thread and all its associated Haskell threads also block. | The thread that main runs in, threads created using forkIO and threads | created for running finalizers or signal handlers are not necessarily | associated with a native thread. However, an implementation might | choose to do so. But the impl may *not* choose a bound native thread. These must be kept inviolate. | When a free foreign exported function is invoked, the implementation | may freely choose what kind of Haskell thread the function is executed | in. It is not specified whether this thread is associated with a | particular OS thread or not. Again, it must not be a bound native thread. | When a foreign imported function is invoked [by Haskell code], the | foreign code is executed in the native thread associated with the | current Haskell thread, if an association exists. If the current | Haskell thread is not associated to a native thread, the implementation | may freely decide which thread to run the foreign function in. | The existing distinction between unsafe, safe and threadsafe calls | remains unchanged. If a bound Haskell thread calls a foreign import that is not labelled 'threadsafe' which calls a bound foreign export does that work? What if the foreign export was not bound? Similarly, if the foreign import was labelled 'threadsafe', would it work? It's not obvious to me. Some kind of semantics would be good. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
RE: Native Threads in the RTS
Simon P.J. writes: Can you give an example of when a native thread is associated with more than one Haskell thread? I gave an example in my previous message. It's when a bound Haskell thread makes a foreign call which re-enters Haskell via a bound foreign export. You don't say (and I'm not sure if you mean) If a bound native thread blocks, all of its associated Haskell threads are blocked too When Bagpuss goes to sleep, all his friends go to sleep too. :-) (apologies to those who never watched British childrens TV in the 70s). Actually, for any given native thread, only one of its bound Haskell threads can be runnable, the others must all be blocked waiting on the result of a foreign call. So you don't really have to worry about a native thread being bound to multiple Haskell threads; this would be quite tricky to implement in the scheduler anyway. Cheers, Simon ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Native Threads in the RTS
I'll write up a new version of the proposal tomorrow. For now, here are some answers and [at the end] a question about the current FFI specification. Simon Peyton Jones wrote: You don't say (but you do mean) A bound Haskell thread can be executed only by its associated native thread No I don't mean that, and I've been avoiding to say that. This is how the GHC implementation will probably handle it, but I do not want to put that in the general specification. Alastair remarked a while ago: For that matter, I'd like it to be possible to implement this spec in Hugs. Hugs is internally single-threaded but this spec is concerned with what happens when Haskell calls out to C and we could arrange to switch into the appropriate native thread when Hugs calls out to C. After all, the only thing which needs to be guaranteed is that foreign functions called by the Haskell thread are executed in the associated native thread. You don't say (and I'm not sure if you mean) If a bound native thread blocks, all of its associated Haskell threads are blocked too If a bound Haskell thread blocks, its associate native thread and all its associated Haskell threads also block. Does this sound clearer: *) Exactly one Haskell thread associated with the native thread is executing. All other associated Haskell threads are blocked. No foreign code is being executed by the native thread. *) The native thread is executing foreign code. No Haskell code is executing in any of the associated Haskell threads. *) The native thread and all Haskell threads associated with it are blocked. | The thread that main runs in, threads created using forkIO and threads | created for running finalizers or signal handlers are not necessarily | associated with a native thread. However, an implementation might | choose to do so. But the impl may *not* choose a bound native thread. These must be kept inviolate. [...] Again, it must not be a bound native thread. Good point, I had overlooked that. If a bound Haskell thread calls a foreign import that is not labelled 'threadsafe' which calls a bound foreign export does that work? What if the foreign export was not bound? Similarly, if the foreign import was labelled 'threadsafe', would it work? It's not obvious to me. Some kind of semantics would be good. Good question. I reread Section 3.3 of the FFI document (RC7), and now I think I cannot clarify my specification in this respect without first asking others to clarify the current specs - can someone explain the distinction between unsafe, safe and threadsafe in the current FFI to me? I think I know what it does in GHC, but what's the general definition? I've read the description in the FFI document, but it's not clear to me. Is there any reason why safe is the default and not threadsafe? After all, safe is less safe (it might cause the whole program to block). To me, safe seems to be an odd middle ground between speed and safety. What is safe guaranteed/allowed to do? Is it _guaranteed_ to block other Haskell threads under certain conditions? Or is that only an artifact of current implementations? Why are implementations allowed to _silently_ fall back to safe when threadsafe is not supported? Isn't that dangerous? If I'm not mistaken, threadsafe calls from bound Haskell threads would have exactly the same overhead as safe calls. Should we make sure that safe calls somehow block other threads? If so, why? Thats all for now Wolfgang ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
